A KPI-Based Framework for Evaluating Sustainable Agricultural Practices in Southern Angola

Abstract

Agricultural production in southern Angola faces challenges due to unsustainable practices, including inefficient use of water, fertilizers, and machinery, resulting in low yields and environmental degradation. Therefore, clear and measurable indicators are needed to guide farmers toward more sustainable practices. The scientific literature insufficiently addresses this issue, leaving a significant gap in the evaluation of key performance indicators (KPIs) that can guide good agricultural practices (GAPs) adapted to the context of southern Angola, with the goal of promoting a more resilient and sustainable agricultural sector. So, the objective of this study is to identify and assess KPIs capable of supporting the selection of GAPs suitable for maize, potato, and tomato cultivation in the context of southern Angolan agriculture. A systematic literature review (SLR) was conducted, screening 2720 articles and selecting 14 studies that met defined inclusion criteria. Five KPIs were identified as the most relevant: gross margin, net profit, water use efficiency, nitrogen use efficiency, and machine energy. These indicators were analyzed and standardized to evaluate their contribution to sustainability across different GAPs. Results show that organic fertilizers are the most sustainable option for maize, drip irrigation for potatoes, and crop rotation for tomatoes in southern Angola because of their efficiency in low-resource environments. A clear, simple, and effective representation of the KPIs was developed to be useful in communicating to farmers and policy makers on the selection of the best GAPs in the cultivation of different crops. The study proposes a validated KPI-based methodology for assessing sustainable agricultural practices in developing regions such as southern Angola, aiming to lead to greater self-sufficiency and economic stability in this sector.

1. Introduction

Intensifying sustainable agricultural production should be the main strategic objective for the coming decades [1]. Crop production with unsustainable practices and poor development in the agricultural sector led Angola to join the Capacity Development for Agricultural Innovation Systems (CDAIS) project, which aims to make agricultural innovation systems more effective and sustainable [2].

Most Angolan farmers depend on the rains to grow a variety of crops. The drought, mainly in the southern region, has caused a sharp decline in agricultural production at a national level, and as a result, has increased the need for imports, mainly of maize [3].

There is a lack of knowledge among farmers in Angola regarding the key performance indicators (KPIs) for selecting good agricultural practices (GAPs). Of the 2,289,644 family farm holdings (FFH), approximately 228,964 (10%) use inorganic fertilizers, and 526,618 (23%) use agricultural practices with organic fertilizers in the cultivation of crop varieties. At the provincial level, the provinces with the highest proportions of FFH using inorganic fertilizers are Huambo with around 29%, Luanda with around 28% involved in agricultural production, and Kwanza South with around 18% [4].

The sustainability of GAPs is generally assessed using agro-environmental indicators [5]. With the growing awareness of environmental problems in recent decades, nitrogen use efficiency and water use efficiency are examples of agro-environmental indicators that have been developed to assess the adverse effects of cropping and farming systems, such as agrochemicals and water pollution by nitrates, phosphates, and pesticides [6].

An important challenge for the research community is to identify clear, scientifically based indicators that are accessible and capable of summarizing multiple dimensions of sustainability to support decision-making, preferably in ways that allow a direct comparison of policy alternatives [1].

Different environmental, economic, social, and technological performance indicators are to be found in the literature and applied in the agricultural sector to quantify the production of different crops in different parts of the world [7,8,9].

In the set of articles found, there was one article that investigated the use of KPIs to study the sustainability of farms in Poland, but it did not carry out an SLR to evaluate the different KPI methods. This shortcoming led us to evaluate the main KPIs capable of guiding GAPs applied to various crops [10].

This literature review examines the environmental, technological, economic, and social performance indicators used in agricultural systems in different countries to enhance sustainability, resilience, and competitiveness. This literature review has three objectives:

• To provide scientific evidence on the impact of KPIs on the selection of GAPs in the agricultural sector and to promote farmers’ income from agricultural production in southern Angola.
• To contribute to the scientific knowledge of agricultural science professors and researchers who discuss the promotion of the agricultural sector through the institutionalization of KPIs and GAPs in southern Angola.
• To identify the environmental, technological, economic, and social KPIs that have made it possible to quantify different sustainable agricultural practices in various countries and to replicate them for the agricultural practices currently developed in southern Angola.

Thus, this study aims to contribute to the ongoing international debate on sustainability metrics in agriculture by addressing a critical gap: the operationalization of GAPs through measurable, context-specific KPIs in southern Angola, and that can be adapted to low- and middle-income countries. Recent global initiatives, such as the FAO’s SAFA Guidelines and the SDG framework, emphasize the need for measurable indicators, but their application does not focus on the adaptation to local agricultural, socio-economic, and environmental contexts. Existing studies in the scientific literature present generic sustainability indicators or focus on high-income countries with robust data systems. On the other hand, this work proposes a replicable framework that integrates an SLR, standardization procedures, and a composite sustainability index tailored to the semi-subsistence farming systems in southern Angola. The use of crop-specific KPIs combined with a practical decision-support perspective distinguishes this approach from prior works and provides a tool for the basis of data-informed policy design and farmer-level decision-making in resource-constrained settings.

2. Methods

To systematically analyze the information related to the KPIs applied in agricultural GAPs, a series of inclusion and exclusion criteria (see

Table 1. Inclusion and exclusion criteria.

Inclusion	Exclusion
The document discussed and focused on GAPs;	The document does not discuss and focus on GAPs;
Studies performed in the agricultural sector;	Studies have not been performed in the agricultural sector;
Studies related to environmental, technological, economic, and social KPIs in the agricultural sector;	Studies related to environmental, technological, economic, and social KPIs in other sectors;
Studies performed in developed and underdeveloped countries;	Studies performed in developed and underdeveloped countries are not available;
Publication date between 2019 and 2024;	Published outside the defined range;
Studies and scenarios of GAPs and KPIs in the agricultural sector are written in English.	Studies and scenarios of GAPs and KPIs in the agricultural sector are written in a language other than English.

) was designed for data extraction, aimed at specific information on the main environmental, technological, economic, and social indicators applied in the agricultural production of different crops.

The objective of our research study led us to include all articles that dealt with KPIs applied to GAPs in the development of agricultural activities. When including articles, restrictions were applied based on the date of publication, i.e., all documents written before the selected date were considered invalid (excluded). Articles referring to studies that addressed the use of KPIs in the health sector, agricultural waste, environmental conservation, agricultural management practices, and agricultural economics were also excluded.

Quantitative and qualitative analysis of previous articles with case studies performed in different regions enabled the identification of themes, content, and patterns in the different KPIs applied to different GAPs. A thematic synthesis framework guided the analysis to categorize these codes into overarching themes, allowing for the identification of commonly adopted KPIs and GAPs in different crop cultures and contexts. A quantitative method was employed to highlight the prevalence of KPI-driven GAPs (e.g., net profit when growing maize with drip irrigation). This integrated qualitative and quantitative approach facilitated a thorough synthesis of KPIs and GAPs, offering a clear summary of the key KPIs that could be adapted to the context of southern Angola’s existing GAPs.

An SLR article provides a wide range of knowledge for different users of the analysis. In these documents, the information provided should be explicit, accurate and specific regarding the purpose and methods employed in developing the SLR (the way each of the studies was identified and selected), and as well as the main findings, including the study characteristics, contributors, and outcomes of the meta-analyses. To perform this SLR, Science Direct served as the primary database. The literature search focused on KPIs related to GAPs in regions with comparable crop profiles. While this study concentrates on southern Angola, research from other regions was also included to enhance diversity and enable comparative analysis. Abstracts of the chosen articles were reviewed to assess their relevance and appropriateness for the study. The bibliographical references of the selected publications were used as a source and a search for articles with similar and relevant themes. The terms included in the search were: “Agriculture” and “Key Performance” and “Indicators”. We used “Good Practices” AND “Agricultural” AND “Key AND Performance Indicator” OR “Agricultural”. The Boolean operators “AND” and “OR” were used to combine the above search terms and obtain the targeted search results. The articles included in the SLR presented GAPs and KPIs applied in the agricultural sector in different countries around the world. The articles met the following inclusion criteria: (1) the article discussed and focused on GAPs, (2) the article was published between 2019 and 2024, (3) the article described GAPs, (4) the article described KPIs, (5) studies related to environmental, technological, economic, and social KPIs in the agricultural sector, (6) studies and scenarios of GAPs and KPIs in the agricultural sector written in English (Appendix A).

