Compost Tea as Organic Fertilizer and Plant Disease Control: Bibliometric Analysis

Abstract

A variety of research reports that compost tea controls plant pathogens and improves plant nutrition and plant growth. Therefore, it can be used to reduce the use of synthetic fertilizers and pesticides. The aim of the study was to characterize and quantify the scientific production in the SCOPUS database on compost tea using bibliometric indicators. A total of 285 published papers related to compost tea were identified. The results show a general increasing trend from 2001 to 2023, with the highest number of publications occurring in 2021. Most of the publications were in the form of original articles, and English was the main language of publication. The top 10 countries with the highest scientific productivity were the United States, Egypt, Spain, Canada, Italy, India, China, Australia, Iran and Malaysia. Zaccardelli, M. and Pane, C. were the authors with the highest productivity with nine articles. In the co-authorship networks, two main networks were registered: the first with Diáñez F., together with Gea F. J., Navarro M.Y. and Santo M., and the second with Zaccardelli M., Celono G., and Pane C. Therefore, the need to adapt more resilient agricultural production systems allows for the consideration of compost tea as an alternative to mitigate environmental problems and soil degradation.

1. Introduction

One of humanity’s greatest challenges will be to produce enough nutritious and healthy food for a growing world population and, at the same time, to ensure environmental, social and economic sustainability [1]. Agricultural soils will have to maintain adequate levels of physical, chemical and biological properties to produce food, fiber and energy without their nutritional balance, health and productive capacity being negatively impacted. It is estimated that the application of synthetic fertilizers was responsible for increasing crop yields by at least 50% in the 20th century [2]. However, the excessive use of synthetic fertilizers and the practice of monoculture have led to soil degradation and environmental problems, as well as to the emergence of soil-borne diseases [3].

It is important to adapt more resilient food production systems based on sustainable agriculture to mitigate environmental problems and soil degradation. Sustainable agricultural development can be adopted by applying different methods, either through organic agriculture or using different principles (crop rotation, polyculture, pest and disease management, urban agriculture, biodynamic agriculture and permaculture) [4]. Organic agriculture is an agricultural system that restricts the use of synthetic fertilizers, pesticides, livestock feed additives, growth hormones and genetically modified organisms. In organic agriculture, soil fertility is protected in the long term by conserving or increasing the levels of organic matter, mainly by applying compost, animal manure and green waste, among others. In addition, soil microbial activity, minimal agricultural mechanization, use of improved varieties and water and soil conservation practices are encouraged [5].

Composting is a method used for the treatment of organic waste, which transforms degradable organic matter into stable compost through microbial action. Compost plays an important role in agricultural fields because it promotes crop growth, improves crop resistance to abiotic or biotic factors, absorbs and immobilizes pesticides and reduces heavy metal contamination due to its high organic carbon content and active functional groups [6].

According to Scheuerell and Mahaffee [7], during the 1990s, water-based compost preparations received interest from producers and researchers, resulting in a diversity of terminologies and preparation methodologies. There are several terms to describe compost preparations: compost tea, aerated compost tea, organic tea, compost extract, aqueous fermented compost extract, amended extracts, steepings and sludges. It is worth mentioning that some are synonymous with each other or are easily confused with other concepts [8].

Therefore, compost tea is a fermented liquid organic preparation of composted materials with tap water in a ratio of 1:5 or 1:10 (v/v) with or without aeration and optionally with nutrient additives. Compost teas are composed of soluble nutrients and useful microorganisms (bacteria, actinomycetes, filamentous fungi, oomycetes and yeasts) that have a synergistic effect in combating phytopathogens, maintaining soil fertility and increasing agricultural yields [9,10,11].

Compost tea has been reported to mitigate damage caused by phytopathogens such as Phytophthora infestans, Plasmopara vitícola, Pseudomonas syringae pv. maculicola, Pseudopeziza tracheiphila, Sphaerotheca fuliginea, S. pannosa var. rosae, Uncinula necator, Venturia inaequalis and Xanthomonas vesicatoria in Arabidopsis, grapevine, potato, tomato, cucumber, apple and rose crops [8]. Hargreaves et al. [12] indicated that supplying compost tea via foliar to strawberry plants provided similar amounts of most macronutrients and micronutrients compared to municipal solid waste compost, ruminant compost and synthetic fertilizers.

On the other hand, for the development of the stages of scientific research, it is necessary to know the potential areas of research and its impact. One of the tools that allow its understanding is the bibliometric technique [13].

Bibliometrics is the application of mathematical and statistical methods to books and other media to demonstrate the historical distribution, determine the use of national or universal research of books and journals and determine, in many local situations, the general use of books and journals. Bibliometric analysis can be applied to identify articles, authors, journals, countries and their connections in published research addressing a particular topic [14]. Bibliometric data analysis can be visualized using the mapping technique. Several bibliometric mapping tools are used, one of which is through the VOSviewer software. VOSviewer allows several types of maps to be made, including co-authorship maps, citation maps, set citation maps, bibliographic linkage maps and co-occurrence maps [15].

In addition to the above, the most related to a bibliometric analysis on compost tea is a systematic review and meta-analysis to compare crop yields with the application of bioproducts (compost tea and anaerobic digestates) against conventional fertilization and without treatment, where approximately 1200 scientific articles were registered; however, according to their inclusion criteria: papers based on scientific experiments including at least one treatment of pure fertilization with anaerobic digestate or compost tea (not combined with any other organic or synthetic fertilizer), papers based on field, greenhouse and pot experiments were selected, papers that measured crop yield as biomass: dry or fresh weight of either the total plant, aerial part or the organ of agronomic interest (e.g., fruit), only 194 articles were preselected, and when reviewing the entire contents, only 35 articles were included for review [16]. Considering that no research discussing a bibliometric analysis with the method of mapping analysis in compost tea research was recorded. The objective of the present study was to characterize and quantify the scientific production in the SCOPUS database on compost tea by means of bibliometric indicators.

