Sustainability of Organic Farming from a Global Perspective: A Critical Analysis of Soil Types and Fertility Status of Organically Managed vs. Chemicalized Vegetable Fields of South India

doi:10.21203/rs.3.rs-3345053/v1

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Sustainability of Organic Farming from a Global Perspective: A Critical Analysis of Soil Types and Fertility Status of Organically Managed vs. Chemicalized Vegetable Fields of South India

https://doi.org/10.21203/rs.3.rs-3345053/v1

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Aims: Globally, organic farming is emerging as a promising alternative to chemicalized agriculture in overcoming agricultural challenges. However, experts often question its efficiency in maintaining soil fertility to sustain the global demand for agricultural production. South India, characterized by its rich agricultural heritage and diverse agroecosystems, stands as a unique test ground for analyzing the problems and prospects of sustainable organic farming. Therefore, the primary goal of the current studywas to assess the soil fertility status of organic vegetable fields concerning various agroclimatic zones, soil types, seasons, organic practices, and vegetable crops in South India as a model for global comparisons.

Method: The approach was an assessment of soil quality parameters such as soil pH, total organic carbon (TOC), soil available nitrogen (SAN), soil available phosphorus (SAP), and soil available potassium (SAK) as the basis of soil fertility status and to compare the soil quality at different levels.

Results: The organic fields didn't exhibit distinctiveness in soil parameters from that of chemicalized fields except for soil-specific variations in some fields. Soil fertility status was at par with chemicalized fields in the region. The region does not consistently monitor nutrient levels concerning soil type, climate, or seasons. Different kinds of organic practiceshad no significant influence on soil quality.

Conclusion: The study highlights the need for global standardization of procedures in organizing organic agriculture as a sustainable model of agricultural production.

Organic Farming

Total organic carbon

NPK

soil pH

Sustainable Agriculture

Soil Fertility

Soil Types

Soil is fundamental to life on the earth and key to achieving many SDGs of the UN (FAO, 2022). Since soil is the primary ecological resource essential to the sustainability of life on the earth, the productive utilization of soil resources reflects humans' progress, prosperity, and sustainability (Dror et al., 2022). The soil health status in crop fields mirrors the sustainability of fertility (Ray et al., 2022). However, fertility is declining in soil systems worldwide (EU Report, 2023), and soil degradation has become a severe global issue threatening global food security (J. Wang et al., 2023). Chemicalized agriculture remains the primary cause of the degradation of soil resources because it is destroying soil's natural ability to self-sustain its biodiversity, health, and productivity (Gamage et al., 2023). Additionally, it damages the general environment by causing pollution of air, water, and land (Alengebawy et al., 2021; Warsame et al., 2023) and harmfully affects global biodiversity.

Loss of soil health and climate crises are interrelated and driven by excessive consumption. Moreover, soil ill-health is the root cause of global environmental instability (Almond et al., 2022). Improving soil health is crucial to regain the lost global ecological stability and the ecosystem's balance. Therefore, the world looks forward to a different, affordable means of food production, quality, and safety for the future UN (FAO, 2022). Accordingly, organic agriculture is emerging as the right choice (Akter et al., 2023). It can ensure the safe production of quality food while contributing to global environmental sustainability, but questions yet prevail on its prospects as a sustainable solution to global food security (Willer et al., 2023).

However, an emergency is felt in intensifying the sustainability of agricultural systems worldwide (S. Zhang et al., 2023) to ensure global food security. The USDA has already initiatives to solve critical organic agriculture issues, problems, and priorities through the integration of research (USDA report, 2023). The major criticism of organic farming is that it is uneconomical, low productive, and unsuitable for a prosperous world (Meemken & Qaim, 2018). However, it is emerging worldwide as a viable alternative for a circular economy (Steenmans & Lesniewska, 2022). Organic agriculture is firmly rooted in the sustainability of soil health and crop productivity (George & Ray, 2022), and it ensures rural development (Kujala et al. 2022). Accordingly, organic food production systems show a steady annual growth of 14% globally. The fast growth in organic agriculture is in tune with the growing consumer needs and positive attitude (Organic Food Global Market Report, 2023) to sustainable economic prosperity over the old cowboy economics (Haas et al., 2020) that ignores circular economic principles. Naturally, many countries currently target an increase in organic agriculture, up to 50% of their total cultivation area by 2030 (Basnet et al., 2023). Naturally, research on organic cultivation in attaining sustainable soil health and productivity is highly relevant worldwide.