The selection of articles in the first phase of our research followed the inclusion and exclusion criteria. Articles that did not meet the selection criteria were excluded as invalid for the research. The initial search with search strings retrieved 2720 articles from the years 2019 to 2024. From these, 367 review articles and 1989 research articles were evaluated, resulting in a total exclusion of 2356 articles. We excluded 31 encyclopedias, 233 book chapters, 30 conference abstracts, 2 case reports, 1 conference briefing, 1 discussion, 9 editorials, 18 mini-reviews, 1 practice guideline, 10 short communications, 1 software publication, and 27 others. The articles with titles of applied energy, twenty nine (29), marine policy, twenty six (26), computers and industrial engineering, nineteen (19), technological forecasting and social change, nineteen (19), cities, seventeen (17), resources, conservation and recycling, seventeen (17), construction and building materials, fifteen (15), Journal of Business Research, fifteen (15), International Journal of Production Economics, fifteen (15), energy conversion and management, fourteen (14), expert systems with applications, fourteen (14), and resource policy, fourteen (14).

Figure 1 shows the SLR process, including identification, screening, and eligibility stages, based on the PRISMA methodology. The selection process began with a review of the titles of 2720 articles and continued with a review of the titles and abstracts. A total of 2663 documents were excluded because they were outside the scope of the topic. The screening process produced 14 documents eligible for full-text review. Additionally, the references of these documents were manually examined to identify any relevant studies not retrieved in the initial search. This study assesses KPIs and GAPs across different countries. The selected articles represent multiple regions, with eight studies from Asia, two from Europe, one from Africa, and three from the Americas.

3. Good Agricultural Practices in Angola

In the agricultural sector, GAPs are important in achieving the objectives of sustainable agriculture and the Sustainable Development Goals, but there is a lack of generalizable knowledge about the reasons why farmers apply sustainable agricultural practices. It is important to know the main motivations that have led farmers to adopt GAPs in the agricultural production chain [11].

Agriculture in Angola is mostly rainfed. The practice of drip irrigation, which is of great importance for increasing productivity and agricultural production, both in quantitative and qualitative terms, is still very limited in the country. During the “cacimbo” or dry season, agricultural activity is very limited since it is a dry period. As such, farming households that practice agricultural production during this period resort to the use of irrigation to grow crops, dedicating themselves mainly to growing vegetables to increase food production. Currently, the main agricultural practices developed in Angola to diversify the economy, mitigate the import of basic food products, and promote exports are as follows [12]:

• Drip irrigation: This GAP involves lateral drip tape irrigation with a spacing significantly greater than 20 cm, in comparison with the intermittent irrigation control treatment, which uses a fixed five-day irrigation interval [13,14,15]. It is a GAP that conserves water in regions facing scarcity, as all the irrigation water is directed precisely to the plant and meets the plant’s needs [13];
• Crop rotation: Crop rotation involves growing leguminous and non-leguminous crops on the same plot of agricultural land. For example, different crops grown by farmers alter the microbial activity and decomposition rates of organic material in the cultivated area, consequently significantly affecting the levels of the soil’s organic carbon fractions [14]. Crop rotation is a good GAP that can help mitigate the negative impacts that can be achieved by some crops that are not resistant to a long period of water absence, thus contributing to the sustainability of agricultural systems. By applying this practice, farmers contribute to soil improvement and conservation, but it can also influence soil organic carbon fractions and arsenic content [15];
• Inorganic fertilizers: The application of fertilizers (e.g., nitrogen and potassium) depends on the type of crop and the availability of water for irrigation to each hectare of agricultural land, e.g., 270 kg of nitrogen are applied to a hectare of herbaceous crops (artificially irrigated), while 110 kg are applied to the same plot of land for woody crops [16];
• Organic fertilizers: Once organic matter has reached the right level of growth, farmers collect it, such as grass, tree branches, and fruit scraps, and deposit it in the growing areas to fertilize the soil. The organic matter contained in organic fertilizers improves the structure and nutrient composition of the soil and gives the soil a high water and nutrient retention capacity. Microbial activity is stimulated by the presence of organic fertilizers in the soil, which improves the processes of decomposition of crop residues, plant growth, and the general cycle of nutrients, contributing to the strengthening of agricultural sustainability and respect for the environment [17,18]. Organic fertilizers are the most economical, and policies should be created to support farmers to adhere to organic GAP, contributing to environmental protection, and the sustainability of alternative development projects [19,20].

Table 2. Main agricultural KPIs according to the respective articles.

	KPIs	1	2	3	4	5	6	7	8	9	10	11	12	13	14
Economic	Financial strength
	Farmer’s income
	Gross margin
	Net profit
	Profit for the producer caused by irrigation
Environmental	Soil conservation
	Primary production for internal inputs
	Energy use efficiency
	Water use efficiency
	Nitrogen use efficiency
Social	Percentage of irrigated agricultural areas
	Land tenure
Technology	Agricultural machinery
	Machine energy

shows the indicators (colored squares) and the numbers corresponding to the year, numbering, and title of the article shown in Appendix A. The numbers with an asterisk are the articles searched manually. It includes the selected KPIs grouped under economic, environmental, and technological dimensions. These indicators were derived from the SLR and serve as measurable variables to evaluate the sustainability of each GAP.

4. Scoring Specific and Measurable Main Agricultural KPIs

Several indicators need to be calculated at different spatial scales to assess their contribution in agriculture to the sustainability issues of the selected agricultural practices. The properties of these indicators must be addressed before the indicators are selected and applied [21].

From the point of view of agricultural sustainability, economic (financial strength, farmer’s income, annual net income, profit for the producer caused by irrigation, gross margin, net profit and payments for environmental measures), environmental (soil conservation, primary production of inputs, efficiency in the use of energy, and efficiency in the use of nitrogen), social (percentage of irrigated agricultural areas, land tenure, uniformity of water distribution, efficiency of fertilizer application, and uniformity of fertilizer distribution) and technological (agricultural machinery and machine energy) factors, it is possible to quantify the performance of drip irrigation practices, crop rotation with inorganic fertilizers and organic fertilizers. Performance indicators are used to provide an assessment of progress towards an established goal or objective of a given agricultural practice [22]. Furthermore, the indicator shows a high sensitivity to changes in the fuel consumption of machinery and irrigation resources in the agricultural sector [23]. The description of each of the KPIs and the GAP is shown in Table 3.

Economic KPIs are tools used by farmers to assess the economic situation of an agricultural business in a region or country for macroeconomic control and to reduce the accuracy of economic indicators to create potential wealth [24]. Farmers can make a valuable assessment of production, as well as learn about emerging economic trends and the situation of companies in the agricultural sector with the help of economic KPIs [25].

Companies in the agricultural sector need to evaluate their environmental performance, measure the circular economy, quantify the carbon index, and calculate the ratio of ecological management in recent years. Teachers and researchers have applied strategies to create environmental KPIs that make it possible to assess sustainability and thus reward managers and workers in the agricultural sector [26].

Social KPIs also provide farmers with recommendations on sustainable practices to implement in their agricultural fields. In addition, they can serve as a tool to audit and guide family and corporate farmers to comply with external certifications, to be internationally competitive, and, at the same time, they can be used by public organizations to assess degrees of sustainability in the agricultural sector [27].

Agricultural technology is crucial to optimizing farming practices and making them more sustainable [28]. Technological indicators assess the performance and impact of different agricultural practices in low- and middle-income countries and help define policies and investment priorities to increase agricultural growth and productivity [29].

Table 4, Table 5, Table 6 and Table 7 show the specific and measurable KPIs for maize, potato, and tomato crops. Table 4 includes the parameters for drip irrigation, while Table 5 does so for crop rotation. Table 6 shows the specific and measurable KPIs for inorganic fertilizers, and Table 7 for organic fertilizers. The selected crops (maize, tomato, and potato) were chosen based on their combined agronomic relevance, dietary importance, and regional production potential in Southern Angola. These crops represent distinct agronomic systems (cereal and vegetables) and are widely cultivated by FFH in the region under study.

The KPI scoring criteria presented in Table 4, Table 5, Table 6 and Table 7 were assigned according to the properties proposed by the United Nations (UN), which are closely related to their identification, definition, and calculation. These properties include being specific (focused on a clear aspect) and measurable (capable of being presented as a reliable numerical value). This raw data was collected from secondary sources for each selected KPI across different GAPs [30,31,32].

Table 3. KPIs applied to the agricultural sector.