2. Materials and Methods

The methodology of this descriptive-retrospective bibliometric study comprised four phases: (1) search, (2) results, (3) software selection and (4) analysis (Figure 1). The SCOPUS database^® Elsevier (Amsterdam, The Netherlands) was used, which provides details of publications, including type of access, year, author name, subject area, type of documents, source title, keywords, affiliation, country, source type and language. The search strategy used was ((TITLE-ABS-KEY (“compost tea”) OR TITLE-ABS-KEY (“infusion compost”))) in both title, abstract and keywords. This search returned a total of 285 documents for further analysis. Data was retrieved on 16 March 2023.

The descriptive analysis of the extracted documents was: scientific productivity, annual growth rate, growth rate, duplication time, journals, subject area, productivity by country, publication efficiency index of the countries, document typologies, institutions, participation index of the institutions, authorship, collaboration index, degree of collaboration and collaboration coefficient (

Table 1. Indicators of scientific productivity calculated in the present study.

Bibliometric Indicator	Description
Annual growth rate (AGR) [17]	$\mathrm{T}\mathrm{C}\mathrm{A}=\left(\frac{{NP}_t-{NP}_{t-1}}{{NP}_{t-1}}\right)\times 100$ where ${NP}_t$ is the number of publications in the year $t$; and ${NP}_{t-1}$ is the number of publications in the year $t-1$.
Relative growth rate per year (RCR) [18]	$\mathrm{T}\mathrm{C}\mathrm{R}=\left(\frac{In{NP}_2-In{NP}_1}{t_2-t_1}\right)$ where $In$ = natural logarithm; ${NP}_1$ and ${NP}_2$ are the cumulative number of publications in the years $t_2$ and $t_1$; $t_2-t_1$ = the difference between the final year and the initial year; here, the year can be taken as the unit of time.
Doubling time per year (TD) [18]	$\mathrm{T}\mathrm{D}=0.693/\mathrm{T}\mathrm{C}\mathrm{R}$ where $\mathrm{T}\mathrm{C}\mathrm{R}$ = relative growth rate.
Publication Efficiency Index (PEI) of countries [19]	$\mathrm{I}\mathrm{E}\mathrm{P}=\frac{\left({TC}_i/\sum TC\right)}{{NP}_i/\sum NP}$ where ${TC}_i$ denotes the total number of citations for the country; $\sum TC$ denotes the total number of citations for all countries; ${NP}_i$ denotes the total number of publications from the country $i$; $\sum NP$ denotes the total number of publications from all countries.
Institutional participation index (IP)	$\mathrm{I}\mathrm{P}=\frac{the term number of publications by the institution}{total number of published documents}$
Collaboration index, degree of collaboration, collaboration coefficient [20]	$Collaboration index=\frac{{\sum }_{j=1}^{A}jfj}{N}$ where $j$ = the number of authors in a publication, i.e., 1, 2, 3…; $fj$ = the number of $j$ publications; $N$ = the total number of publications in a year; $A$ = the total number of authors per publication. $degree of collaboration=1-\frac{f1}{N}$ $f1$ = the number of publications by a single author; $N$ = total number of publications in a year. $collaboration coefficient=1-\frac{{\sum }_{j=1}^A\left(\frac{1}{j}\right)fj}{N}$ $j$ = the number of authors in an article, i.e., 1, 2, 3…; $fj$ = the number of $j$ articles written; $N$ = the total number of articles published in a year; $A$ = the total number of authors per article.

). Likewise, citation analysis was performed as citation and dissemination metrics for the 10 most-cited articles on compost tea.

With the software Harzing’s Publish or Perish 8.0 [21], the bibliometric indicators were analyzed: total numbers, total articles, citations, average citations per year, average citations per article, average citations per author per year, average articles per author, average authors per article, h index, g index, contemporary h index (hc), individual h index (hI), normalized hI index, AWCR index, AW index, e index, hm index, annual hI index, H amplitude and G amplitude [22].

Bibliometric maps were elaborated with VOSviewer software version 1.6.10 of co-occurrences of keywords, co-authorships between authors, co-authorships between countries and citations between journals [15].

3. Results

3.1. Bibliometric Indicators

The period covered by the publications on compost tea in SCOPUS was 27 years (1996–2023), and 285 publications were found, in which 896 authors (1161 authorships), 155 journals, 2251 keywords, 53 countries and 605 organizations were registered. The 285 publications exceeded the minimum sample size required (200 articles) to perform a bibliometric analysis [23]. Regarding the language of publications, there was a large predominance of research in English (97.19%), followed by Spanish (1.75%). The citation score of the publications was 4147 citations, corresponding to 153.59 citations per year and 14.55 citations per published paper. The publications showed an h-index of 32 (32 documents have been cited at least 32 times) and a g-index of 53 (

Table 2. Data collected from scientific productivity on compost tea in SCOPUS.

Indicators	Results
Document	285
Citations	4147
Years	27 (1996–2023)
Citations/year	153.59
Quotes/articles	14.55
Quotes/authors	1384.43
Citations/authors/year	51.27
Articles/author	97.65
Authors/article	4.06
Index h	32
Index g	53
Index hc	22
Index hl	8
Normalized hl index	17
AWRC	486.25
AW Index	22.05
Table of contents e	35.26
Index hm	19.89
Annual hl index	0.63
Coverage H	54.7
Coverage G	68.8

).