Although it is presumed that soils under organic farming build up soil fertility and have high nutrient scores (Bautze & Dietemann, 2022), contrary reports say that soils under organic farming are depleted of their fertility (Kolbe, 2022). Comparative studies of soil systems under organic and conventional agriculture can overcome the controversy. Such comparative studies are also essential to understanding the actual impact of organic agriculture on soil systems (Kujala et al., 2022). Moreover, soil and crop management practices always decide soil quality, productivity, and sustainability in any agricultural field (Aulakh et al., 2022). In the above contexts, continuously monitoring changes in soil fertility parameters over diverse soil systems under conventional and organic farming has become significant worldwide.

Organic farming occurs worldwide, not in just one mode. Organic practices of diverse kinds under different labels, such as organic farming, natural farming, sustainable farming, zero-budget farming, and the like, are often carried out by farmers in a single climatic zone in the same or different soil types. Therefore, comparative studies of the soil quality of differently organically and chemically managed farms of various labels in identical and different soil types have become essential to provide a definite idea of how soil fertility and productivity change over the repetitive growth of crops under specific soil management practices. Studying the soil fertility parameters of crop fields that are managed organically in different ways is crucial to knowing how such farming systems work to promote sustainable agriculture (Siddiqui, 2021). Additionally, investigating the seasonal impact of environmental factors on soil fertility (Omer et al., 2018) of such differently managed fields is also crucial for good generalizations on identifying specific factors influencing the sustainability of soil health.

The primary concern in organic farming is to keep the physicochemical qualities of soils stable and consistently productive while maintaining soil health (Schrama et al., 2018). The positive influence of organic farming on soil's physical (Williams et al., 2017; Hesse et al., 2023) and chemical properties (Wesołowska et al., 2022) are well known. The physicochemical soil quality parameters determine soil fertility, crop productivity, and soil quality status (Y. Liu et al., 2023) of all soil types. It reflects the ability of soils to produce specific crops and their yield under all circumstances. However, such data comparing physicochemical soil fertility parameters of organically managed fields over diverse soil types, different organic farming practices, and specific crops under identical climatic regions are unavailable, especially in the tropics, like India. Overall, the secrets of organic farming practices' long-term viability and productivity may be better understood by analyzing the soil's physicochemical status (Kolbe, 2022) in differently managed organic fields vs. chemicalized fields on different soil types.

In the above contexts, a systematic comparative survey of the physicochemical soil characteristics of different soils (soil series) of the broad tropical south India, spread over the three states of Kerala, Karnataka, and Tamil Nadu, is carried out. The study included fields of five different vegetable crops differently organically and chemically cultivated in two different seasons. Data is generated on the physicochemical soil fertility status of different organically managed soils under five different vegetable crops on different soil types compared to the same crops and soils under chemicalized management practices. The soil fertility parameters investigated in the present study include soil pH, total soil organic carbon (TOC), soil-available nitrogen (SAN), soil-available phosphorus (SAP), and soil-available potassium (SAK). Apart from a general understanding of soil fertility status and health, the generated data can potentially develop significant ideas on the sustainability of organic agricultural practices promoting soil health and fertility.

The main objectives of the current random survey were three-fold. The primary goal was to generate an idea of the diversity of organically cultivated soil types (soil orders and soil series per the USDA soil taxonomic system) over the south Indian states of Kerala, Karnataka, and Tamil Nadu for global information. The second goal was to identify the nutritional status of organically managed fields over the different soil types and their seasonal variations (monsoon and post-monsoon seasons) in a region. The third goal was to evaluate soil fertility parameters concerning various organically managed crop fields under diverse organic cultivation practices in south India.

Since various organic farming practices have a crucial option to guarantee food security in comparable warm tropical regions in light of the projected effects of climate change and global warming, the method of investigation and the findings remain timely and highly relevant. The significant achievement of the project is that it enabled the generation of comparative soil data on diverse organic and chemicalized fields in a tropical region, which will help develop organic agriculture as a sustainable solution in other areas and global environmental sustainability for the future.