KPI		Description
Economic	Financial strength	With this indicator, it is possible to assess the annual income of suppliers of products from the field so that they can adhere to payment by check. It also assesses the investments made in large land holdings so that sub-suppliers can be provided with product storage facilities to avoid difficulties when selling [33].
	Farmer’s income	The quantification of the yield of each hectare of land is carried out by subtracting the modelled fixed costs of production and the total variables from the total income, including subsidies for organic farming [34].
	Gross margin	It allows you to quantify the gross benefit (the total market value of production) minus the total costs paid (cost of inputs purchased and labor hired) [35].
	Net profit	Net profit (NP) is quantified from the marginal profit (MP) for an irrigated olive grove, considering the current cost of irrigation water for the olive grove and a high cost of water [36].
	Payments for environmental and climate measures	Payments related to the area for agro-environment–climate measures (AECM) and the type of production, in organic and conventional farming [37].
	Profit for the producer caused by irrigation	Allows the product of the economic productivity of irrigation water to be quantified in cubic meters of irrigation water applied to the field [38].
Environmental	Soil conservation	This indicator was calculated by adding up the areas covered by native grasslands, swamps, woodlands, or native forests and dividing by the area of the cadastral unit [39].
	Primary production of inputs	The amount of energy generated by the agricultural system in the form of the production of different products per unit of energy produced during farming by farmers [10].
	Energy use efficiency	This is calculated by applying the crop yield in combination with its coefficient and energy equivalent. This indicator allows farmers to assess the degree of sustainability of different products in the cultivation system of crop varieties, i.e., from an energy point of view, the actual ratio of the energy input of each component to the total energy output for each treatment [40].
	Water use efficiency	This KPI allows farmers to quantify the ratio between crop yield and evapotranspiration for each season [41].
	Nitrogen use efficiency	Nitrogen use efficiency is quantified based on total N absorption in the fertilized treatment, total N absorption in the unfertilized treatment, and the rate of N fertilizer application [42].
Social	Percentage of agricultural area irrigated	The ratio between the area of irrigated land and the total area of land cultivated by farmers [43].
	Land tenure	This is access to state land and how this land is owned or traded. Land tenure refers to the rights and origins of land and its functions, duties, laws, responsibilities, transition, ownership, and security [44].
Technological	Agricultural machinery	It is relatively more efficient and economical than labor and has a positive impact on farmers’ agricultural income [9].
	Machine energy	During their manufacture, agricultural machinery involves indirect energy. This indicator quantifies the sum of the total weight of the machine, the specific number of working hours for each run, and the number of applications in field operation divided by the useful life of the machine [45].

Table 3. KPIs applied to the agricultural sector.

KPI		Description
Economic	Financial strength	With this indicator, it is possible to assess the annual income of suppliers of products from the field so that they can adhere to payment by check. It also assesses the investments made in large land holdings so that sub-suppliers can be provided with product storage facilities to avoid difficulties when selling [33].
	Farmer’s income	The quantification of the yield of each hectare of land is carried out by subtracting the modelled fixed costs of production and the total variables from the total income, including subsidies for organic farming [34].
	Gross margin	It allows you to quantify the gross benefit (the total market value of production) minus the total costs paid (cost of inputs purchased and labor hired) [35].
	Net profit	Net profit (NP) is quantified from the marginal profit (MP) for an irrigated olive grove, considering the current cost of irrigation water for the olive grove and a high cost of water [36].
	Payments for environmental and climate measures	Payments related to the area for agro-environment–climate measures (AECM) and the type of production, in organic and conventional farming [37].
	Profit for the producer caused by irrigation	Allows the product of the economic productivity of irrigation water to be quantified in cubic meters of irrigation water applied to the field [38].
Environmental	Soil conservation	This indicator was calculated by adding up the areas covered by native grasslands, swamps, woodlands, or native forests and dividing by the area of the cadastral unit [39].
	Primary production of inputs	The amount of energy generated by the agricultural system in the form of the production of different products per unit of energy produced during farming by farmers [10].
	Energy use efficiency	This is calculated by applying the crop yield in combination with its coefficient and energy equivalent. This indicator allows farmers to assess the degree of sustainability of different products in the cultivation system of crop varieties, i.e., from an energy point of view, the actual ratio of the energy input of each component to the total energy output for each treatment [40].
	Water use efficiency	This KPI allows farmers to quantify the ratio between crop yield and evapotranspiration for each season [41].
	Nitrogen use efficiency	Nitrogen use efficiency is quantified based on total N absorption in the fertilized treatment, total N absorption in the unfertilized treatment, and the rate of N fertilizer application [42].
Social	Percentage of agricultural area irrigated	The ratio between the area of irrigated land and the total area of land cultivated by farmers [43].
	Land tenure	This is access to state land and how this land is owned or traded. Land tenure refers to the rights and origins of land and its functions, duties, laws, responsibilities, transition, ownership, and security [44].
Technological	Agricultural machinery	It is relatively more efficient and economical than labor and has a positive impact on farmers’ agricultural income [9].
	Machine energy	During their manufacture, agricultural machinery involves indirect energy. This indicator quantifies the sum of the total weight of the machine, the specific number of working hours for each run, and the number of applications in field operation divided by the useful life of the machine [45].

Table 4. Specific and measurable KPIs in drip irrigation for maize, potato, and tomato crops.

KPI		Specific (S)	Measurable (M)
Economic	Financial strength	Not available	Not available
	Farmer’s income	Not available	Not available
	Profit for the producer caused by irrigation	Not available	Not available
	Gross margin	Drip irrigation	The cultivation gross margin varies for maize from 638.5 to 1151.7 EUR/ha [46,47] potato from 1396.19 to 2605.90 EUR/ha [48] tomato from 2609.19 to 7104.78 EUR/ha [48,49]
	Net profit	Drip irrigation	The growing net profit varies for maize from 279.6 to 4944 EUR/ha [50,51] potato from 1706.65 to 2933.95 EUR/ha [52] tomato from 1652.27 to 6352.18 EUR/ha [48,53]
	Payments for environmental and climate measures	Not available	Not available
Environmental	Soil conservation	Not available	Not available
	Primary production with internal inputs	Not available	Not available
	Energy use efficiency	Not available	Not available
	Water use efficiency	Drip irrigation	Water use efficiency in cultivation varies for maize from 23.70 to 49% [41] potato from 40 to 46.1% [54,55] tomato from 7.8 to 21.4 [56,57]
	Nitrogen use efficiency	Drip irrigation	Nitrogen use efficiency in cultivation varies for maize from 66.2 to 90% [58,59] potato from 48.60 to 81.67% [60] tomato from 50 to 73% [61]
Social	Percentage of agricultural area irrigated	Not available	Not available
	Land tenure		Not available
Technological	Agricultural machinery	Not available	Not available
	Machine energy	Drip irrigation	Machine energy use in cultivation varies for maize from 15,340 to 21,146 MJ/ha [62] potato from 64.8 to 6880 MJ/ha [63,64] tomato from 728.7 to 11,791.7 MJ/ha [65,66]

Table 4. Specific and measurable KPIs in drip irrigation for maize, potato, and tomato crops.

KPI		Specific (S)	Measurable (M)
Economic	Financial strength	Not available	Not available
	Farmer’s income	Not available	Not available
	Profit for the producer caused by irrigation	Not available	Not available
	Gross margin	Drip irrigation	The cultivation gross margin varies for maize from 638.5 to 1151.7 EUR/ha [46,47] potato from 1396.19 to 2605.90 EUR/ha [48] tomato from 2609.19 to 7104.78 EUR/ha [48,49]
	Net profit	Drip irrigation	The growing net profit varies for maize from 279.6 to 4944 EUR/ha [50,51] potato from 1706.65 to 2933.95 EUR/ha [52] tomato from 1652.27 to 6352.18 EUR/ha [48,53]
	Payments for environmental and climate measures	Not available	Not available
Environmental	Soil conservation	Not available	Not available
	Primary production with internal inputs	Not available	Not available
	Energy use efficiency	Not available	Not available
	Water use efficiency	Drip irrigation	Water use efficiency in cultivation varies for maize from 23.70 to 49% [41] potato from 40 to 46.1% [54,55] tomato from 7.8 to 21.4 [56,57]
	Nitrogen use efficiency	Drip irrigation	Nitrogen use efficiency in cultivation varies for maize from 66.2 to 90% [58,59] potato from 48.60 to 81.67% [60] tomato from 50 to 73% [61]
Social	Percentage of agricultural area irrigated	Not available	Not available
	Land tenure		Not available
Technological	Agricultural machinery	Not available	Not available
	Machine energy	Drip irrigation	Machine energy use in cultivation varies for maize from 15,340 to 21,146 MJ/ha [62] potato from 64.8 to 6880 MJ/ha [63,64] tomato from 728.7 to 11,791.7 MJ/ha [65,66]

Table 5. Specific and measurable KPIs in crop rotation for maize, potato, and tomato.

KPI		Specific (S)	Measurable (M)
Economic	Financial strength	Not available	Not available
	Farmer’s income	Not available	Not available
	Profit for the producer caused by irrigation	Not available	Not available
	Gross margin	Crop rotation	The gross margin in cultivation varies for maize from 1000 to 6000 EUR/ha [67] potato from 1000 to 5056 EUR/ha [68] tomato from 500 to 3000 EUR/ha [69]
	Net profit	Crop rotation	The growing net profit varies for maize from 122.63 to 379.77 EUR/ha [70,71] potato from 20 to 8100 EUR/ha [72,73] tomato from 484.40 to 7185.2 EUR/ha [74]
	Payments for environmental and climate measures	Not available	Not available
Environmental	Soil conservation	Not available	Not available
	Primary production with internal inputs	Not available	Not available
	Energy use efficiency	Not available	Not available
	Water use efficiency	Crop rotation	Water use efficiency in cultivation varies for maize from 8 to 18% [75] potato from 12.5 to 35.3% [76,77] tomato from 15.5 to 23.4% [78]
	Nitrogen use efficiency	Crop rotation	Nitrogen use efficiency in cultivation varies for maize from 46 to 85% [79] potato from 6.14 to 50% [80,81] tomato from 12.5 to 64% [82,83]
Social	Percentage of agricultural area irrigated	Not available	Not available
	Land tenure	Not available	Not available
Technological	Agricultural machinery	Not available	Not available
	Machine energy	Crop rotation	Machine energy in maize cultivations varies for maize from 1000 to 8000 MJ/ha [84] potato from 1120 to 2780 MJ/ha [85,86] tomato from 826.70 to 1067.56 MJ/ha [87]

Table 5. Specific and measurable KPIs in crop rotation for maize, potato, and tomato.