3.2. Annual Distribution and Growth Trend in Scientific Productivity

The distribution of publications and number of citations per year on compost tea in SCOPUS is shown in Figure 2, and it can be highlighted that the publications show an irregular increasing trend over the last 20 years (2004–2023). The first references of publications on compost tea as a source of nutrients and/or control of phytopathogens in plant species date back to 1996, with the article “The effect of water extracts of spent mushroom compost on apple scab in the field”, which aimed to control apple scab (cv. McIntosh) caused by Venturia inaequalis by applying anaerobically fermented teas for seven days using two sources of compost based on spent mushroom (Agaricus bisporus) substrate [24].

There was only one publication in 1996, and its peak was in 2012, with 25 publications. This trend shows an increase in the importance of this topic for researchers, followed by industries, both of which paid attention to compost tea due to the global value of environmental problems, and this trend continues to show linear growth. This increase is due to the global interest in generating solutions to manage organic solid waste by transforming organic waste into amendments that contribute to soil fertility and plant health, thus encouraging sustainable agricultural production. Compost tea is the extraction of compounds that are soluble in water, in addition to the microorganisms present in the compost. The soluble compounds in compost tea are absorbed by plants and favor the development of beneficial microorganisms that reduce the incidence of plant diseases [25].

In 2002, researchers started to show great interest in compost tea. After 2004, except for the years 2007, 2008, 2013, 2015, at least 10 articles have been published. The year with the highest scientific production was 2021, with 26 publications, and the lowest production was 1997 to 2000, with zero publications. Of the 285 publications, 71 (26.32%) were published in open access journals and showed a linear function of $y=0.7523x-0.7302,R^2=0.6226$. The mean annual production was 10.18 publications, with a range of 26, standard deviation of 7.84 and coefficient of variation of 76.99%. The number of overall citations increased rapidly between 2002 and 2014 (Figure 2).

The publications were cited 4147 times, with an average of 14.55 citations per publication; 2009 had 15 publications with the highest number of citations (438), and the years with the lowest number of citations were 1997 to 2000, with zero citations. Ninety-six percent of the total number of publications (275 publications) received at least one citation (Figure 2). The number of citations is an indicator of visibility, use, dissemination or impact of a scientific publication. There is a positive correlation between the number of citations received and scientific quality. The number of publications and the prestige of the journal in which it is published establish indicators to measure the trajectory, scientific depth and capacity of each research community.

After analyzing scientific productivity in five-year periods, the trend showed an increase between 2009–2013 (71) and 2019–2023 (99), representing 59.65% of the total number of publications. In the initial years 1994–1998, only one publication was contributed. In the periods 2004–2008 and 2009–2013, the highest number of citations (1254 and 1338, respectively) were observed. There was an average of 19.80 publications per year in 2019–2023, which was higher than the rest of the five-year periods (

Table 3. Scientific production on compost tea in five-year blocks (n = 285).

Years	NP	% NP	PPA	Cumulative NP	Accumulated NP	Total of Appointments
1994–1998	1	0.35	0.20	1	0.35	34
1999–2003	10	3.51	2.00	11	3.86	747
2004–2008	43	15.09	8.60	54	18.95	1254
2009–2013	71	24.91	14.20	125	43.86	1338
2014–2018	61	21.40	12.20	186	65.26	624
2019–2023	99	34.74	19.80	285	100.00	199

NP = number of publications; PPA = average publications per year.

).

3.3. Types of Publications

Table 4. Typology of scientific productivity on compost tea (n = 285).

Typology	Total of Publications	Total % of Publications	Total of Citations	Average Citations per Publication
Article	241	84.56	3606	14.96
Conference Paper	21	7.37	77	3.67
Review	12	4.21	416	34.67
Book Chapter	6	2.11	46	7.67
Editorial	2	0.70	1	0.50
Note	2	0.70	1	0.50
Short Survey	1	0.35	0	0.00

shows the different document typologies in which the publications on compost tea were distributed in SCOPUS. Seven typologies were recorded. Of the 285 documents retrieved, 241 (85.56%) correspond to articles, 21 (7.37%) are conference papers and 12 (4.21%) are reviews. In terms of average citations per publication, the highest value was recorded for review-type publications, with 12 publications and 416 citations, representing 34.67 average citations per publication, followed by articles with 241 publications, 3606 citations and 14.96 average citations per publication.

3.4. Annual Growth Rate

Growth rate is a necessary metric in any scientific field. The growth in the number of publications in a particular discipline is often a metric of annual increase or decrease, which was recorded in this study [26]. Figure 3 shows the growth rate of publications during 1996–2023 between −64.00 and 200.00 on compost tea. The 285 total publications showed a positive average annual growth rate of 25.04%. This is due to the interest of the scientific community in identifying and explaining the effects of supplying compost tea on agricultural crops. Such supply increased plant yields by at least 92% compared to the control (no fertilization) and reduced by 10% to synthetic fertilization [16]. The growth rate was positive for the years 2002–2006, 2009, 2011, 2012, 2014, 2016–2019 and 2021, with a fluctuation between 9.09% and 200%, while 2007, 2008, 2010, 2013, 2015, 2020, 2022 and 2023 recorded negative rates ranging from −14.29 to −64.00. The years 1997–2000 showed a different view with a neutral rate (0%). The years 2002 and 2009 showed the highest growth rate (200%), followed by 2014 (111.11%) and 2012 (108.33%), while 2013 (−64%) showed the lowest negative growth rate of all (Figure 3).

3.5. Relative Growth Rate and Doubling Time per Year

The relative growth rate (RCR) is the increase in the number of publications per unit of time [18]. Figure 4a indicates the TCR of compost tea for the period 1996–2023. The maximum value of TCR was 0.92 in the year 2002, followed by the year 2003 with a value of 0.76. Similarly, the lowest value occurred in the years 1996–2000 with the same value of zero. The TCR was not stable during the study period. The RCR for the years 2001, 2002 and 2003 were the highest (mean value 0.80), and in the block 2003–2008, the mean growth rate gradually decreased from 0.79 to 0.10. Although there was a slight increase in 2009, the TCR decreased from 0.24 in 2009 to 0.14 in 2010, 0.07 in 2013 and 0.04 in 2023. This means that the number of publications decreased slowly.