Area of study

Chemicalized and organic agricultural fields of the three states of South India are included in the study. Chemicalized agriculture is the mainstream farming process in the region. It considers soil as a medium of plant growth. Farmers often unscrupulously apply chemical fertilizers and other agrochemicals to such fields to maximize productivity, ignoring their impact on soil health or natural biodiversity. However, organic farming is emerging as a novel mode of agriculture in the region, but only in rare fields, to avoid the toxicity and other environmental issues of chemicalized farming. Only a few farmers in South India practice organic farming because of the skeptical attitude to it as a non-productive agriculture. In the current investigation, the soil characteristics of organic farms are compared with a few representative chemicalized fields parallel to organic farms on similar soils in the area. A limitation of the current study is that although organic farms of six districts of Kerala are systematically sampled, those of Karnataka and Tamil Nadu are sampled by only one district each.

Moreover, organic farms of South India included in the present study are limited to certified farms per INDOCERT or Lacon Quality Certification Private Limited (Organic certification agencies approved by the USDA and Government of India) available in the eight districts investigated of the region. The system followed in the current systematic sampling of organic farms was based on the farm's registrations in the certified agency's register. Altogether, 218 fields are examined each season; 149 are from Kerala, 44 from Karnataka, and 25 from Tamil Nadu. (Fig. 1). The organic fields investigated were 170, whereas the chemicalized fields included parallel to organic fields for comparison were 48.

Specific vegetables studied

No data was available on the kind of organic vegetable crops in the study area of South India. Therefore, an initial survey was carried out to identify organic vegetables to be selected for the study. A general survey of the spread of organically cultivated vegetables in the study area revealed that Chilli (Capsicum annum L.) is the most widely organically produced of all the vegetables, followed by Brinjal, Okra, and Tomato. Altogether, 51 sites of Chilli, 32 places of Brinjal (Solanum melogena L.), 22 Okra, and 20 fields of Tomato could be identified for the current study. Additionally, various small-scale Cucurbitaceae and Fabaceae vegetable (10 different kinds) cultivating sites adding up to 45 were also observed, which are treated together as one group of 'other vegetables' in the present study for convenience of analysis.

Identification of soil types

At the time of collection of soil samples from fields, geographical coordinates of the sampling fields were noted using Google Earth with a mobile phone (Redmi MI 10T). Based on the geographic coordinates, the corresponding soil series per the USDA system of each sampled field was identified from official government documents (Benchmark Soils of Kerala, 2009; Soil Survey and Land Use Organization, 2013).

Soil sampling

Soil sampling was conducted in two seasons. The two seasons included in the study were summer (January to June) and monsoon (July to December) periods. Organic vegetable fields included in the present study belonged to 14 different soil series. All five groups of vegetable fields were not available in all the 14 different soil series studied. For each soil series studied, a minimum of three chemicalized vegetable fields were also included for comparison. A soil sample was always collected from the rhizosphere area of a specific vegetable crop in a field. The soil was gathered from a maximum 30-centimeter radius around the plant's base (0–30 cm depth) at three plant-growing field spots using a hand shovel and mixed thoroughly into a composite sample. Approximately 1 kg of the composite soil sample from a field was put into a clean labelled cloth bag (serially numbered) for each vegetable studied from the field and transported to the laboratory for subsequent analysis. The samples were kept open for air drying in the laboratory. In all cases, the average of the three measurements of a composite sample was taken as the representative value. The 218 composite samples of each season represented organically (170)/chemically (48) managed soils from five different vegetable fields belonging to 14 different soil series. Altogether, 436 composite soil samples were collected in two seasons over four years (August 2018 to December 2022).

Soil chemical analysis: The investigated soil fertility parameters comprised soil pH, total organic carbon (TOC), soil-available nitrogen (SAN), soil-available phosphorus (SAP), and soil-available potassium (SAK). The pH of the soil was determined in the field after mixing with distilled water in a 1:2.5 ratio and measuring it with a pH meter (Hanna pH meter HI 5522). All the other soil analyses were carried out in the laboratory using air-dried samples.

The soil-available nitrogen (SAN) was quantified using the micro-diffusion method (Sparks et al., 1996). The present study employed spectrophotometric analysis (Thermo Fisher, Varioskan Flash multimode reader) to estimate soil-available phosphorus (SAP). The estimation of soil available potassium (SAK) was carried out using the Flame Photometer (Systronics Flame photometer 128, Systronics India Ltd, Ahmadabad, Gujarat) after acid digestion of soil samples, following the methodology outlined by Jackson (1973). Organic carbon was estimated using the Walkley and Black wet digestion method (1934).

