3.1. General Overview of the Data
The general overview of the retrieved data is presented in Table 1
. Literature searched in the Web of Science database on conservation agriculture using the query mentioned in the methodology section yielded 3023 published documents between 1990 and 2021. In addition, 670 sources (journals, book chapters, proceedings, etc.) published documents on ‘Conservation agriculture. The topic recorded an average annual growth rate of 21.77% over the years, while each document has received 25.12 citations. The total citation count stood at 106,085 on 19th October 2022.
A total of 8692 authors across the World have written a document on this topic either as primary Authors or co-author. Each document has 4.79 co-authors on average, which indicates a relatively higher degree of collaboration. Generally, more authors per document indicate the interdisciplinary nature of the article, as authors from different subject groups often collaborate to conduct in-depth research [13
]. The claim is also supported by the international co-authorship percentage of 46.87%, which means many authors across different countries have collaborated to produce a document. This scenario is often witnessed when the topic has a global scope of research rather than regional interest [13
Among different document types, article or original research article is the prominent type contributing 77.37% of the total document count, followed by review article (8.93%) and book chapter (6.74%). To date, nine books have been written on the topic, while 149 proceedings paper is also published. To further discuss the obtained results, they were separated into the following sections: Annual scientific production, source analysis, author analysis, and document analysis.
3.2. Annual Scientific Production
Annual scientific production in CA is represented in Figure 1
. Initial years included in the search showed very slow development, and until the early 2000s, the curve started taking a spike. Although in 2003, many documents were published (n = 36), it was short-lived. From 2007, gradual development in the annual number of scientific publications was observed. The number of publications, only ten in 2006, leaped to 32 in 2007, and the curve touched the highest peak of 534 publications in 2020. However, in 2021, a sharp publication decline can be seen when 449 documents were published. The annual growth rate of 21.77 percent in the context of the number of annual scientific productions was also obtained to be considerably high. However, due to the decline of publication in 2021, a steady increase cannot be predicted unless it is confirmed that the decline in 2021 is a temporary dip due to current issues (such as COVID-19) or a permanent decline due to reduced research interest.
Although we have been analyzing the bibliometric data since 1990 because there were not many publications before the period, a quick look at the documents published before the 90s could be useful to understand the research trend. As per the Web of Science database, the first document published on this topic is “Procedures of Studying Returns from Conservation Farming”, a note published in the Journal of Farm Economics [19
]. However, the note did not receive any citations (in the WoS database) to date. From 1945 to 1990, 11 more documents were published. The term “conservation agriculture” was finalized in 1997 at a regional workshop organized by FAO on the topic of Soil Management and Conservation—Efficient Tillage Methods for Soil Conservation in Ibadan, Nigeria; before that, “conservation agriculture” was primarily focused on only no-tillage practices. This may account for the rise in publications after 2000. The FAO held its first regional workshop on CA in Harare in 1998. During this workshop, a code of practice for CA was drafted, outlining the three interconnected CA principles currently known and how they might be used in a real-world situation [20
]. The European Conservation Agriculture Federation (ECAF) and the Food and Agriculture Organization of the United Nations (FAO) hosted the First World Congress on Conservation Agriculture in Madrid, Spain, in 2001. Seven other World Congresses have since taken place: in Foz do Iguassu, Brazil (2003); Nairobi, Kenya (2005); New Delhi, India (2009); Brisbane, Australia (2011); Winnipeg, Canada (2015); Rosario, Argentina (2017), and Bern, Switzerland (2021). After each of these congresses, the number of published documents on CA increased with the only exception of 2021. In the 8th congress in Bern, Switzerland, many new steps were taken to advance CA in the future; as a result, it can be assumed that the publication decline in 2021 is temporary and will rise again.
3.3. Analysis of Source
A total of 670 unique sources (journals, conference proceedings, etc.) have been identified and have published at least one document on CA. However, the productivity (number of articles published by the source on CA) of these sources greatly differs. Bradford’s law is a well-known method to classify sources based on their contribution [21
]. Bradford’s law is used to classify all the sources into three groups; a highly productive nuclear or core zone (Zone 1), a moderately productive (Zone 2), and a less productive (Zone 3). Although the contribution of each zone is nearly equal, the number of sources varies significantly. The nuclear zone (Zone 1) constitutes a minimum number of sources yet a very high contribution.