The doubling time (TD) is an indicator of the time, expressed in years, that has to elapse for the scientific production of a given subject to double [27]. Figure 4b shows the TD, which increased from 1 (2001) to 7.59 (2022), without considering the year 2023 due to the fact that when the bibliometric search was performed, it was in its first quarter. As the TCR decreased, the TD increased during the research period.

3.6. Journals and Subject Areas

The present bibliometric review obtained 155 journal records indexed in SCOPUS in the field of compost tea during the period 1996–2023.

Table 5. Top ten most productive journals on compost tea in SCOPUS.

Magazine	TP	TC	CPP	CiteSore	SNIP	SJR	Country
Acta Horticulturae	19	60	3.158	0.500	0.226	0.163	Belgium
Biocycle	18	82	4.556	0.200	0.000	0.103	United States
Compost Science and Utilization	14	334	23.857	2.600	0.699	0.265	United Kingdom
Scientia Horticulturae	6	235	39.167	7.000	1.721	0.844	Netherlands
Biological Agriculture and Horticulture	5	95	19.000	3.400	0.718	0.464	United Kingdom
Crop Protection	5	189	37.800	4.500	1.474	0.700	United Kingdom
Hortscience	5	36	7.200	2.600	0.875	0.444	United States
Journal Of Plant Nutrition	5	25	5.000	3.400	0.928	0.510	United States
European Journal of Plant Pathology	4	68	17.000	3.700	1.019	0.529	Netherlands
Environmental Science and Pollution Research	4	39	9.750	6.600	1.154	0.831	Germany

TP = total publications; TC = total citations; CPP = average citations per publication; SJR = Scientific Journal Rankings.

lists the 10 most prolific journals that published four or more papers. These 10 prolific journals have published 85 (29.82%) of the total literature as of 16 March 2023. Articles published in these 10 prolific journals have been cited 1163 (28.04%) times. The remaining 145 journals collectively produced 200 (70.17%) articles and received 2984 (71.96%) citations in the period 1996–2023. Most of the journals with publications on compost tea were classified in agricultural and biological sciences with 221 (49.55%), environmental sciences with 100 (22.42%), biochemistry, genetics and molecular biology with 31 (6.96), immunology and microbiology with 17 (3.81) and engineering with 15 (3.36) (Figure 5).

The journal Acta Horticulturae (ISSN: 0567-7572) published the highest number of articles (19 (6.67%)) and received 60 citations. It is edited by the International Society for Horticultural Sciences with the thematic areas of agricultural sciences and biology: horticulture. The second most productive journal was Biocycle (ISSN: 0276-5055), with 18 (6.32%) and 82 citations, published by JP Press, Inc. in the subject areas of agricultural sciences and biology: soil science and environmental sciences: waste management and disposal.

The journal Compost Science and Utilization (E-ISSN: 2326-2397), with 14 publications, recorded the highest number of citations (334), with an average number of citations of 23.857, followed by Scientia Horticulturae (ISSN: 0304-4238), with 6 publications, 235 citations and 39.167 average citations per publication, and Crop Protection (ISSN: 0261-2194), with 5 publications, 189 citations and 37.80 average citations per publication. In addition, the journal Scientia Horticulturae, with six publications, showed the highest Cite Sore value (7.00), indicating a higher average number of citations per publication. The SJR-2021 (Scimago Journal Rankings) and SNIP-2021 (Source Normalized Impact per Paper) values of the latter two journals are at the highest value. Three journals belong to the United Kingdom (3); likewise, three are from the United States and two from the Netherlands, while the countries of Belgium and Germany registered one journal each (Table 5).

3.7. Countries with the Highest Number of Publications

The 285 papers on compost tea were published in 53 countries. The United States of America was the country with the highest scientific production, with 66 articles (23.16%), followed by Egypt with 28 (9.82%) and Spain with 24 (8.24%) publications. These three countries are located on different continents; the United States represents the Americas, Egypt is the highest-ranked country in Africa and Spain is the country with the most publications among other European countries. Overall, the latter countries accounted for 41.22% of the scientific publications, indicating that each continent has a leading country in compost tea research. Subsequently, six countries published articles in the range of twenty-three and ten; six countries published articles in the range of eight and five (

Table 6. Efficiency index of the most productive countries’ publications on compost tea.

Ranking	Country	TP	TC	CPP	IEP
1st	United States	66	1338	20.27	1.38
2st	Egypt	28	208	7.43	0.51
3st	Spain	24	361	15.04	1.03
4st	Canada	23	492	21.39	1.46
5st	Italy	23	274	11.91	0.81
6st	India	13	220	16.92	1.16
7st	China	13	68	5.23	0.36
8st	Australia	10	301	30.10	2.06
9st	Iran	10	56	5.60	0.38
10st	Malaysia	8	223	27.88	1.90
11st	Mexico	8	58	7.25	0.50
12st	Tunisia	7	79	11.29	0.77
13st	United Kingdom	6	267	44.50	3.04
14st	Indonesia	6	7	1.17	0.08
15st	Trinidad and Tobago	5	101	20.20	1.38
	Others	67	4053	60.49	1.07
	Total	317	4640	14.65

TP = number of publications; TC = total citations; CPP = average citations per publication; IEP = publication efficiency index.

).