KPI		Specific (S)	Measurable (M)
Economic	Financial strength	Not available	Not available
	Farmer’s income	Not available	Not available
	Profit for the producer caused by irrigation	Not available	Not available
	Gross margin	Crop rotation	The gross margin in cultivation varies for maize from 1000 to 6000 EUR/ha [67] potato from 1000 to 5056 EUR/ha [68] tomato from 500 to 3000 EUR/ha [69]
	Net profit	Crop rotation	The growing net profit varies for maize from 122.63 to 379.77 EUR/ha [70,71] potato from 20 to 8100 EUR/ha [72,73] tomato from 484.40 to 7185.2 EUR/ha [74]
	Payments for environmental and climate measures	Not available	Not available
Environmental	Soil conservation	Not available	Not available
	Primary production with internal inputs	Not available	Not available
	Energy use efficiency	Not available	Not available
	Water use efficiency	Crop rotation	Water use efficiency in cultivation varies for maize from 8 to 18% [75] potato from 12.5 to 35.3% [76,77] tomato from 15.5 to 23.4% [78]
	Nitrogen use efficiency	Crop rotation	Nitrogen use efficiency in cultivation varies for maize from 46 to 85% [79] potato from 6.14 to 50% [80,81] tomato from 12.5 to 64% [82,83]
Social	Percentage of agricultural area irrigated	Not available	Not available
	Land tenure	Not available	Not available
Technological	Agricultural machinery	Not available	Not available
	Machine energy	Crop rotation	Machine energy in maize cultivations varies for maize from 1000 to 8000 MJ/ha [84] potato from 1120 to 2780 MJ/ha [85,86] tomato from 826.70 to 1067.56 MJ/ha [87]

Table 6. Specific and measurable KPIs with inorganic fertilizers in maize, potato, and tomato crops.

KPI		Specific (S)	Measurable (M)
Economic	Financial strength	Not available	Not available
	Farmer’s income	Not available	Not available
	Profit for the producer caused by irrigation	Not available	Not available
	Gross margin	Inorganic fertilizers	The gross margin in cultivation varies for maize from 706 to 1187 EUR/ha [88] potato from 412 to 1537 EUR/ha [89] tomato from 1419.26 to 8332.87 EUR/ha [90,91].
	Net profit	Inorganic fertilizers	Net profit cultivation varies for maize from 1220.56 to 1853.01 EUR/ha [92] potato from 621.38 to 2414.26 EUR/ha [93] tomato from 2218.38 to 3904.15 EUR/ha [94].
	Payments for environmental and climate measures	Not available	Not available
Environmental	Soil conservation	Not available	Not available
	Primary production with internal inputs	Not available	Not available
	Energy use efficiency	Not available	Not available
	Water use efficiency	Inorganic fertilizers	Water use efficiency in cultivation varies for maize from 60 to 80% [95] potato from 5.9 to 47.8% [96] tomato from 22.63 to 62% [97,98]
	Nitrogen use efficiency	Inorganic fertilizers	Nitrogen use efficiency in cultivation varies for maize from 42 to 68% [82] potato from 48.60 to 81.67% [99] tomato from 15 to 69% [100]
Social	Percentage of agricultural area irrigated	Not available	Not available
	Land tenure	Not available	Not available
Technological	Agricultural machinery	Not available	Not available
	Machine energy	Inorganic fertilizers	Machine energy in cultivations varies for maize from 589.38 to 1739.5 MJ/ha [45] potato from 866 to 950 [101] tomato from 142.69 to 530.3 MJ/ha [102,103]

Table 6. Specific and measurable KPIs with inorganic fertilizers in maize, potato, and tomato crops.

KPI		Specific (S)	Measurable (M)
Economic	Financial strength	Not available	Not available
	Farmer’s income	Not available	Not available
	Profit for the producer caused by irrigation	Not available	Not available
	Gross margin	Inorganic fertilizers	The gross margin in cultivation varies for maize from 706 to 1187 EUR/ha [88] potato from 412 to 1537 EUR/ha [89] tomato from 1419.26 to 8332.87 EUR/ha [90,91].
	Net profit	Inorganic fertilizers	Net profit cultivation varies for maize from 1220.56 to 1853.01 EUR/ha [92] potato from 621.38 to 2414.26 EUR/ha [93] tomato from 2218.38 to 3904.15 EUR/ha [94].
	Payments for environmental and climate measures	Not available	Not available
Environmental	Soil conservation	Not available	Not available
	Primary production with internal inputs	Not available	Not available
	Energy use efficiency	Not available	Not available
	Water use efficiency	Inorganic fertilizers	Water use efficiency in cultivation varies for maize from 60 to 80% [95] potato from 5.9 to 47.8% [96] tomato from 22.63 to 62% [97,98]
	Nitrogen use efficiency	Inorganic fertilizers	Nitrogen use efficiency in cultivation varies for maize from 42 to 68% [82] potato from 48.60 to 81.67% [99] tomato from 15 to 69% [100]
Social	Percentage of agricultural area irrigated	Not available	Not available
	Land tenure	Not available	Not available
Technological	Agricultural machinery	Not available	Not available
	Machine energy	Inorganic fertilizers	Machine energy in cultivations varies for maize from 589.38 to 1739.5 MJ/ha [45] potato from 866 to 950 [101] tomato from 142.69 to 530.3 MJ/ha [102,103]

Table 7. Specific and measurable KPIs for growing maize, potatoes, and tomatoes with organic fertilizers.

KPI		Specific (S)	Measurable (M)
Economic	Financial strength	Not available	Not available
	Farmer’s income	Not available	Not available
	Profit for the producer caused by irrigation	Not available	Not available
	Gross margin	Organic fertilizers	The cultivation gross margin varies for maize from 24.97 to 139.21 EUR/ha potato from 2850 to 6288 EUR/ha [104] tomato from 355.57 to 3640.28 EUR/ha [105]
	Net profit	Organic fertilizers	Net profit from cultivation varies for maize from 812.81 to 3500.97 EUR/ha [50] potato from 830.2 to 7211.96 EUR/ha [106,107] tomato from 6339.29 to 9248.59 EUR/ha [108].
	Payments for environmental and climate measures	Not available	Not available
Environmental	Soil conservation	Not available	Not available
	Primary production with internal inputs	Not available	Not available
	Energy use efficiency	Not available	Not available
	Water use efficiency	Organic fertilizers	Water use efficiency in cultivation varies for maize from 9.9 to 58.4% [50] potato from 31 to 77% [109,110] tomato from 25 to 40% [111]
	Nitrogen use efficiency	Organic fertilizers	Nitrogen use efficiency in cultivation varies for maize from 20 to 45% [112] potato from 33 to 44.1% [113,114] tomato from 10 to 69% [115]
Social	Percentage of agricultural area irrigated	Not available	Not available
	Land tenure	Not available	Not available
Technological	Agricultural machinery	Not available	Not available
	Machine energy	Organic fertilizers	Machine energy in cultivation varies for maize from 237.60 to 6217.4 MJ/ha [116,117] potato from 1582.61 to 3206.48 MJ/ha [116,118] tomato from 835 to 13,006 MJ/ha [119]

Table 7. Specific and measurable KPIs for growing maize, potatoes, and tomatoes with organic fertilizers.