Statistics

Soil quality parameters of organic and chemicalized fields are statistically compared. In addition, soil quality parameters of different organically managed fields are compared between seasons (summer and monsoon), specific organic management systems (four different management practices), soil types (14 soil series), and vegetable crops (five different crops) to check whether such factors have any specific influences on soil fertility concerning specific soil quality parameters in the organic fields. An independent sample t-test enabled analysis of the variations in soil parameters within the groups included in the study (organic farming types, vegetable crops, soil series). The difference in soil parameters between various groups was analyzed using One-way ANOVA. The statistical analyses were conducted utilizing IBM SPSS software version 27 (SPSS Inc. Chicago, IL).

Diversity of soil types (Soil Series) under Organic Farms in South India

Organic farming of vegetables was found in a wide variety of 14 soil types (soil series) among the eight districts of South India examined in the current study. Among the 14 different soil series observed, one each, the Bevinahalli soil series belonged to Shimoga district, Karnataka state (38 organic and six chemicalized fields), and Periyanaikenpalayam soil series belonged to Coimbatore district, Tamil Nadu state (22 organic and three chemicalized fields). The rest 12 different soil series belonged to six districts of Kerala such as Kottayam (three soil series; 12 organic and nine chemicalized fields), Alappuzha (one soil series; 12 organic and three chemicalized fields, Idukki (one soil series; 11 organic and three chemicalized fields), Thrissur (two soil series; 18 organic and six chemicalized fields), Palghat ( two soil series; 19 organic and six chemicalized fields) and Wayanad (two soil series; 39 organic and nine chemicalized fields) districts of Kerala state. The details of the samples, soil series, districts, and state of the collection are depicted in Fig. 2.

Organic agricultural types in South India: The diverse types of organic farm practices identified in the current investigation in South India included four different kinds: organic farming (OF), zero-budget natural farming (ZBNF), natural farming (NF), and sustainable farming (SF). In general, 69.41% of fields belonged to OF, 13.52% to ZBNF, 12.94% to NF, and 4.1% to SF. Although the classification of organic farming into four different categories is followed in South India, the present study observed that many farming procedures have overlapping components in such classes. Therefore, a brief description of the different organic practices per the description of such farmers is described below:

(1)OF is traditional farming using organic fertilizers such as compost, animal dung or poultry litter or manure, green manure, and oil cakes while avoiding all kinds of chemical fertilizers and other agrochemicals in farming.

(2)ZBNF in South India is a specific organic practice popularized by an Indian organic agriculturist, Subhash Palekar. It is a kind of biodynamic farming, utilizing distinct microbial cultures known as Jeevamrut and Bijamrut, used in India for centuries as soil fertility enhancers.

(3)NF is a traditional agricultural practice focusing on minimized artificial additions to soil and utilizing the natural potential of soils in agricultural production following the ecological theory; it is a 'do-nothing farming,' a farming avoiding even tillage.

(4)SF is a modified modern agriculture that focuses on applying organic fertilizers while minimizing the application of all kinds of toxic agrochemicals, including chemical fertilizers in farming, to conserve the soil and vegetational biodiversity and improve soil health.

Soil quality parameters of diverse fields: All the details of average specific soil quality parameters of the studied organic and chemicalized fields are shown season-wise in supplementary tables; S-1 shows details of 170 samples per season for organic fields, and S-2 shows details of 48 per season for chemicalized fields. Details of the soil parameters per specific categories are also shown in supplementary tables: specific-organic-management-wise:- S-3 (23 Palekar's method-fields), S-4 (118 OF fields,) S-5 ( 22 NF fields) and S-6 (7 SF fields); per vegetable-crop-wise:- S-7 (51 organic fields of Chilli), S-8 (32 organic fields of Brinjal), S-9 (22 organic fields of Okra), S-10 (20 organic fields of Tomato) and S-11 (45 organic fields of other vegetables clubbed together); per soil-series-wise:- S-12 (organic fields), and S-13 (chemicalized fields).

Descriptive statistics of all the studied soil parameters per entire organic and chemicalized fields (Table 1), different kinds of organic practices (Table 2), different kinds of vegetable fields (Table 3), and various soil orders/series (Table 4) are shown below.