The total number of publications, adding the contributions from each country, is 329, which is higher than 285, indicating that there was collaborative work between different countries. Information and communication technologies have facilitated the linkage and scientific work between researchers, experts or technicians from different latitudes, which was recorded in this research. The United States of America received the highest number of citations (1338 (32.26)) in the 66 documents. However, the average citations per publication (20.27) of papers from the United States is lower than the average citation of papers from the United Kingdom, which registered the highest value of 44.50, with six documents. The next order of countries with the highest average citations were Australia with 30.10 in 10 publications, Malaysia with 27.88 in 8 publications, Canada with 21.39 in 23 publications and Trinidad and Tobago with 20.20 in 5 publications. In contrast, Mexico is in 11th position, with 8 publications and 58 citations, which represents an average citation of 7.25 (Table 6).

The publication efficiency index (PEI) is the measure of research quality and indicates whether the impact of publications in a country in a field of research is compatible with the research offerings. An IEP value >1 for a country indicates that the impact of publications is greater than the research supply devoted to them for that particular country and vice versa. It is obtained by dividing the percentage of citations “citation” by the percentage of publications “offers”. Countries such as the United States, Spain, India, Australia, Malaysia, United Kingdom, Trinidad and Tobago have an IEP greater than 1, which means that the impact of their publications exceeds their own supply (percentage of publications); on the contrary, in the rest of the countries, the impact of their publications is less than what is offered (Table 6).

3.8. Institutions

Of the total number of publications, 75 (26.32%) were open access, registering 896 authorships with affiliations in 160 institutions from 53 countries.

Table 7. Top most productive institutions on SCOPUS compost tea (n = 285).

Ranking	Institution	TP	TC	CPP	IP	Country
1st	United States Department of Agriculture	10	483	48.30	3.51	United States
2st	CREA-Centro di Ricerca per l’orticoltura, Pontecagnano	9	159	17.67	3.16	Italy
3st	University of Almeria	9	117	13.00	3.16	Spain
4st	Dalhousie University	8	159	19.88	2.81	Canada
5st	National Research Centre	8	87	10.88	2.81	Egypt
6st	Oregon State University	7	387	55.29	2.46	United States
7st	University of Basilicata	7	150	21.43	2.46	Italy
8st	Università degli Studi di Napoli Federico II	7	85	12.14	2.46	Italy
9st	University of Hawai‘i at Mānoa	6	244	40.67	2.11	United States
10st	University of Tasmania	6	69	11.50	2.11	Australia
11st	Kafrelsheikh University	6	68	11.33	2.11	Egypt
12st	Soil, Water and Environment Research Institute SWERI	6	62	10.33	1.50	Egypt
13st	Agricultural Research Center	6	55	9.17	1.33	Egypt
14st	Universidad Autónoma Agraria Antonio Narro	6	46	7.67	1.11	Mexico
15st	Faculty of Agriculture	6	40	6.67	0.96	Egypt

TP = total publications, TC = total citations, CPP = average citations per publication, IP = participation index.

shows the top 15 institutions that published more than five papers, totaling 107 (37.54%) publications and 2211 (53.32%) citations of the total scientific literature related to compost tea. The three institutions with the highest number of publications were found to be the United States Department of Agriculture (USDA) (Washington, DC, USA), with 10 publications, 483 citations and 48.30 average citations per publication, meaning it is the institution with the highest productivity in the field of compost tea literature, with a productivity index of 3.5, followed by CREA—Centro di Ricerca per l’orticoltura, Pontecagnano (Pontecagnano Faiano, Italy), with 9 publications, 159 citations, 17.67 average citations per publication and a productivity index of 3.16, and the University of Almeria (Spain), with 9 publications, 117 citations, 13 average citations per publication and a productivity index of 3.16.

Five institutions (33.33%) of the fifteen most productive are located in Egypt. Twenty percent of the institutions are from the USA (United States Department of Agriculture, Oregon State University and University of Hawai‘i at Mānoa), which have received more citations—483, 387 and 244, respectively—compared to the rest of the institutions; therefore, it can be interpreted that research on compost tea has been dominated by researchers from the USA. It is worth mentioning that only one Mexican institution registered more than 5 publications, positioned in 14th place with 6 publications, 46 citations and 7.67 average citations per publication.

3.9. Most Prolific Authors

According to the analysis of authorships on compost tea, a total of 896 authors were registered. The 13 authors with more than four publications are presented in

Table 8. Authors with the highest number of publications on compost tea.

Ranking	Author	NP	TC	CPP	h-Index	Lotka Productivity Index Log(n)	Country
1st	Zaccardelli M.	9	159	17.67	28	0.95	Italy
2st	Pane, C.	9	159	17.67	20	0.95	Italy
3st	Diánez F.	9	117	13.00	18	0.95	Spain
4st	Santos M.	9	117	13.00	17	0.96	Spain
5st	Celano G.	6	149	24.83	29	0.78	Italy
6st	Warman P. R.	6	147	24.50	28	0.78	Canada
7st	Siddiqui Y.	5	211	42.20	23	0.70	Malaysia
8st	Meon S.	5	211	42.20	21	0.70	Malaysia
9st	Scharenbroch B. C.	5	97	19.40	20	0.70	United States
10st	Gea F. J.	5	81	16.20	16	0.70	Spain
11st	Navarro, M. J.	5	81	16.20	15	0.70	Spain
12st	Evans K. J.	5	65	13.00	16	0.70	Australia
13st	Preciado-Rangel P.	5	30	6.00	12	0.70	Mexico

NP = number of publications; TP = total publications; TC = total citations; CP = average citations per publication.

. These 13 authors collectively published 83 (29.12%) of the total publications and received 1624 (39.16%) citations. The top authors with more than six publications were Pane, C., (orcid.org/0000-0001-8666-2424), Zaccardelli, M., Díanez, F. (orcid.org/0000-0002-1890-6594) and Santos M. (orcid.org/0000-0002-9626-3279) with nine publications. Meon S. and Siddiqui Y. from Universiti Putra Malaysia, with five publications, were the most-cited authors, with 211 citations, representing 5.09% of the total citations. Preciado-Rangel P. (orcid.org/0000-0002-3450-4739), with 5 publications and 30 citations, was the only one with a Mexican institution (Tecnológico Nacional de México, Mexico City, Mexico). This indicates that the number of researchers and research related to compost tea is still limited, which can be observed with a productivity index of Lotka < 1, resulting in an intermediate level of productivity (2 to 9 publications) (Table 8).