KPI		Specific (S)	Measurable (M)
Economic	Financial strength	Not available	Not available
	Farmer’s income	Not available	Not available
	Profit for the producer caused by irrigation	Not available	Not available
	Gross margin	Organic fertilizers	The cultivation gross margin varies for maize from 24.97 to 139.21 EUR/ha potato from 2850 to 6288 EUR/ha [104] tomato from 355.57 to 3640.28 EUR/ha [105]
	Net profit	Organic fertilizers	Net profit from cultivation varies for maize from 812.81 to 3500.97 EUR/ha [50] potato from 830.2 to 7211.96 EUR/ha [106,107] tomato from 6339.29 to 9248.59 EUR/ha [108].
	Payments for environmental and climate measures	Not available	Not available
Environmental	Soil conservation	Not available	Not available
	Primary production with internal inputs	Not available	Not available
	Energy use efficiency	Not available	Not available
	Water use efficiency	Organic fertilizers	Water use efficiency in cultivation varies for maize from 9.9 to 58.4% [50] potato from 31 to 77% [109,110] tomato from 25 to 40% [111]
	Nitrogen use efficiency	Organic fertilizers	Nitrogen use efficiency in cultivation varies for maize from 20 to 45% [112] potato from 33 to 44.1% [113,114] tomato from 10 to 69% [115]
Social	Percentage of agricultural area irrigated	Not available	Not available
	Land tenure	Not available	Not available
Technological	Agricultural machinery	Not available	Not available
	Machine energy	Organic fertilizers	Machine energy in cultivation varies for maize from 237.60 to 6217.4 MJ/ha [116,117] potato from 1582.61 to 3206.48 MJ/ha [116,118] tomato from 835 to 13,006 MJ/ha [119]

Table 2 includes all candidate KPIs identified from the literature review, while Table 3, Table 4, Table 5, Table 6 and Table 7 present the refined set of indicators selected based on data availability, measurability, and relevance to the studied GAPs. Only five indicators (gross margin, net profit, water use efficiency, nitrogen use, and machine energy) were retained and used for standardization. These five indicators were chosen due to: (1) availability of consistent data from reliable sources, (2) quantitative comparability across practices, and (3) relevance to sustainability dimensions (economic, environmental, technological).

Therefore, the most suitable KPIs for which S, M, and total (T) data can be found are economic (gross margin and net profit), environmental (water use efficiency and nitrogen use efficiency), and technological (machine energy), according to the data presented in

Table 8. Score of the KPIs applied in the agricultural sector according to the S and M of Table 4, Table 5, Table 6 and Table 7.

KPIs	Drip Irrigation			Crop Rotation			Inorganic Fertilizers			Organic Fertilizers
	S	M	T	S	M	T	S	M	T	S	M	T
Gross margin	0.5	0.5	1	0.5	0.5	1	0.5	0.5	1	0.5	0.5	1
Net profit	0.5	0.5	1	0.5	0.5	1	0.5	0.5	1	0.5	0.5	1
Water use efficiency	0.5	0.5	1	0.5	0.5	1	0.5	0.5	1	0.5	0.5	1
Nitrogen use efficiency	0.5	0.5	1	0.5	0.5	1	0.5	0.5	1	0.5	0.5	1
Machine energy	0.5	0.5	1	0.5	0.5	1	0.5	0.5	1	0.5	0.5	1

. Social KPIs, such as percentage of agricultural area irrigated and land tenure, initially considered during the KPI selection process were excluded from the final model due to limited up-to-date data availability and lack of quantitative comparability. These constraints limited their inclusion. The maximum weight of each KPI in Table 8 is one (1) when the KPI has two KPI properties (specific and measurable). Some of the KPIs in Table 4 are not included in Table 8 (for example, payments for environmental and climate measures) because they are not specific (S) to a particular culture, like southern Angola, and measurable (M). Moreover, weights in Table 8 were assigned equally to all KPIs in all GAPs. This decision reflects an initial assumption of equal importance that can be changed using stakeholders input or empirical weighting methods.

5. Mathematical Formulation

In many scientific papers, motivated by a variety of circumstances, the process of standardization has been used. In assessing the sustainability of a GAP, the main motivation for standardization is to transform the measurements of KPIs applied to different GAPs in the cultivation of crop varieties, normally obtained in different units, into an appropriate and common unit of measurement, to compare them or prepare them for inclusion in an aggregate sustainability score [120,121].

Five specific indicators were selected first, as shown in Table 8, and then an index was formulated to quantify the sustainability of each KPI. It is then necessary to relate them in such a way that they can be presented or expressed in a single quantifiable value.

The analysis of the results in Table 4, Table 5, Table 6 and Table 7 allows us to emphasize the heterogeneity of data sources and potential comparability issues across countries and crops. While all data were drawn from peer-reviewed publications and reputable institutional reports, regional differences in measurement methods or reporting may introduce variation. To moderate this, we used normalized ranges and excluded outliers to improve consistency. Equation (1) makes it possible to relate them using an association function for each of these KPIs. The procedure is carried out for the minimum and maximum values of the economic, environmental, and technological KPIs to obtain a range of values for the overall sustainability index.

For each indicator:

-

Select a maximum, max(x_i), and a minimum, min(x_i), for each indicator separately for the value ranges of these values (minimum and maximum).

-

Evaluate the increase and decrease in S_ij as x_ij grows. The indicator directionality allows us to classify each KPI as either a benefit (positive indicator, where higher values are desirable) or a cost (negative indicator, where lower values are preferable).

The increase in the membership function S_ij increases with the indicator x_i, their relationship is expressed by the following:

$$S_{ij}=\left\{\begin{array}{c}{\displaystyle \frac{x_{ij}-\min \left(x_i\right)}{\max \left(x_i\right)-\min \left(x_i\right)}}\times 100, \mathrm{when} x_{ij}, j \mathrm{is} a \mathrm{positive} \mathrm{indicator}\\ \\ {\displaystyle \frac{{\mathrm{m}\mathrm{a}\mathrm{x}\left(x_i\right)-x}_{ij}}{\mathrm{m}\mathrm{a}\mathrm{x}\left(x_i\right)-\mathrm{m}\mathrm{i}\mathrm{n}\left(x_i\right)}}\times 100, \mathrm{when} x_{ij}, j \mathrm{is} a \mathrm{negative} \mathrm{indicator}\end{array}\right.$$

(1)

In Equation (1), S_ij and x_ij represent, respectively, the standardized value and the original value for indicator i (i = 1, 2, …, n) in reference to sample j (j = 1, 2, …, m), min(x_i) and max(x_i) are the maximum and minimum values among all the samples for indicator i. The overall sustainability index (S) is the sum of the various indicators, considering the weight (0.5) that each one has in Equation (1) of the index. Considering m indicators for selecting a sustainable agricultural practice, the final mathematical expression is given by Equation (2):

$$S\left(s;0.5\right)=\sum _{i=1}^{m}0.5·{\mathrm{S}}_{\mathrm{i}\mathrm{j}}$$

(2)

The data from the economic, environmental and technological KPIs in Table 4 and the S and M conditions in Table 8 allowed us to develop

Table 9. Maximum (max(x_i)), minimum (min(x_i)) and test values of the KPI metrics in drip irrigation practice.

KPIs	Maize		Potato		Tomato
	$\mathbf{m}\mathbf{i}\mathbf{n}\left({\mathit{x}}_{\mathit{i}}\right)$	$\mathbf{m}\mathbf{a}\mathbf{x}\left({\mathit{x}}_{\mathit{i}}\right)$	$\mathbf{m}\mathbf{i}\mathbf{n}\left({\mathit{x}}_{\mathit{i}}\right)$	$\mathbf{m}\mathbf{a}\mathbf{x}\left({\mathit{x}}_{\mathit{i}}\right)$	$\mathbf{m}\mathbf{i}\mathbf{n}\left({\mathit{x}}_{\mathit{i}}\right)$	$\mathbf{m}\mathbf{a}\mathbf{x}\left({\mathit{x}}_{\mathit{i}}\right)$
$\mathrm{Gross} \mathrm{margin} (\mathrm{E}\mathrm{U}\mathrm{R}$/ha)	638.5	1151.7	1396.19	2605.90	2609.19	7104.78
Value to test the metric	1056.68 [122]		2368 [123]		3287.4 [124]
$\mathrm{Net} \mathrm{profit} (\mathrm{E}\mathrm{U}\mathrm{R}$/ha)	279.6	4944	1706.65	2933.91	1652.27	6352.18
Value to test metric	983.77 [125]		2765.85 [126]		2145.45 [127]
Water use efficiency (%)	23.70	49	40	46.1	7.8	21.4
Value to test metric	32.5 [128]		45.9 [129]		18 [130]
Nitrogen use efficiency (%)	66.2	90	48.60	81.67	50	73
Value to test metric	68.5 [128]		61 [131]		68 [132].
Machine energy (MJ/ha)	15,340	21,146	64.8	6880	728.7	11,791.7
Value to test metric	16,506 [133]		1290 [134]		1300.6 [135]

,

Table 10. Maximum (max(x_i)), minimum (min(x_i)) and test values of the KPI metrics in crop rotation practice.