Table 1

Descriptive Statistics of Soil quality parameter for total samples in general
Organic farms
Parameter	Minimum	Maximum	Range	Mean	SD	CV
pH	5.43	8.88	3.45	6.97	.57	.330
C (%)	.39	4.6	4.21	1.9	.74	5.622
N (kg ha^− 1)	37.70	732.67	694.97	184.61	134.47	18085.85
P (kg ha^− 1)	2.13	193.33	191.2	53.33	43.07	1856.39
K (kg ha^− 1)	200	2600	2400	856.05	419.31	176025.15
Chemicalized farms
pH	5.48	8.88	3.4	7.05	.645	.418
C (%)	.57	3.48	2.91	2.001	.818	3.306
N (kg ha^− 1)	56	900.67	844.67	279.50	216.08	46698.77
P (kg ha^− 1)	3.10	173.33	170.23	67.54	54.11	2929.89
K (kg ha^− 1)	320	1900	1580	924.16	428.51	184347.6

Table 2

Descriptive Statistics of soil quality parameters for different organic management systems
Organic Farming
Parameter	Minimum	Maximum	Range	Mean	SD	CV
pH	5.48	8.88	3.4	6.99	.587	.345
C (%)	.45	4.6	4.15	1.95	.764	.583
N (kg ha^− 1)	37.7	672	634.3	186.74	132.71	17613.27
P (kg ha^− 1)	3.40	193.33	189.93	55.03	41.94	1759.38
K (kg ha^− 1)	200	2600	2400	815.52	377.35	142381.74
ZBNF
pH	5.91	7.8	1.89	7.065	.36	.12
C (%)	.46	4.68	4.22	2	.658	.432
N (kg ha^− 1)	60.67	499.33	438.66	151.09	88.69	7865.91
P (kg ha^− 1)	3.40	193.33	184.93	61.54	56.29	3168.56
K (kg ha^− 1)	440	2600	2160	1055.21	557.15	310416.12
Natural Farming
pH	5.78	8.19	2.41	6.93	.705	.497
C (%)	.39	4.19	3.8	1.66	.745	.55
N (kg ha^− 1)	51.33	714	662.67	190.58	149.17	22251.68
P (kg ha^− 1)	2.13	96	93.87	38.55	24.47	598.78
K (kg ha^− 1)	300	2500	2200	863.63	448.20	200887.72
Sustainable Farming
pH	6.63	7.89	1.26	7.06	.45	.20
C (%)	.87	3.28	2.41	1.69	.49	.24
N (kg ha^− 1)	70	732.67	662.67	239.99	222.55	49528.50
P (kg ha^− 1)	11.67	166.67	155	44.26	65.02	2918.16
K (kg ha^− 1)	380	1633.33	860.94	374.98	141.72	20085.97

Table 3

Descriptive Statistics of soil quality parameters for different vegetables
Chilli
Parameter	Minimum	Maximum	Range	Mean	SD	CV
pH	5.78	8.43	2.65	6.94	.054	.0029
C (%)	.39	4.68	4.29	1.94	.826	.682
N (kg ha^− 1)	46.67	732.67	686	202.22	157.44	24787.35
P (kg ha^− 1)	2.13	193.33	191.2	52.20	44.63	1991.83
K (kg ha^− 1)	200	2100	1900	859.84	396.77	157426.43
Brinjal
pH	5.91	8.88	2.97	6.99	.57	.324
C (%)	.57	4.53	3.96	1.93	.79	.62
N (kg ha^− 1)	51.30	672	620.7	190.63	139.30	19404.49
P (kg ha^− 1)	3.50	166.67	163.17	57.73	44.10	1944.10
K (kg ha^− 1)	250	2600	2350	928.32	432.40	186969.76
Okra
pH	5.72	8.11	2.97	6.97	.56	.313
C (%)	.39	4.19	3.8	1.76	.79	.6241
N (kg ha^− 1)	46.67	574	527.33	174.09	123.43	15234.96
P (kg ha^− 1)	4.10	166.67	162.07	46.35	35.92	1290.24
K (kg ha^− 1)	200	2500	2300	821.66	454.02	206134.16
Tomato
pH	5.84	7.95	2.11	7.03	.50	.25
C (%)	1.10	3.63	2.53	1.82	.56	.31
N (kg ha^− 1)	37.70	574	536.3	181.64	129.01	16643.58
P (kg ha^− 1)	2.80	193.33	190.53	65.15	53.52	2864.39
K (kg ha^− 1)	300	1620	1320	817.70	338.98	114907.44
Other vegetables
pH	5.43	8.47	3.04	7.06	.64	.40
C (%)	.39	4.68	4.29	1.96	.66	.43
N (kg ha^− 1)	56	672	616	166.84	106.77	11399.83
P (kg ha^− 1)	3.50	166.67	163.17	41.76	57.55	3312
K (kg ha^− 1)	260	2600	2340	834.22	457.84	209617.46