In addition, single-author publications accounted for 31 (10.88%) of the total. The contributions with two and three authors were 47 (16.49%) and 40 (14.04%), respectively. The lowest proportions of authorships and number of citations were 11 and 14 authors contributing only one publication. Publications with four authors (63 (22.10%)) were the most cited, with a total of 956, which represents 23.05% of the total citations. This means that publications with five or fewer authors may present better quality, resulting in a higher number of citations compared to publications with six or more authors (Figure 6).

3.10. Collaboration Index (CI), Degree of Collaboration (GC) and Collaboration Coefficient (CC)

The study period was also divided into six blocks of five-year periods. The CI is the average number of authors per publication.

Table 9. Number of publications distributed according to the number of authors and by period.

Years	TP	PSA	PMA	Total Number of Authors of Multi-Authored Publications	Total by Author in Single-Author Publications	Total of Authors	IC	GC	CC
1994–1998	1	0	1	3	0	3	3.00	1.00	0.67
1999–2003	10	6	4	14	6	20	2.00	0.40	0.26
2004–2008	43	15	28	97	15	112	2.60	0.65	0.43
2009–2013	71	3	68	258	3	261	3.68	0.96	0.66
2014–2018	61	4	57	254	4	258	4.23	0.93	0.68
2019–2023	99	3	96	507	3	510	5.15	0.97	0.75

TP = total publications; PSA = single-author publications; PMA = multi-author publications; IC = collaboration index; GC = degree of collaboration; CC = collaboration coefficient.

presents the average CI of 3.44, and the highest CI was noticed in the years 2019–2023 with 5.15, followed by the years 2014–2018 with 4.23, while the lowest was in the years 1999–2003 with 2.00, implying that the research teams are between two and six authorship patterns in the compost tea field. The CI was increasing over time, which shows that the increase in publication trend is directly proportional to the increase in authorships in collaborative publications.

The GC was found to be between 0.40 and 1.00, with an average value of 0.81 during the study period. It was observed that the GC value increased during the time period, indicating that collaborative works are increasing in the field of compost tea literature. The CC for the periods 1999–2003 (0.26) and 2004–2008 (0.43) were less than 0.5, indicating weak collaboration among the authors. The CC value for the last two five-year periods—2014–2018 (0.68) and 2019–2023 (0.75)—indicates better collaboration among the authors. Overall, the collaboration among authors was 0.58 in the scientific literature on compost tea; therefore, there was better collaboration among authors during the study period.

3.11. Most-Cited Publications

A higher number of citations in a publication suggests that its quality is very good because it has an impact on the knowledge of a given topic.

Table 10. Top ten most-cited publications related to compost tea in SCOPUS.

Authors	Title	Magazine	Year	Citation	Country	Type
Litterick et al. [8]	The role of uncomposted materials, composts, manures, and compost extracts in reducing pest and disease incidence and severity in sustainable temperate agricultural and horticultural crop production—A review	Critical Reviews in Plant Sciences	2004	181	United Kingdom	Review
Quilty and Cattle [28]	Use and understanding of organic amendments in Australian agriculture: A review.	Soil Research	2011	169	Australia	Review
Scheuerell and Mahaffee [7]	Compost tea: Principles and prospects for plant disease control.	Compost Science and Utilization	2002	168	United States	Article
Scheuerell and Mahaffee [29]	Compost tea as a container medium drench for suppressing seedling damping-off caused by Pythium ultimum	Phytopathology	2004	124	United States	Article
Pant et al. [30]	Vermicompost extracts influence growth, mineral nutrients, phytonutrients and antioxidant activity in pak choi (Brassica rapa cv. Bonsai, Chinensis group) grown under vermicompost and chemical fertilizer	Journal of the Science of Food and Agriculture	2009	117	United States	Article
Larkin [31]	Relative effects of biological amendments and crop rotations on soil microbial communities and soilborne diseases of potato	Soil Biology and Biochemistry	2008	108	United States	Article
Carballo et al. [32]	Characterization of different compost extracts using Fourier-transform infrared spectroscopy (FTIR) and thermal analysis	Biodegradation	2008	95	Spain	Article
Al-Dahmani et al. [33]	Suppression of bacterial spot of tomato with foliar sprays of compost extracts under greenhouse and field conditions	Plant Disease	2003	88	United Arab Emirates–Canada	Article
Pant et al. [34]	Biochemical properties of compost tea associated with compost quality and effects on pak choi growth	Scientia Horticulturae	2012	84	United States	Article
Scheuerell and Mahaffee [11]	Variability associated with suppression of gray mold (Botrytis cinerea) on geranium by foliar applications of nonaerated and aerated compost teas	Plant Disease	2006	82	United States	Article

presents the 10 most-cited articles, with a citation range of 82–181, totaling 1216, which represents 29.32% of the total citations on compost tea. Four papers received 82–95 citations, three papers 108–124 citations and four papers 168–181. Sixty percent of these papers were published in the United States between 2002 and 2012. It is interesting to mention that the two most-cited publications are classified as review articles. Authors Scheuerell S., affiliated with Evergreen State College, Olympia, WA, USA, and Mahaffee, W., affiliated with USDA Agricultural Research Service, Washington, DC, USA, contributed three of the top ten most-cited publications.