KPI	Maize		Potato		Tomato
	$\mathbf{m}\mathbf{i}\mathbf{n}\left({\mathit{x}}_{\mathit{i}}\right)$	$\mathbf{m}\mathbf{a}\mathbf{x}\left({\mathit{x}}_{\mathit{i}}\right)$	$\mathbf{m}\mathbf{i}\mathbf{n}\left({\mathit{x}}_{\mathit{i}}\right)$	$\mathbf{m}\mathbf{a}\mathbf{x}\left({\mathit{x}}_{\mathit{i}}\right)$	$\mathbf{m}\mathbf{i}\mathbf{n}\left({\mathit{x}}_{\mathit{i}}\right)$	$\mathbf{m}\mathbf{a}\mathbf{x}\left({\mathit{x}}_{\mathit{i}}\right)$
$\mathrm{Gross} \mathrm{margin} (\mathrm{E}\mathrm{U}\mathrm{R}$/ha)	1000	6000	1000	5056	500	3000
Value to test the metric	2860 [136]		5020 [137]		2699.35 [138]
$\mathrm{Net} \mathrm{profit} (\mathrm{E}\mathrm{U}\mathrm{R}$/ha)	112.63	379.77	20	8100	484.40	7185.2
Value to test metric	260.40 [139]		792.36 [140]		4571.28 [141]
Water use efficiency (%)	8	18	12.5	35.3	15.5	23.4
Value to test metric	12 [142]		27 [143]		20 [144]
Nitrogen use efficiency (%)	46	85	33	53	12.05	61
Value to test metric	60 [145]		34.3 [132]		29.3 [146]
Machine energy (MJ/ha)	1000	8000	1120	2780	826.70	1067.56
Value to test metric	6600 [147]		2130.01 [148]		871 [149]

,

Table 11. Maximum (max(x_i)), minimum (min(x_i)) and test values of the KPI metrics in the inorganic fertilizer practice.

KPI	Maize		Potatoes		Tomatoes
	$\mathbf{m}\mathbf{i}\mathbf{n}\left(\mathit{x}\right)$	$\mathbf{m}\mathbf{a}\mathbf{x}\left(\mathit{x}\right)$	$\mathbf{m}\mathbf{i}\mathbf{n}\left(\mathit{x}\right)$	$\mathbf{m}\mathbf{a}\mathbf{x}\left(\mathit{x}\right)$	$\mathbf{m}\mathbf{i}\mathbf{n}\left(\mathit{x}\right)$	$\mathbf{m}\mathbf{a}\mathbf{x}\left(\mathit{x}\right)$
$\mathrm{Gross} \mathrm{margin} (\mathrm{E}\mathrm{U}\mathrm{R}$/ha)	706	1187	412	1537	1419.26	8332.87
Value to test the metric	831 [150]		1026.80 [151]		2699.35 [138]
$\mathrm{Net} \mathrm{profit} (\mathrm{E}\mathrm{U}\mathrm{R}$/ha)	1220.56	1853.01	621.38	2414.26	2218.38	3904.15
Value to test metric	1411 [152]		1240.02 [140]		2914.19 [153]
Water use efficiency (%)	60	80	5.9	47.80	22.63	62
Value to test metric	65 [154]		25 [155]		45.33 [156]
Nitrogen use efficiency (%)	42	68	48.60	81.67	15	24
Value to test metric	60 [157]		68.25 [158]		22 [159]
Machine energy (MJ/ha)	589.38	1739.5	866	950	142.69	530.3
Value to test metric	1245.6 [160]		910 [161]		440.66 [162]

and

Table 12. Maximum (max(x_i)), minimum (min(x_i)) and test value of the KPI metrics in organic fertilizer practice.

KPIs	Maize		Potato		Tomato
	$\mathbf{m}\mathbf{i}\mathbf{n}\left({\mathit{x}}_{\mathit{i}}\right)$	$\mathbf{m}\mathbf{a}\mathbf{x}\left({\mathit{x}}_{\mathit{i}}\right)$	$\mathbf{m}\mathbf{i}\mathbf{n}\left({\mathit{x}}_{\mathit{i}}\right)$	$\mathbf{m}\mathbf{a}\mathbf{x}\left({\mathit{x}}_{\mathit{i}}\right)$	$\mathbf{m}\mathbf{i}\mathbf{n}\left({\mathit{x}}_{\mathit{i}}\right)$	$\mathbf{m}\mathbf{a}\mathbf{x}\left({\mathit{x}}_{\mathit{i}}\right)$
$\mathrm{Gross} \mathrm{margin} (\mathrm{E}\mathrm{U}\mathrm{R}$/ha)	24.97	139.21	2850	6288	355.57	3640.28
Value to test the metric	125 [163]		3945 [164]		429 [165]
$\mathrm{Net} \mathrm{profit} (\mathrm{E}\mathrm{U}\mathrm{R}$/ha)	812.81	3500.97	830.2	7211.96	6339.29	9248.59
Value to test metric	990.85 [125]		1181.0 [140]		8901.47 [166]
Water use efficiency (%)	9.9	58.4	31	77	25	40
Value to test metric	40 [167]		65 [168]		29.1 [169]
Nitrogen use efficiency (%)	20	45	33	44.1	10	69
Value to test metric	40.9 [170]		39.5 [171]		22.6 [172]
Machine energy (MJ/ha)	237.60	6217.4	1582.61	3206.48	835	1766
Value to test metric	1870 [173]		3182.65 [174]		871 [149]

of maximum (max(x_i)), minimum (min(x_i)) values of KPIs for the practices of drip irrigation, crop rotation, organic fertilizers and inorganic fertilizers for the maize, potato and tomato crops.

In the process of standardization, the maximum, minimum, and value to test the metrics for gross margin, net profit, water use efficiency, nitrogen use efficiency, and machine energy are required. The manual search of case studies allowed us to obtain the value to test the metrics from Table 9, Table 10, Table 11 and Table 12 to identify which is the best practice that farmers should apply in the cultivation of maize, potato, and tomato for more sustainable, economical, and environmentally friendly agriculture.

The values to test the metrics shown in Table 9, Table 10, Table 11 and Table 12 are based on standardized indicators using equal weights and data from secondary sources. Whilst this ensures comparability, it may not reflect local priorities or conditions. Future work must include a sensitivity analysis to test how different assumptions affect the results. The framework supports the comparison between GAPs. The scalability may be limited by constraints related to digital literacy, market access, extension service capacity, and infrastructure. So, its scalability from southern Angola to the rest of the country or other countries with similar agricultural landscapes requires local adaptation and validation to ensure its relevance and effectiveness in real-world conditions.

6. Application of the Graphical Tool Developed to Case Studies

Based on the results of the mathematical formulation, where the maximum and minimum values for the overall sustainability index were obtained for each test scenario, the most relevant indicators were defined, as well as the mix of energy carrier systems that should be considered in the electricity supply portfolio.

To support the practical application of the proposed sustainability assessment framework, a visual tool was developed to assist local stakeholders—particularly farmers, technicians, and extension agents—in comparing GAPs based on standardized KPI values and the resulting sustainability index. This analogue dashboard is presented in the form of a triangular graph representing the sustainability performance of GAPs applied to cultivation. Each vertex of the triangle represents one sustainability dimension (economic, environmental, and technological). The plotted points correspond to specific GAP scenarios, with their position indicating the balance achieved between the three dimensions. The closer a point is to a given vertex, the greater its relative contribution in that category. Thus, the suitability of each GAP to the sustainability dimensions is given by the larger area that is covered. This graphical tool enables side-by-side interpretation of the performance of each GAP across economic, environmental, and technological dimensions.

Given the limited digital infrastructure and literacy in the region under study, the tool was deliberately designed as non-digital, with the objective to be an easy-to-use support instrument, suitable for field-level use and farmer-training sessions. By interpreting technical results into a simplified and accessible format, the visual dashboard increases the usability of the assessment outcomes and promotes more informed decision-making in real agricultural settings.

The following sections discuss the results obtained from the case studies using the proposed framework.

6.1. Evaluation of the Standardized KPIs for the Maize Crop

The minimum and maximum values from Table 9, Table 10, Table 11 and Table 12 of the GAPs of drip irrigation, crop rotation, inorganic fertilizers and organic fertilizers in maize cultivation when considering the case studies as the metric to test for all the economic, environmental and technological KPIs normalized using Equation (1), are shown in Figure 2. This figure, as the following ones for other case studies, shows the conceptual framework adopted for KPI selection and sustainability assessment. It integrates the three dimensions of sustainability (economic, environmental, technological) evaluated in this research work into a unified evaluation process. This visual representation, besides providing an easy and expeditious way to evaluate sustainability in countries like Angola, supports the transparency and reproducibility of the indicator selection process. The following considerations should be highlighted:

(1)

The high gross margin required to grow maize with GAPs of drip irrigation, crop rotation and the use of inorganic fertilizers lowers its sustainability indices.

(2)

The low gross margin required to grow maize with GAPs that use organic fertilizers contributes to the sustainability index compared to GAPs that use drip irrigation, crop rotation, and inorganic fertilizers.

(3)

Maize cultivation with GAPs that use organic fertilizers and drip irrigation produces higher profits, which increases its indicator value.

(4)

The low net profit values in maize cultivation with inorganic fertilizers and crop rotation contribute to its low sustainability index.

(5)

Sustainability in maize production with organic and inorganic fertilizer GAPs in maize cultivation is based on water use efficiency compared to drip irrigation and crop rotation GAPs.