The analysis indicates only 15 sources (all journals), constituting Zone 1 (Table 2
). It means only 2.23% of sources published 33.80% of documents in CA. The classification of sources based on Bradford’s law has several advantages; such as any researcher wants to search a minimum number of sources/journals where he/she can find the relevant highest number of documents; searching in Zone 1 sources will be the best strategy [13
]. Among the 15 most contributing sources, the Soil and tillage research journal has the maximum contribution of 190 articles, accounting for 6.285% of the total article share, followed by Field crops research (3.837%) and Agriculture ecosystem and environment (3.473% contribution) journal. Sustainability, which is 4th on our list, is a multidisciplinary open-access journal that has published a significantly higher number of articles in a shorter time span on a diverse range of topics yet primarily focusing on the sustainable production system. The majority of the sources had a high impact factor of more than 2; however, ‘The Indian Journal of Agricultural Sciences’ although having less impact factor of 0.39, had a noteworthy number of publications (57 publications), contributing about 1.886% of the total publications and ranked 7th in the list. All of these enlisted sources are peer-reviewed and reputed among the scientific community, which can be assumed from their respective impact factor indicates that documents published in those sources are cited regularly. By identifying these top source journals using bibliometric analysis, a portrait of the journal is created, demonstrating a description that goes beyond the surface and revealing the journal’s quality, maturity, and productivity in any discipline [22
]. The Soil and tillage research got the highest attention of CA researchers primarily because of the minimum tillage aspect of CA. On the other hand, the Field crop research published articles on crop management, cropping system, and yield attributes related to CA. Agriculture, ecosystem, and environment, as well as Sustainability journal primarily focus on the environmental aspect of CA, essentially a multidisciplinary scope, ranging from the ecosystem service of CA to its role in climate change, adoption practices of CA, and impact on the soil environment.
depicts the number of publications for the top five journals over time. The Soil and Tillage Journal has seen the most growth in publications. The trend also indicates that the higher preferences of the author side to publish in top journals over the years have increased.
The result of the co-citation analysis of sources is depicted in Table 3
, and the network is visualized in figure below. Soil and tillage research is the top source based on total citations, which had received a total of 11,696 local citations (cited within the documents included in our dataset). The total link strength of this journal indicates the total strength of co-citation of a given source with other sources, which is depicted as the size of the respective circles in Figure 3
. It means the higher the number of sources citing a particular source, the higher the total link strength (size of the circle). The local citation of soil and tillage research is much higher than the agriculture ecosystem and environment.
Nevertheless, the total link strength is not much different, indicating that both of these sources have been cited in an equally large number of other sources, although soil and tillage research has benefited from a higher number of articles and each article receives more citations. The VOS viewer has partitioned all the sources into three clusters depicted in three different colors of the circles in Figure 3
. Cluster 1 is depicted in red, cluster 2 in green, and cluster 3 in blue. The formation of the cluster is based on groups co-citing each other. The red cluster is a journal that publishes articles on different physical, chemical, and biological aspects of soil science. The second cluster is depicted in green; this group’s journals are largely environmental science journals and multidisciplinary journals. The third cluster is blue, primarily agronomy and crop production science journals. It is also worth noting that cluster formation follows a decreasing trend, which means the maximum number of sources are enlisted in cluster 1, then cluster 2 and cluster 3. The largest number of sources hence focuses on soil science, while fewer sources focus on purely agronomic perspective. However, that does not mean that little research is conducted keeping crops in mind; rather, those journals which include crops also include other aspects such as soil or environment and are published in journals with multidisciplinary scopes.