Table 4

Descriptive Statistics of soil quality parameters for different soil series
Kottapuram series
Parameter	Minimum	Maximum	Range	Mean	SD	CV
pH	5.78	6.91	1.13	6.319	.460	.211
C (%)	1.33	4.19	2.86	2.1	.94	.88
N (kg ha^− 1)	116.67	494.7	378.03	252	110.15	12133.02
P (kg ha^− 1)	.39	4.68	4.29	9.94	12.90	166.41
K (kg ha^− 1)	360	1100	740	644	260.56	67891.5
Paippara series
pH	6.12	6.86	.74	6.553	.22	.048
C (%)	.96	4.53	3.57	2.33	1.23	1.51
N (kg ha^− 1)	170.67	714	543.33	375.83	195.93	38388.56
P (kg ha^− 1)	7.6	45.30	37.7	21.8	16.1	259.21
K (kg ha^− 1)	400	2500	2100	936.66	714.04	509853.1
Thidanadu series
pH	6.76	6.91	.15	6.83	.08	.0064
C (%)	.46	2.15	1.69	1.29	.72	.51
N (kg ha^− 1)	140	294	154	207.67	79.61	6337.7
P (kg ha^− 1)	3.80	24.70	20.9	13.7	10.9	118.8
K (kg ha^− 1)	986.67	1440	453.33	1216.66	196.29	38529.7
Alappuzha Series
pH	7.06	7.80	.64	7.33	.2	.04
C (%)	.92	3.21	2.29	2.19	.73	.53
N (kg ha^− 1)	37.7	163	125.3	105.03	41.44	1717.2
P (kg ha^− 1)	3.40	4.8	1.4	3.8	.38	14.44
K (kg ha^− 1)	640	2100	1460	993.03	393.94	155188.7
Pampadumpara series
pH	5.43	5.72	.29	6.03	.31	.06
C (%)	1.41	2.28	.87	1.85	.24	.05
N (kg ha^− 1)	93.33	527	433.67	244.3	130.05	16913.00
P (kg ha^− 1)	6.80	32.20	25.4	14.33	7.8	60.8
K (kg ha^− 1)	200	700	500	450.45	152.36	23213.5
Mannur series
pH	6.14	7.2	1.06	6.76	.23	.05
C (%)	.91	3.98	3.07	2.41	1	1
N (kg ha^− 1)	74.67	732.67	660	173.5	162.11	26279.65
P (kg ha^− 1)	40.66	193.33	152.67	96.72	44.71	1998.98
K (kg ha^− 1)	280	1633.33	1353	954.81	305.58	93379.1
Thirunavayapuram series
pH	6.7	7.89	1.19	7.22	.43	.18
C (%)	.87	2.08	1.21	1.54	.42	.17
N (kg ha^− 1)	70	289.33	219.33	167.53	79.54	6326.6
P (kg ha^− 1)	11.67	166.67	155	43.5	62.3	3881.29
K (kg ha^− 1)	380	900	520	654	158.61	25157.13
Koratti series
pH	6.3	8.88	2.58	7.89	.73	.53
C (%)	1.59	3.63	2.04	2.72	.71	.50
N (kg ha^− 1)	98	266	168	163.12	49.25	2425.5
P (kg ha^− 1)	45	116.67	71.67	85.39	20.68	427.6
K (kg ha^− 1)	380	1400	1020	915.75	266.87	71219.5
Manathala series
pH	6.1	7.92	1.82	6.97	.57	.32
C (%)	1.98	3.390	1.41	2.72	.44	.19
N (kg ha^− 1)	102.67	256.67	154	151.66	55.05	3030.5
P (kg ha^− 1)	27.67	45	17.33	35.73	5.75	33.06
K (kg ha^− 1)	420	1200	780	768	290.92	84634.4
Kozhykully series
pH	6.5	6.97	.47	6.74	.22	.04
C (%)	1.70	1.96	.26	1.81	.1	.01
N (kg ha^− 1)	149.33	298.67	249.34	224	71.89	5168.1
P (kg ha^− 1)	44	46.67	2.67	45.41	1.16	1.34
K (kg ha^− 1)	680	900	220	785	91.46	8364.9
Pulpally series
pH	5.91	8.19	2.28	7.26	.46	.21
C (%)	.39	4.68	4.29	1.79	.62	.38
N (kg ha^− 1)	46.67	672	625.33	159.61	142.87	20411.8
P (kg ha^− 1)	11	193.33	182.33	62.54	44.40	1971.3
K (kg ha^− 1)	440	2600	2160	1002.91	472.82	223558.7
Battuvady series
pH	6.38	8.19	1.81	7.4	.65	.42
C (%)	.39	1.99	1.6	1.32	.44	.19
N (kg ha^− 1)	56	154	98	106.33	28.13	791.2
P (kg ha^− 1)	36	96	60	62.54	19.81	392.4
K (kg ha^− 1)	300	1620	1320	814.76	456.58	208465.2
Bevinahalli series
pH	5.99	7.80	1.81	6.95	.34	.11
C (%)	.39	4.68	4.29	1.66	.61	.37
N (kg ha^− 1)	46.67	672	625.33	149.76	131.77	17363.3
P (kg ha^− 1)	11	193.33	182.33	65.07	44.08	1960.6
K (kg ha^− 1)	300	2600	2300	978.59	459.62	211250.5
Periyanaikenpalayam series
pH	6.20	7.89	1.69	6.86	.34	.11
C (%)	.88	2.69	1.81	1.72	.48	.23
N (kg ha^− 1)	163.33	574	410.67	280.77	107.77	11614.3
P (kg ha^− 1)	15	143.33	128.33	44.92	24.92	621.00
K (kg ha^− 1)	300	880	580	555	151.21	22864.46