(6)

The low amount of nitrogen supplied via fertilizers in the maize crop in the crop rotation and drip irrigation GAPs contributes to low sustainability.

(7)

The overall sustainability indices for maize cultivation systems with drip irrigation, inorganic fertilizer use, and crop rotation are based on water use efficiency, compared to other GAPs.

(8)

The reduced machine energy requirements in maize cultivation with GAPs that use organic and inorganic fertilizers, and drip irrigation, also influence its overall sustainability index concerning the crop rotation GAP.

6.2. Evaluation of Standardized KPIs for Potato Cultivation

The minimum and maximum values from Table 9, Table 10, Table 11 and Table 12 of the GAPs of drip irrigation, crop rotation, inorganic fertilizers, and organic fertilizers in potato cultivation when considering the case studies as the metric to test for all the economic, environmental, and technological KPIs normalized using Equation (1), are shown in Figure 3. The following considerations should be highlighted:

(1)

The high gross margin required for potato cultivation with GAPs of crop rotation and use of organic fertilizers lowers its sustainability indices.

(2)

The low gross margin required for potato cultivation with GAPs in the application of drip irrigation and inorganic fertilizers contributes to the sustainability index compared to other GAPs.

(3)

Potato cultivation with GAPs in the use of drip irrigation, and inorganic fertilizers produces higher profits, benefiting its sustainability indicator value compared to other GAPs.

(4)

The sustainability of potato production with the organic fertilizer GAP is based on water use efficiency compared to the GAPs of drip irrigation, crop rotation, and inorganic fertilizers.

(5)

The high amount of nitrogen supplied via fertilizers concerning drip irrigation GAPs and the utilization of inorganic fertilizers which promotes their overall sustainability indices.

(6)

The lower sustainability index of the crop rotation and inorganic fertilizer GAPs in potato production has low nitrogen use efficiency compared to the other GAPs.

6.3. Evaluation of Standardized KPIs for Tomato Cultivation

The minimum and maximum values from Table 9, Table 10, Table 11 and Table 12 of the GAPs of drip irrigation, crop rotation, inorganic fertilizers and organic fertilizers in tomato cultivation when considering the case studies as the metric to test for all the economic, environmental and technological KPIs normalized using Equation (1), are shown in Figure 4. The following considerations should be highlighted:

(1)

Growing tomatoes with GAPs using crop rotation and organic fertilizers produces higher profits, which benefits their indicator value.

(2)

The capital costs required to grow tomatoes with GAPs using inorganic fertilizers, and drip irrigation penalize their sustainability indices, conditioning farmers to not adhere to these GAPs and opt for more sustainable GAPs.

(3)

The value of water use efficiency in tomato cultivation in rotation with other crops is one of the indicators that promotes the sustainability of this practice, and increases the overall index in relation to the drip irrigation GAP.

(4)

The amount of nitrogen supplied via fertilizers (nitrogen use efficiency) in tomato cultivation in the drip irrigation, crop rotation and organic fertilizer GAPs contributes to high sustainability.

(5)

Low machine energy use in tomato cultivation with GAPs in the use of organic and inorganic fertilizers, and crop rotation influences its overall sustainability index in relation to the drip irrigation GAP.

7. Discussion

The perception of and adherence to available and effective KPIs applied in companies in the agricultural sector can lead to the sustainable development of GAPs and promote positive results in the cultivation of different crops by farmers, such as more efficient production. In this study, it is possible to identify and evaluate the main KPIs, their quality, and how they can be applied to GAPs. Fourteen scientific articles were selected with a variety of KPIs to measure the sustainability of GAP in different countries around the world. The results revealed that the majority of KPIs were developed in drip irrigation practices, crop rotation, inorganic fertilizers, and organic fertilizers. The KPIs found are applied to GAPs in regions with maize, potato, and tomato crops, and it may be possible to adapt them to a specific context and location. Furthermore, normalization was the most used method to validate the economic (gross margin and net profit), environmental (water use efficiency and nitrogen use efficiency), and technological (machine energy) KPIs selected for each of the GAPs and crops.

Several studies presented a set of KPIs for factors including economic (financial strength, farmer’s income, gross margin, net profit and profit for the producer caused by irrigation), environmental (soil conservation, primary production to internal inputs, efficiency in the use of energy, efficiency in the use of water and efficiency in the use of nitrogen), social (percentage of irrigated agricultural areas and land tenure), and technological (agricultural machinery and machine power). However, they did not have an instruction guide to help farmers interpret the results and carry out practical tests, which would be a fundamental guiding element for farmers to monitor and assess whether GAPs are sustainable. Thus, it is necessary to validate the KPIs. This is essential as it facilitates monitoring, standardized assessment of sustainability, and improved agricultural production.

Previous systematic literature reviews have updated a comprehensive list of 133 key sustainable performance indicators (economic, social, and environmental) that can be used to measure the sustainability performance of GAPs [27]. The KPIs developed in various studies help promoters in the agricultural sector to quantify the results of agricultural production. They make it possible to quantify a set of GAPs applied to crop varieties, to help assess productivity progress, target strategic objectives, and lead us to identify the main areas in the agricultural production chain where improvements are needed to support decision-making [175].

The studies included in this review were carried out on different farms and covered different regions of the world, providing a broad perspective of GAPs based on KPIs. Indicators should be adapted to the local context when they are developed and tested in a specific context or country, as each region has unique agricultural systems and information infrastructures [175].

The solution to the problems relating to the lack of water for irrigation in areas with cyclical droughts, the need for water, and the limited water resources that condition and limit the cultivation of potatoes lies in the application of the drip irrigation GAP. This GAP allows water to reach the stem and root of the plant directly, thus reducing water waste and increasing productivity [176]. Growing maize and potatoes using organic fertilizers can increase soil organic carbon levels and improve the biological and physical functioning of the soil [177]. Organic fertilizers also keep the soil healthy and increase plants’ resistance to adverse environmental conditions, as well as improving their properties [178]. Organic farms perform significantly better economically, with organic farms showing the highest growth rate in profitability [179].

Growing maize and potatoes in rotation with other crops increases productivity and mitigates the pressure from weeds, pathogens, and insects [180]. Several studies have shown that the annual crop rotation scheme offers a favorable balance across the sustainability dimensions in the case of tomato production, significantly increasing soil quality, tuber yield, and reducing the water footprint [181].

The use of inorganic fertilizers in agriculture significantly increases the yield and growth of tomatoes [182,183].

The economic (gross margin and net profit), environmental (water use efficiency and nitrogen use efficiency), and technological (machine energy) KPIs were applied to drip irrigation practices, crop rotation, inorganic fertilizers, and organic fertilizers in maize, potato and tomato crops like those grown in Angola. Although these KPIs are applied to GAPs in countries with different socio-economic, technological and social conditions to Angola, they can be replicated for the southern Angolan context as they are applied to crops like those grown in southern Angola. The application of these KPIs in GAPs in the cultivation of maize, potatoes, and tomatoes will allow farmers to quantify (i) the gross margin, i.e., the crop yield, the selling price of the crop harvest, the by-product yield and the selling price of the by-product [46]; (ii) net profit, i.e., the profit from growing different crops to determine the best agricultural practice [51]; (iii) water use efficiency: the ratio between marketable yield and the total evapotranspiration that depends on irrigation efficiency, that is, the volume of water applied per hectare, allowing comparison of irrigation effectiveness across different practices in terms of productivity per unit of water used [169]; (iv) nitrogen use efficiency, i.e., optimizing the total absorption of N in fertilizer integration on land to improve crop yields in agricultural production [146]; and (v) machine energy, i.e., the direct energy used during various agricultural activities, such as creating beds, planting crops, harvesting, threshing and transportation pertaining to the cultivation of different crops [40].

So, the initial objectives of this study were fully accomplished and, in several aspects, exceeded. The research provided scientific evidence on the role of KPIs in selecting GAPs. It offers a structured and replicable methodology, including standardized indicators and a sustainability index, which can support the widespread use of KPIs in agricultural planning and education. Furthermore, it identified and validated a comprehensive set of environmental, technological, economic, and social KPIs, adapting them to the southern Angolan context with practical application to key crops. A mathematical framework for KPI standardization and sustainability assessment is proposed with a simple and clear visual representation that helps and enhances decision-making for farmers.

It should be noted that solutions based on economic, environmental, and technological KPIs have been widely seen as an opportunity for selecting better GAPs in corn, potato, and tomato cultivation in developed and developing countries. In recent years, KPIs applied in the agricultural sector were presented in readable dashboards and have been rapidly adopted by several countries around the world, with governments and policymakers increasingly recognizing their potential as visual tools to quickly represent the performance of an agricultural enterprise. These dashboards present the latest KPIs on a single screen in a concise, easy-to-understand format [184].