3.4. Analysis of Authors
Analysis of authors is a crucial section of bibliometric analysis. While the total number of authors on this topic counted as 8692, not all authors are equally productive. The productivity of authors is estimated using Lotka’s law, a classical procedure in bibliometrics that describes the frequency of publication by an author in a given field [23
]. Generally, only a few authors produce more documents and a large pool of authors with very few documents. The portrayal of Lotka’s law in Figure 4
indicates that 76.8% of the authors have produced only one document, 1.2% of the authors have produced 5 documents, 0.2% of the authors have produced 10 documents, and 0.1% of authors have produced 15 documents. There are only four authors who have written more than 50 articles. Table 4
enlists the top ten most prominent authors who had significantly contributed to the publications on conservation agriculture. Jat M. L. of CIMMYT had made 78 publications, the highest on the topic, followed by Thierfelder, C. (67); Govaerts, B. (55); Lal, R. (56); Das, T.K. (42); Verhulst, N. (38); Jat, H.S. (37); Giller, K.E. (33); Corbeels, M. (32), and Parihar, C.M. (32). It can be seen from the Table 4
that an author with a higher number of publications does not necessarily mean a highly cited author or vice versa. The h-index, defined as the highest value of h
for which the specified author or journal has published at least h
papers, each of which has been referenced at least h
times, is another crucial metric.
There is disagreement in the literature on which metric—their h-index or their number of publications or citations—is more effective in determining how relevant a certain author is to a particular field of study. Hence, we have also represented the top 10 authors based on citations (Figure 5
a) and h-index (Figure 5
b). Based on citations, Corbeels, Marc has the highest citations of 1365, followed by Giller, Ken E (1293 citations) and Tittonell, Pablo (910 citations). The ranking of authors based on the h-index does not follow the ranking of the most productive authors or the authors with the highest citations. Corbeels, mark, although he has the highest citation count, does not have many publications, which is the reason for his less h-index.
On the other hand, Jat Mangi Lal has authored many documents on CA but received moderate citation counts; the balance of both factors made him the Author with the highest h-index. The quantity of citations, rather than the quantity of publications, is more important since it gauges a researcher’s influence, according to [24
]. In a nutshell, while the number of publications indicates the Author’s productivity, the number of citations predicts the Author’s influence [25
]. However, considering the number of publications as a measure of an author’s scientific influence in a particular field can be prone to error because, in some cases, the Author is in his early career whose scientific trajectory is still in the early growth stage [26
illustrates the co-authorship network of the most prominent authors. The size of the letter and circle indicate the Author’s total link strength (degree of co-authorship). The authors with the higher co-authorship have a larger circle and letter size. The distance between the authors (circle) indicates their relatedness based on the co-occurrence link. The top 10 authors have been included in the map who have authored at least 10 documents and have at least 50 citations. The authors have been grouped under 8 distinct clusters and connected by 153 links with a total link strength of 852. The top 10 authors with the highest collaboration are Jat, M.L. (156); Govaerts, B. (106), Parihar C.M. (82), Jat S.L. (73); Jat H.S. (65), Singh, A.K. (62); Choudhary M. (61); Dutta, A. (60); Therfelder, C. (59); and Deckers, J. (58).
While the topic of CA has 8692 authors, only 50 have been included in the network, which shows a greater opportunity for a higher degree of collaboration in the future.
From the 7034 authors (Table 1
), a network containing only 60 authors shows that the field can be enlarged. The international co-authorship percentage of 46.87% indicates a good number of authors collaborating internationally, which can be improved further. The network analysis identified eight different clusters based on co-authorship. Cluster 1 has the highest number of authors, 13, followed by cluster 2 with 8 authors, cluster 3 with 7 authors, cluster 4 with 6 authors, cluster 5 with 6 authors, Clusters 6 and 7 each with 4 authors, and finally, cluster 8 with only two authors.