Soil pH

The soil pH of the currently investigated organic fields in south India generally varied from 5.53 to 8.88 in the monsoon and 5.43 to 8.50 in the summer. In contrast, the soil pH of chemicalized fields varied from 5.50 to 8.88 in the monsoon and 5.48 to 8.50 in the summer (Table 1). ANOVA reveals that the difference in pH of all the surveyed fields (organic and chemicalized) was insignificant in both seasons (P > 0.05). Moreover, the difference in pH between the organic and chemicalized fields was insignificant in both seasons (P > 0.05). However, when fields are clubbed together into different categories, the difference in soil pH over the different organic management systems (Table 2) and fields of varying vegetable types (Table 3) was insignificant in both seasons (P > 0.05). However, in both seasons, the difference in pH between soil series (Table 4) was significant (P < 0.05) among organic and chemicalized fields. The paired t-test shows that among the organic vegetable farms, the variations over nine soil types, such as Paippara, Alappuzha, Pampadumpara Mannur, Koratti, Manathala, Kozhikully, Pulpally, and Bevinahalli, contributed to the significant difference between soil series (P < 0.05). There were no seasonal variations in the soil pH of the fields among organic or chemicalized fields and all the other groups included in the current study.

Total organic carbon (TOC)

The TOC of organic fields ranged from 0.39–4.6% in the monsoon and 0.39–3.9% in the summer, whereas that of chemicalized fields varied from 0.57–3.4% in the monsoon and 0.88–3.3% in the summer (Table 1). In general, ANOVA shows that the variations in TOC of all the fields, irrespective of categorizations, are insignificant in both seasons (P > 0.05) among fields of chemicalized and organic management practices. Moreover, a seasonal difference in TOC was insignificant in such fields. Similarly, TOC between fields of organic and chemicalized practices and between fields of diverse organic practices (Table 2) or different vegetable types (Table 3) was also insignificant (P > 0.05). However, the TOC over soil series (Table 4) differed significantly. Still, paired t-test revealed that the difference was only among seven-soil series such as Alappuzha, Pampadumpara, Mannur, Thirunavayapuram, Koratti, Manathala and Kozhykully (P < 0.05). It may be noted that most of the above soil series, except Mannur, were significantly different in soil pH. The seasonal difference in the TOC of different soil series was insignificant.