To increase robust productivity, agricultural sustainability, and select better agricultural GAPs in the cultivation of corn, potatoes, and tomatoes, southern Angola needs to opt for the following KPIs:

• Gross margin: This allows farmers to relate total revenue as a percentage to determine the productivity of one euro (EUR 1) of revenue generated [185]. For example, the revenue, production costs, and services provided for one hectare of maize grown with organic fertilizers is EUR 139.21 [104].
• Net profit: This quantifies the company’s income after paying the workers’ wages, and the cost of buying seeds, farmyard manure, weed control, and pesticides. For example, a cost of EUR 85.936 for corn production using organic fertilizers on a one-hectare plot of land generates a total return of EUR 3520.6 and a net profit of EUR 2661.25 [50].
• Water use efficiency: This calculates the sum of the total water applied by irrigation, the water used in the profile, and rainfall during the growing season [186]. For example, total irrigation for growing maize using organic fertilizers generates a water use efficiency of 58.4% [50].
• Nitrogen use efficiency: This allows the yield per unit of nutrient absorbed to be quantified. The average value of nitrogen absorption in maize cultivation with organic fertilizers generates a nitrogen use efficiency of 40.9% [170].
• Machine energy: This allows the machine’s energy on each hectare during cultivation operations, the success of the tool’s and machine’s work to be quantified [187]. It allows the calculation of the total diesel used during various agricultural activities, such as creating seedbeds, planting crops, harvesting, threshing, and transportation [45]. For example, it was possible to quantify the machine energy value of 871 MJ/ha in the tomato and rice rotation in Marvdasht in Iran [87].

GAPs using irrigation systems (sprinkling, gravity, ditch or furrow, flooding, and bucket or watering cans) and the use of chemical fertilizers are the most common in potato and tomato cultivation in various provinces of southern Angola [4]. Although these GAPs are applied in developed countries with modern technology, it is necessary for southern Angola to apply the sustainable and ecologically correct GAPs guided by the environmental, economic and technological KPIs for sustainable agriculture and to take a significant step forward in the agricultural sector: The practice of using organic fertilizers in maize cultivation can reduce the adverse effects on the environment, maintaining its natural cycles in the recovery process and improving the quality of food. This practice is economically viable when farmers can get a higher price for their product [188]. The practice of drip irrigation in potato cultivation is often preferred over other irrigation methods because of the high efficiency of water application due to reduced losses, surface evaporation, and deep percolation [189]. This method of irrigation allows water to be applied directly to the plant, as the low amount of water can meet all the plant’s needs [13]. The practice of crop rotation in tomato cultivation guarantees long-term yield stability and maintains soil fertility. Rotating tomato cultivation compared to monoculture can reduce the intensive use of pesticides and synthetic fertilizers and mitigate greenhouse gas emissions [190].

It is intended that our analysis may provide valuable insights into the understudied aspects of KPIs and GAPs in several African countries. First of all, research relating to KPIs applied to GAPs in developed and developing countries is still incipient and has received little attention from researchers, academics, governments, and decision-makers in the agricultural sector to increase economic development and productivity. Secondly, sustainability-oriented GAPs in southern Angola based on KPIs is an important research topic. Finally, the KPIs for the sustainable agricultural practices of crop rotation and organic fertilizers (maize cultivation), drip irrigation and organic fertilizers (potato cultivation) and crop rotation and inorganic fertilizers (tomato cultivation) applied in other countries need to be reproduced in southern Angola for sustainable agriculture.

It is pertinent to perform more in-depth investigations of similar studies for a complete appraisal of the overall impact of the KPIs applied to good agricultural practices in maize, potato and tomato cultivation in companies in the agricultural sector, with greater attention to the possible adverse effects that the present GAPs pose as obstacles to the progress of agricultural production, preventing their adoption. By understanding the impact of the application of KPIs on the development of GAPs, it is possible to highlight areas with potential for improvement to boost sustainable agricultural development in southern Angola.

Nevertheless, the academic community, political decision-makers, and the civil society in southern Angola need to apply the KPIs to the GAPs of drip irrigation, crop rotation, organic and inorganic fertilizers in the cultivation of maize, potatoes, and tomatoes to promote sustainable agricultural development.

This article has some limitations. Although the SLR was very exhausting, it has allowed us to retrieve some articles with very similar case studies that were not found because they were not indexed in the databases where the research was carried out or because they were released on the websites of organizations, governmental institutions, or academic societies.

The manual search process consisted of analyzing and verifying reference lists during the full-text selection stage, along with employing a manual search code in Google Scholar to address these issues.

Furthermore, research performed in different areas where KPIs applied to the agricultural sector or GAPs are grouped may not have been retrieved. Furthermore, the application of KPIs to different GAPs might have been underestimated in certain aspects. The majority of the KPIs were applied to GAPs with crops and technologies that do not match the realities of southern Angola.

The SLR allowed us to identify several studies that propose indicator-based tools for evaluating sustainable agriculture, although most of them rely on complex data inputs or multi-criteria methods that are difficult to operationalize in low-resource settings. In contrast, the proposed framework emphasizes simplicity, transparency, and applicability with minimal data requirements, which enhances its feasibility in FFH systems like those in Southern Angola. Moreover, our approach tailors KPI selection and standardization to crop-specific and regional contexts. This local adaptation represents a distinctive contribution to the literature on sustainability evaluation in African agricultural systems.

8. Conclusions

In the literature analyzed in this research, there is potential to use economic, environmental, and technological KPIs to monitor and evaluate the quality and sustainability of GAPs. These identified KPIs can be used as a database to monitor and evaluate GAPs and decision-makers in the agricultural sector, small and large farmers opting for economic, environmental and technological KPIs to quantify the practice of drip irrigation, crop rotation, inorganic fertilizers and organic fertilizers in the cultivation of maize, potatoes, and tomatoes. The results of this study show that most of the KPIs in the included articles were developed to quantify gross margin, net profit, nitrogen use efficiency, water use efficiency, and machine energy in sustainable agricultural practices. Therefore, the majority of KPIs do not present evidence to be validated because they were not specific and did not present a specific value that would make it possible to quantify a particular indicator in a sustainable GAP.

In this study, the values of the economic, environmental and technological KPIs reflect the most appropriate and sustainable GAPs for growing maize, potatoes and tomatoes. The use of organic fertilizers is the best practice for growing maize. This practice requires lower production costs and provides higher net profit (a gross margin of 139 EUR/ha has a net profit of 3500 EUR/ha) and higher water use efficiency of around 58%. Growing maize on one hectare requires only 6217 MJ of machine energy. As an example, the use of organic manure improves the growth performance of young coffee under low water levels while the application of inorganic fertilizers results in more growth at higher water levels [191].

In potato cultivation, the practice of drip irrigation is the most sustainable with a very low cost of production compared to other GAPs; for example, potato production on one hectare of land has a cost of production of EUR 2605 and a net profit of EUR 2933.91. The drip irrigation GAP allows a nitrogen use efficiency of 81% and a machine energy of 6880 MJ. In Tanzania, growing cabbage using drip irrigation is sustainable and has generated positive profits. What is more, drip irrigation is restricted to ground-level crops such as carrots, onions, and French beans, and this practice would significantly improve the yields of these crops [192].

The high and low values of gross margin (3000 EUR/ha) and machine energy (871 MJ/ha), respectively, allow the crop rotation GAP to be the best sustainable practice in tomato cultivation. In this practice, the yield is very high and significant (in one hectare of tomatoes, there is around EUR 7185.2 of net profit) and it enables a nitrogen use efficiency of 61%. In South Africa, two levels of tillage, no-till and conventional tillage were introduced, with two crop rotations, sweet sorghum–pasture vetch–sweet sorghum and sweet sorghum–fallow–sweet sorghum. No-till increased the concentration in the soil of total nitrogen, total organic nitrogen, magnesium, and sodium by 3.19% to 45% compared to conventional tillage [193], contributing to improved nutrient availability for subsequent crop cycles.

The application of the proposed KPI-based framework in southern Angola faces several limitations. Regional variations in data availability and quality can limit the accuracy and comparability of results, while economic limitations may restrict the adoption of certain GAPs. In addition, factors such as local farming traditions, literacy, and risk perceptions, can pose barriers to the acceptance of the recommended GAP. The dataset used in this study includes information sourced from countries with agronomic and economic conditions that, while similar in some respects, differ from the specific context of Angola. This introduces limitations in terms of representativeness and may affect the direct applicability of the results. Thus, field-level validation, stakeholders’ engagement, and phased implementation strategies adapted to local realities are required to overcome these limitations.

Academics and researchers are encouraged to conduct further studies on KPIs for GAP in developing countries that share similar climatic conditions, soil types, energy resources, human resources, technology levels. Comparative analysis with KPIs for GAPs used in developed countries are needed to identify and understand the barriers that delay the application of KPIs for quantifying the sustainability of GAP in developing countries. Additionally, future studies should focus on strengthening the empirical foundation of KPI sets, developing robust mathematical formulations, and rigorously testing their applicability and effectiveness within GAP systems.