Cluster 1 (red color) comprises authors from India and neighboring Asian countries, and most of their documents are related to the field of Agronomy, followed by Soil science and Environmental science, respectively, as per the Web of Science subject category. The most prominent topic of this cluster revolves around conservation agriculture practices in Indo-Gangetic plains with a predominantly Rice-Wheat cropping system. The most frequent aspects being studied in general are the effect of CA on crop yield under intensive cropping systems and the soil’s physical, chemical, and biological properties, especially soil organic carbon. Although the aim of the bibliometric analysis is only the quantitative evaluation of the scientific literature based on the top number of documents, top sources, authors, country, document, etcetera; however, a short overview of documents by the authors clustered in a single group helps to understand why they have been clustered together better. The conclusions from cluster one indicate that, for resource-poor farmers of IGP, CA provides better economic return as well as resource use efficiency in the rice-wheat cropping system; the wheat shows economic benefit faster compared to rice, where it takes 4–5 years to attain the monetary advantage of CA [27
]. The wide adoption of CA could also help India to lower the crop residue burning issue [31
]. The improvement of soil physical, chemical, and biological properties after the adoption of CA is also evaluated by the authors of this cluster [32
Cluster 2 (green color) includes authors who have largely conducted their research in African nations, and documents are mostly from agronomy, multidisciplinary and agricultural policy. The major topics in this group are sustainable intensification and adoption among small-holding farmers. The documents by this cluster of authors contradict cluster 1; they argue that CA is unsuitable for resource-constrained small-holding farmers as it does not provide substantial benefits [34
]. The reason is primarily that the adoption of CA generally requires high inputs, especially fertilizer; even then, the yield stability is not guaranteed, especially under a rainfed scenario where drought or high-intensity rainfall is frequent [35
]. Other prominent challenges observed in Sub-Saharan Africa (SSA) are lack of crop rotation and residue retention [37
]. Therefore, the contrasting outcome in IGP with SSA is probably due to the adoption of CA under irrigated IGP and rainfed conditions in SSA.
The authors of cluster 3 (blue color) researched the effect of CA on soil properties regarding crop production, climate change, and carbon sequestration. This cluster includes authors from around the World but primarily from Mexico. Their research outcome concludes that CA is largely beneficial for soil health. However, farmers’ profit is questionable because most experiments are holistic up to the farm level, which needs to be extended to larger areas to gather data on a global level for better clarity of outcome [38
]. Cluster 4 (yellow color) is also based in Africa, especially Zimbabwe, has authors from other countries, and focuses on topics ranging from agronomy to environmental aspects of policy-making, although agronomy is the major subject. Among topics, these group reports individual components of CA and its advantages [41
]. Other remaining clusters have only a few authors as well as citations.
3.5. Analysis of Documents
The top 15 most cited documents are depicted in Table 5
, which had a great role in the development of the evolution of the topic over the years. The highest cited document in CA is titled “Farmers’ adoption of conservation agriculture: A review and synthesis of recent research”, which points out that despite several modest benefits of CA over conventional practices especially considering soil health, the adoption is rather limited because of financial viability and lack of knowledge and technology to the marginal farmers [44
“Soil carbon 4 per mille”, the second most cited document, has 804 citations, although it has the highest total citation per year of 134 among all the top documents. The article focuses on the ambitious global research initiative launched during the climate change conference held in Paris, 2015, which calls for an initiative to increase soil organic matter by 0.4% each year as an immediate measure to limit climate change; in which among several strategies, one such is the adoption of CA wherever possible [45
]. One of the most frequently cited documents on positive aspects of CA, especially regarding sustainable agriculture, is by Hobbs [33
], which holds the third place with 777 citations. The article provides a detailed account of the advantages of CA over conventional agriculture or even conservation tillage. Coming to challenges faced by the adoption of CA among small-holding farmers; Gillers and co-authors [34
] pointed out the decline of yield in initial years, increased requirement for labor when herbicides are not used, and shifting of gender roles; all of which need to be evaluated under diverse ecological and socioeconomic condition. Selectively adopting individual principles, especially only zero-tillage, usually yields less than conventional.
In contrast, the assured yield advantage can only be possible when all the principles are adequately applied, the feasibility of which is once again questionable, especially considering marginal farmers [46
]. Other positive aspects include intensifying the cropping system sustainably [47
], maintaining soil quality, and mitigating soil degradation [48
]. The major bottleneck of CA, although it performs best in rainfed dry areas, most farmers of those areas find it hard to adopt because of its resource-intensive nature [55
]. The top 15 documents (the majority of the articles are review article) hence provide good insight into CA from both positive and negative aspects and indisputably emphasizes conducting research in diverse agroecological and socioeconomic condition to attain a global perspective. In this regard, collaboration among researchers across the globe could be a better option, as indicated in the network analysis of the author section of this article.
