Research Advances in Intensification of Soil Health in Agriculture

Soil supports the primary production of plants, which is the basis of the food chain, and nourishes all living things on land. Furthermore, soil provides ecosystem-specific services, such as biological production, atmospheric gas composition, water quality, and biodiversity. In other words, without healthy soil, the planet cannot be healthy. Soil health is defined as “the ability of the soil to sustain the productivity, diversity, and environmental services of terrestrial ecosystems” [1]. However, of the nine key indicators of planetary health that can sustain human society, seven—climate change (CO₂ emissions), alteration of biosphere integrity, (biodiversity loss), variation in freshwater (rivers flow and soil moisture content), land system change (forest cover), alteration of biogeochemical fluxes (nitrogen and phosphorus loadings), introduction of new pollutants (anthropogenic pollution such as microplastics and radioactive wastes), and ocean acidification—are estimated to have exceeded the “planetary boundary” of the safe operating zone, with only two remaining within it: aerosol loading and stratospheric ozone depletion [2].

Agriculture contributes significantly to CO₂ emissions through deforestation and the destruction of natural grasslands, and is the main cause of biodiversity loss and increased nitrogen and phosphorus loads. It is directly and indirectly related to the seven global issues. While 30% of the Earth’s surface is used for agriculture and 10% is used for cropland, 23% of total land is degraded due to inappropriate soil management [3]. Furthermore, the conversion of natural ecosystems into cropland has recently increased, particularly in South America, Asia, and Africa [4,5,6]. One factor contributing to such expansion is the decline in crop yields due to decreased rainfall [6]. However, in South America, the biological productivity of cropland is lower than that of natural land, which has been pointed out as a cause of land degradation. On the other hand, in Asia and North America, biological productivity has increased on expanded cropland, but this has led to greater consumption of water resources [4].

There is a significant correlation between the proportion of agricultural land in a watershed and the nitrogen load discharged into rivers. Excessive application of nitrogen chemical fertilizers increases N₂O emissions. In particular, peatland drainage significantly increases CO₂ and N₂O emissions. In other words, the expansion of agricultural land increases the nutrient load in the hydrosphere and greenhouse gas emissions in the atmosphere [7].

Sustainable intensification is needed, meaning producing more food from the same agricultural area while reducing environmental impact. To achieve this, it is important to improve soil health on agricultural land. Soil health is essential for improving crop productivity, strengthening ecosystem resilience, and reducing environmental impact, as well as for achieving a balanced agricultural and natural ecosystem. The importance of conservation agriculture, which maximizes natural soil functions and restores ecosystem services, is emphasized [8]. In recent years, it has been recognized that the carbon sequestration potential of soils has sufficient scope to prevent global warming [9], so conservation agriculture approaches that sequester carbon in soils are gaining importance.

Therefore, this Special Issue, “Feature Review in Agricultural Soils—Intensification of Soil Health”, examines issues related to soil conservation and fertility management to improve soil health in agricultural soils. The collection includes three articles on soil conservation (remediation of saline–alkaline soils [10], sustainability of rotational shifting cultivation [11], and development of technologies of countermeasures to soil contamination [12]), and five articles on fertility management (fertility management system for soil nitrogen fertility assessment [13]; basic research on soil phosphorus fertility assessment: availability of organic phosphorus by enhancing phosphatase activity [14]; availability of inorganic phosphorus by biochar [15]; effects of organic amendments: effects on soil physicochemical properties [16]; and effects on soil carbon sequestration and soil structure [17]).

Intensification of soil health for soil conservation and restration: Soil conservation and restoration are key techniques for reducing soil degradation, increasing soil resilience and promoting soil health.

(1) For saline–alkaline soils, soil improvement measures closely related to soil health have been developed and implemented, based on physical, chemical, and biological indicators. The specific measures vary depending on regional characteristics, such as salinity, alkalinity, soil ionic composition, pH, hydrological processes, groundwater levels, soil properties, and regional policies. The three major Chinese regions with saline–alkaline soils use different techniques depending on the region. However, with the maturation of technology and the dissemination of information, comprehensive measures for various saline–alkaline soils have been widely promoted [10].

(2) Fires associated with rotational shifting cultivation tend to reduce soil porosity, clay content, cohesion, and cation-exchange capacity; increase sand content, pH, available phosphorus, and organic nitrogen; and increase the diversity of some bacteria while reducing fungal communities. Soil erosion is also a major concern. Effective management strategies, including controlled burning, appropriate land zoning, and sustainable agricultural practices such as agroforestry, cover crops, and crop rotation, are essential to mitigate the negative impacts on soil health and microbial communities [11].