The article entitled “The role of uncomposted materials, composts, manures, and compost extracts in reducing pest and disease incidence and severity in sustainable temperate agricultural and horticultural crop production—A review” published in the Critical Reviews in Plant Sciences (ISSN: 0735-2689) was the most cited (181), which aimed to evaluate the evidence of the impact of (i) non-composted plant residues, (ii) composts, (iii) manures, (iv) compost extracts and teas on pest and disease incidence and severity in agricultural crops [8]. The second most-cited paper (169) was “Use and understanding of organic amendments in Australian agriculture: A review” published in Soil Research (ISSN: 1838-675X), which had three objectives: (i) to classify the wide range of organic manures available in Australia, (ii) to explore the benefits and efficacy of applying different groups of organic manures in plant-soil systems and (iii) to discuss the slow adoption of organic manures in Australia [28].

3.12. Keyword Network

In total, 2251 keywords were found and organized in the network shown in Figure 7 are those that obtained at least five co-occurrences. The size of the nodes (the larger the node and the keyword, the higher the co-occurrence) represents the co-occurrence, and the smaller the distance between the nodes, the stronger the relationship they have. The same color of nodes and keywords indicates that they are from the same cluster (related keywords). The VOSviewer software generated four clusters, representing the 143 keywords with at least five co-occurrences. The keywords that headed the clusters are compost tea (red, link strength 870, 133 co-occurrences), compost (green, link strength 808, 89 co-occurrences), composting (blue, link strength 805, 81 co-occurrences) and soil (yellow, link strength 355, 27 co-occurrences). On the periphery of the co-occurrence network, among all keywords, the keywords waste management (blue, link strength 23, five co-occurrences), biological materials (green, link strength 69, five co-occurrences), pH (yellow, link strength 66, five co-occurrences), Capsicum annuum (red, link strength 29, five co-occurrences) can be observed more segregated and therefore less investigated (Figure 7a).

The keywords indicated by the authors in the compost tea publications (and which had at least five co-occurrences in the central VOSviewer database) were included in the word cloud (Figure 7a). Of the 828 keywords with the highest co-occurrences by authors, only 22 reached the threshold of at least five co-occurrences, generating a total strength that ranged from 3 to 97. The seven keywords by authors with at least 10 occurrences were compost tea (104), compost (29), biological control (15), vermicompost (12), disease suppression (11), Solanum lycopersicum (11) and humic acid (10) (Figure 7b).

3.13. Author Co-Authorship Network

In the co-authorship network, based on the 285 publications, a threshold of five was established as the minimum number of documents by an author, registering a total of 13 authors grouped into seven groups. The size of the nodes corresponds to the number of papers published by an author. The thickness of a link between two authors corresponds to the number of papers they have co-authored. Cluster 1 (red) is led by Diáñez F., together with Gea F. J., Navarro M.Y. and Santo M., while cluster 2 (green) is formed by Zaccardelli M., Celono G., and Pane C. and, cluster 3 (blue) Meon S. and Siddiqui Y. (Figure 8). Among them, Zaccardelli M. from CREA-Centro di Ricerca per l’orticoltura, Pontecagnano, Pontecagnano Faiano, Italy, with an h-index of 28, with nine publications and 159 citations positioned him as the author with best indicators on compost tea research [35,36,37,38,39,40,41,42,43] (Figure 8).

The main collaborators of the author Zaccardelli M. were Pane C. and Celano G., who participated in the publication of six articles generating a linking force of 15; the most recent article they participated in this co-authorship network was published in the journal Applied Sciences (ISSN-E: 2076-3417, CiteScore 2021: 3.7, SJR 2021 0.507, SNIP 2021: 1.026) entitled “Effects of organic additives on chemical, microbiological and plant pathogen suppressive properties of aerated municipal waste compost teas” evaluating the hypothesis that the use of additives modulates the quality of compost teas, influencing their agronomic and crop defense properties. The research objective was to characterize the chemical, physical and microbiological quality of a set of aerated compost teas obtained from green/municipal waste compost with two different additives, whey and molasses, in increasing concentration [35].

3.14. Cross-Country Co-Authorship Network

Overall, 53 countries participated in the publications on compost tea between 1996 and 2023. To highlight the countries with the highest unit weight in the submission of publications on the investigated topic, the selection criteria were a minimum of five papers published in a specific country. Figure 9 shows the network among 17 countries of co-authorships expressed in eight clusters. The first cluster presented a collaboration between authors from three North American countries (USA, Canada and Mexico) and one Asian country (Iran). The second cluster was mainly between Spain, Argentina and the United Kingdom. It is worth mentioning that the countries are indicated by a label and a circle. The more important a country is, the larger its label and circle. The distance between two circles indicated the strength of the link. The smaller the distance, the greater the number of co-authored papers published between these countries.

3.15. Inter-Journal Citation Network

The analysis of the most influential journals on compost tea was generated from the citations received using VOSviewer bibliometric mapping (Figure 10). From a total of 155 journals, two clusters were generated with journals that published at least five papers. The first cluster (green) was led by the journal Compost Science and Utilization, with 14 publications, 334 citations, 23.86 average citations per publication and a linkage strength of 30, which shows the importance of this journal due to the fact that the authors’ preference was taken to select it as the first choice for publication. The most-cited article in the journal Compost Science and Utilization on compost tea was “Compost tea: principles and prospects for plant disease control”, published in 2002 and cited 168 times [7]. According to the classification made by SCOPUS in the Scientific Journal Ranking of the year 2021, the journal Compost Science and Utilization appears with an SJR of 0.265 points, while the journal Scientia Horticulturae led the second cluster (red), with 6 publications, 235 citations, 39.17 average citations per publications and a link strength 23, and presented the highest SJR value 0.844 compared to journals with publications on compost tea. The most-cited article from the journal Scientia Horticulturae was “Biochemical properties of compost tea associated with compost quality and effects on pak choi growth”, published in 2012 and recorded 84 citations [34]. It is important to mention that these journals are not the ones with the highest number of publications, but they are the main ones in terms of the number of citations received.