3.6. Analysis of Countries
As mentioned in Table 6
, the country with the maximum number of publications is the USA (n = 658), making up a significant 21.92 percent of the total publications included in the study. The USA also holds the top position in the list regarding total citations from any region on conservation agriculture publications. India (n = 491), Australia (n = 276) and France (n = 247) ranked second, third and fourth with 16.35%, 9.19%, and 8.23% of the publications, respectively. A list of the top 20 countries based on the number of publications is a blending of developed and developing countries. The list consists of developed countries such as the USA, Australia, France, and China and, at the same time, includes developing regions such as India, Zimbabwe, Bangladesh, etc. This is a clear indication of the acceptability and implementation of the research topic, i.e., conservation agriculture in all parts of the World, and based on dynamics, it can be labeled a topic with increasing popularity and growth.
Much like a list of the top 20 countries based on several publications, similar attributes were found for the list of top 20 countries based on the number of total citations. As mentioned earlier, the USA has the highest number of total citations (n= 12771) and is almost double that of the second country on the list, i.e., India (n = 6714). United Kingdom (n = 5029) and France (n = 4833) stand third and fourth on the list, respectively. However, in the context of average article citations per year, the Netherlands is the highest, with 55.98 citations per year. Mexico ranks second with 46 average article citations per year, and the United Kingdom ranks third with 37.25 average article citations per year.
Conservation agriculture is a potential topic with a strong collaboration network among the countries (Figure 7
). Network analysis of the database for the study classifies the countries into six clusters (classified based on colors in Figure 6
). The USA is the most important node in the network and links with other important countries such as India, China, Brazil, France, Mexico, etc. Other important nodes in the network having a good number of publications and string linkage with other countries are India, Australia, Mexico, Zimbabwe, Kenya, France, etc. This network indicates strong research collaboration across the World and signifies good research potential on the topic.
3.7. Analysis of Keywords
shows the most relevant keywords related to conservation agriculture. It can be seen that conservation agriculture has the maximum (845) number of occurrences, followed by zero-tillage (302) and agriculture (187).
The presence of keywords such as climate change, carbon sequestration, and sustainable agriculture provides a clear indication of how research on conservation agriculture has a strong association with the topic of climate change and is gaining momentum in this field. Keywords such as soil, no-tillage, and soil quality reveal how the publications have tried to focus on the environmental aspects and try to implement conservation agriculture safely and sustainably. Conservation agriculture is also highly related to social issues; thus, including keywords such as sustainable intensification and food security is imperative.
The network analysis provides a clearer view of top keywords and their association (Figure 8
). The network analysis identified eight clusters. In the largest cluster (in red), CA is the most highly used core keyword and is often used with carbon sequestration, no-tillage, conventional tillage, soil organic matter, cover crop, conservation tillage, crop residue, and soil health. Among these keywords, no-tillage/conservation tillage and cover crop/crop residue cooccured because of core principles of CA. The second largest cluster represents green, climate change, adoption, adaptation, climate-smart agriculture, Sub-Sahara, or Sub-Saharan African Nations. The adoption of CA in Sub-Saharan African countries was hard; as Giller et al. (2009) [34
] pointed out in their widely cited review paper, the adoption of CA in those countries is a matter of debate, as opposed to the higher adoption claimed by many organizations; the seemingly higher adoption rate is probably due to the temporal effect of the project or due to lucrative offers of input from the government side as the majority of the farmers reverts to conventional farming practice once the project/input supply is over. However, despite all odds, more and more countries in Sub-Saharan Africa are adopting CA, sometimes in small pockets [50
]. However, increasing the adoption is necessary for the fight against climate change and reducing climate change vulnerability which can be achieved by increasing soil carbon stock [58
]. The third cluster, visualized in purple, demonstrates the relationship between sustainable and conservation agriculture. In the coming days, achieving sustainability in agriculture will be a major issue, as the land resource is continuously decreasing while the demand for food is increasing, which needs to be addressed using CA, especially in India and Sub-Saharan Africa.