(3) The number of articles on soil health related to agricultural soil contamination increased from 27 in 2020 to 54 in 2024 (Web of Science search), confirming the growing interest in soil health. Furthermore, the article emphasizes the need to integrate the latest AI-based technologies into soil health research, such as machine learning, natural language processing, and computer visualization for monitoring soil contaminants. These tools can be used as preventive measures to minimize the adverse effects of contaminants on soil [12].

Improving soil health through soil fertility management: Soil fertility management techniques require the restoration of soil physicochemical and biological properties, which are depleted by cultivation, and appropriate fertilizer application. To reduce the use of chemical fertilizers, it is important to effectively utilize the available nutrients accumulated in the soil as well as ensure the appropriate use of organic resources.

(1) Improving soil health in agricultural fields requires a fertilization management system that can adapt to variations in soil fertility within a region and within a field. As part of the Japanese Hokkaido Clean Agriculture Project, the use of compost as a partial substitute for chemical fertilizer reduces the amount of chemical fertilizer applied. Remote sensing technology has also been integrated to address spatial variations in soil properties between fields with different levels of nitrogen fertility. Specifically, for topdressing of winter wheat, satellite imagery is used to estimate the nitrogen uptake in each field based on the NDVI, while simultaneously measuring nitrogen fertility through soil analysis. This allows for optimal and variable nitrogen application levels within the field and enables the development of waste-efficient nitrogen application techniques using variable-rate fertilizer broadcasters [13].

(2) In soil, the main source of phosphorus is found in organic matter. Therefore, it is important to enhance the activity of acid and alkaline phosphatase (APase), which is essential for the release of phosphorus from organic matter through hydrolysis. Previous research has demonstrated a strong correlation between APase and soil pH, with positive effects of clay content, organic matter, microbial biomass carbon, and nitrogen. Proper soil management practices, such as balanced use of organic fertilizer, optimal soil moisture levels, reduced tillage, crop rotation, and the use of beneficial plant microorganisms, help increase both APase activity and soil pH. Further research on the contribution of APase activity to crop productivity is needed to apply phosphorus to fertilizer management [14].

(3) Inorganic phosphorus is also an important component of the soil phosphorus pool. The effects of biochar application on the availability and mobility of inorganic phosphorus depend on the biochar properties (raw material, pyrolysis temperature and time, C:N ratio, pH, ash content, and phosphorus content) and soil properties (pH, soil texture, and phosphorus content). Biochar application significantly increased various inorganic phosphorus fractions and soil available phosphorus. Except for biochar derived from wood residues with a high C/N ratio (>200), biochar significantly increased available phosphorus (water-extractable soil phosphorus, Olsen phosphorus and phosphorus bound to soil calcium compounds). Furthermore, application of biochar derived from crop residues significantly increased soil phosphorus associated with iron and aluminum oxides [15].

(4) The application of organic soil amendments (compost, vermicompost, biochar, olive pomace, etc.) plays a crucial role in improving soil physicochemical quality by increasing soil organic matter content, promoting aggregate formation, and improving soil structure in the short term. They have been shown to have positive effects on water retention capacity, pH level, nutrient availability, and carbon sequestration. However, some studies have not found any change in soil quality due to organic soil amendments. This suggests that the effects of organic soil amendments vary depending on the initial level of soil quality, the application rate of organic soil amendments, and the cropping system, and that strengthening soil management at the local level is important [16].

(5) Improving soil health not only improves crop yields and reduces environmental impacts but also achieves a positive carbon balance in the field (increasing soil carbon). Fertilization management has been examined for this purpose. In four managed grasslands across Japan, compost carbon application at a rate greater than 2.5 t C ha⁻¹ y⁻¹ was necessary to maintain standard yields and achieve a net positive carbon balance. Although compost application can reduce chemical fertilizer use, fertilization generates N₂O emissions, and an additional 1 t C ha⁻¹ y⁻¹ of compost carbon is estimated to be needed to offset the greenhouse effect of N₂O emissions. Without compost application, soil organic carbon was expected to decrease. In these fields, the ratio of soil organic carbon to clay content (SOC/Clay) dropped to less than 1/13 after 39 to 68 years, indicating significant soil structural degradation [17].

In conclusion, proper organic matter management is essential to improving agricultural soil health. Soils degraded by salinization, alkalinization, slash-and-burn farming, and heavy metal contamination are prone to soil erosion due to sparse vegetation and poor microbial activity. Protecting sparsely vegetated soils is a key challenge, and implementing vegetation restoration technologies is a priority. Since vegetation recovery in degraded soils takes a long time, it is crucial to identify the areas to be protected and introduce technologies to accelerate restoration.

Furthermore, improved agricultural fertility management is essential for sustainable intensification. A sufficient supply of organic carbon and an adequate nutrient supply are fundamental to improving soil health. Reducing the use of chemical fertilizers requires developing fertility management systems that respond to the variability in soil fertility across fields based on basic research on improving soil phosphorus and nitrogen fertility.