4. Discussion

In this study, using descriptive analysis software such as Harzing’s Publish or Perish and visualization software such as VOSviewer for extraction and visualization allowed us to identify research hotspots and development trends in the field of compost tea impact on plant nutrition and plant pathogen control. However, the data results are limited to publications indexed in the SCOPUS database. Compost tea is the term given by a growing number of researchers (United States and Europe) to the filtered product of water-fermented compost. Compost teas are produced by recirculating water through loose compost or a porous bag of compost suspended over or within a tank with the intention of maintaining aerobic conditions [7].

The results revealed an increasing and irregular trend in the number of publications on compost tea over the last 20 years. This trend started with a single publication in 1996 [24] and peaked in 2012 with 25 publications [34,36]. The year 2002 marked a turning point in the interest of the scientific community in compost tea. Since 2004, with the exception of a few years, at least 10 articles have been published annually. The data show that the set of publications received a total of 4147 citations. It is important to note that the number of citations is directly related to the scientific quality of the publications. The increases in citations received in the publications on compost tea have been relevant and have captured the attention of other researchers and the scientific community.

Continued growth in publications and citations could lead to further development of compost-tea-based strategies to address environmental and agricultural challenges in the future [7].

The main areas such as agriculture, horticulture and restoration of degraded soils are where the effects of both foliar and edaphic application of compost tea have been published the most [16]. The distribution of documentary typologies and average citations in publications on compost tea reveal the relevance of articles and reviews in this scientific field. The analysis of the growth rate of publications on compost tea has revealed patterns and fluctuations in scientific interest throughout the studied period. The period 2004–2008 was the most relevant in the emphasis on explaining the methodologies, importance and diversity of the microbial community present in compost tea for the control of phytopathogens [8,11,29,31,32].

Single-authored publications with a maximum of four authors may have a higher scientific quality and be more likely to receive a higher number of citations. This could be due to greater cohesion and focus in collaborative work, as well as greater individual attribution and recognition of the researchers involved. These findings may be useful in guiding future research and collaborations in the field of compost tea to foster higher quality and visibility of scientific publications. Currently, the compost tea research field focuses on waste management, biological materials, pH concentration and Capsicum annuum. The top five countries in terms of the number of articles published were the United States, Egypt, Spain, Canada and Italy. The most popular keywords were compost tea, compost, biological control, vermicompost, disease suppression, Solanum lycopersicum and humic acid.

Compost teas are used as a source of macro/micronutrients and beneficial microorganisms, as well as to control pests and plant diseases. Compost teas do not provide a substantial amount of macro or micronutrients when applied at low volumes. However, when applied at high volumes, e.g., 1000 L/ha, they are a significant source of essential plant nutrients [28].

According to Scheuerell and Mahaffee [29], the effects of aerated compost tea and non-aerated compost tea with and without additives vary in the suppression of cucumber wilt disease caused by Phythium ultimum. These same authors indicate that the most efficient in wilt suppression in cucumber was aerated compost tea with seaweed and humic acid additives. The application of aerated compost tea to the soil and the combination of compost tea with beneficial microorganisms (mycorrhizae and a mixture of microorganisms) are an alternative for modifying the characteristics of the soil microbial community and significantly reducing soil-borne diseases [31]. The solubility of inorganic forms present in composts affects the composition of compost teas; the concentration of soluble inorganic nutrients increases in compost teas with respect to compost. This issue is of utmost importance when using compost teas as a source of fertilizer [32].

Foliar application of compost teas significantly reduces the population of Xanthomonas vesicatiroia on tomato leaves. The efficacy of compost teas is not affected by oxygen concentrations in the suspension during extraction, compost maturity or sterilization by filtration or autoclaving. The degree of control provided by compost teas via foliar did not differ in comparison to that obtained with the plant activator acibenzolar-S-methyl [33]. Compost teas formulated from short fermentation cycles ranging from 24 to 36 h have limited usefulness for disease control, and fermentation periods of 7 to 14 days are required for consistent control of diseases caused by phytopathogens [11]. Compost quality impacts nutrient extraction efficiency, microbial activity, phytohormones and total nutrient content in the compost tea. Differences in the quality of compost teas affect plant growth and nutrient status [34].

Compost tea is a promising strategy for managing organic waste and promoting sustainable agricultural production [2]. The extraction of soluble compounds and the beneficial microorganisms present in the compost allow plants to absorb nutrients and favor the growth of antagonistic microorganisms that reduce the incidence of diseases [10].

Interestingly, the provision of compost tea has been associated with significant increases in plant yields, reaching an increase of at least 92% compared to the control group (no fertilization) and a 10% reduction compared to synthetic fertilization [16]. The use of compost tea as part of a strategy to manage plant health requires research by farmers and experts in composting, plant pathology, ecology, molecular microbiology, fermentation science, plant physiology, plant breeding, soil science and horticulture [7].

5. Conclusions

This study identified the publication trend, countries, subject areas and co-authorship network in compost tea publications. A limited number of scientific publications were found in the SCOPUS database. The set of results presented can be a reference for future research proposals that favor an increase in the visibility of publications in which the effects on soil microbiology, plant nutrition, soluble compounds and micro-organisms antagonistic to phytopathogens are evaluated and explained with the application of compost tea both via foliar and edaphic as an alternative to mitigate environmental problems and water and soil resources. Keywords to consult the state of the art of compost tea could be compost tea, biological control, compost, vermicompost, suppression of plant diseases, tomato and humic acids. For the search of research lines related to compost tea, the terms to be investigated could be the variability of pH, water conservation or management, changes in the organic matter content in agricultural soils, the quality and yields of vegetables with the application of compost tea both via foliar or edaphic.