Soil Available Nitrogen (SAN)

The amount of SAN among fields showed wide variations in both organic and chemicalized farms in South India. In the organic farms, SAN varied from 42.00 kg ha^− 1 to 704.70 kg ha^− 1 in the monsoon and 37.70 kg ha^− 1 to 732.67 kg ha^− 1 in the summer. In the chemicalized farms, SAN of fields showed variations from 70.00 kg ha^− 1 to 868.00 kg ha^− 1 in the monsoon and 56.0 kg ha^− 1 to 900.67 kg ha^− 1 in the summer. ANOVA shows that the differences in SAN of fields, in general, were significant (P < 0.05) among organic and chemicalized practices in both seasons. However, the SAN of organic fields was significantly lower than chemicalized fields in both seasons (P < 0.05). However, when fields are clubbed into various other groups, the fluctuations in SAN over the various organic management systems and different types of vegetable fields were not significant in both seasons (P > 0.05). The SAN over the different soil series was statistically significant in chemicalized and organic practicing farms. However, a paired t-test showed that the difference in SAN of ten soil series such as Kottapuram, Alappuzha, Mannur, Thirunavayapuram, Koratti, Manathala, Pulpally, Battuvady, Bevinahalli, and Periyanaikenpalayam soil series (P < 0.05) contributed to the difference. Significant seasonal differences in SAN were visible in the above nine series.

The Mannur series generally shows the highest fluctuation in SAN in both the monsoon and summer seasons. Among the tested organic fields, a majority (84.7% of fields) may be categorized as having low (280 > kg ha^− 1) SAN levels, and 11.76% of fields may be classified as fields of medium (280–560 kg ha^− 1) SAN levels. In contrast, only 3.52% of fields show high (560 < kg ha^− 1) SAN levels. However, in the chemicalized fields, only 75% may be categorized as low SAN levels, 12.5% as medium SAN levels, and 12.5% as high SAN levels. However, seasonal differences in SAN were visible in both kinds of fields.

Soil-available phosphorous (SAP)

The range of SAP observed in organic fields was 2.13 mg kg^− 1 (summer) to 193.33mg kg^− 1 (monsoon), whereas the content in the chemicalized area was 3.10 mg kg^− 1 (summer) to 173.33 (monsoon) mg kg^− 1. ANOVA shows that the differences in SAP of fields, in general, were insignificant (P > 0.05) among organic and chemicalized practices in both seasons. ANOVA revealed that seasonal SAP variation was negligible in organic and chemicalized fields (P > 0.05). However, the SAP between organic and chemicalized fields was insignificant in both seasons. Similarly, fluctuation in SAP was insignificant (P > 0.05) between different organic farming types and farms of varying vegetable types in both seasons. ANOVA shows that the variations in SAP of different soil types were significant in both seasons. However, the paired t-test shows that SAP of varying soil types significantly differed among six soil series only, such as Paipara, Pampadumpara, Mannur, Koratti, Pulpally, and Bevinahalli (P < 0.05). Among the different soil types, the Pulpally series shows the highest fluctuation (78.66 ± 53.6053.42 mg kg^− 1) in the monsoon season, whereas, in the summer season, Thirunavayapuram series shows the highest change (34.54 ± 53.42 mg kg^− 1). In both seasons, 71.7% of organic fields consistently displayed high (> 25 kg ha^− 1) levels of SAP, 15.88% of fields showed medium range (10–25 kg ha^− 1), and 12.35% of fields showed low (< 10 kg ha^− 1) SAP content. However, among chemicalized farms, 75% of farms exhibited high, 6.25% indicated medium range, and 18.75% showed low SAP content levels in the fields in both seasons.

Soil-available potassium (SAK)

The SAK values in organic fields ranged from 200kg ha^− 1 (summer) and 2600 kg ha^− 1 (summer)^, and that of chemicalized farms ranged from 320.0 kg ha^− 1 (monsoon) to 1900 kg ha^− 1 (summer). ANOVA shows variations in SAK of fields in general were insignificant (P > 0.05) in organic and chemicalized fields in both summer and monsoon seasons. Similarly, the variations in SAK over different kinds of organic management fields and different kinds of vegetable crops were also insignificant in both seasons (P > 0.05). However, SAK of varying soil types was significant in both seasons. But paired t-test shows that the difference in SAK between soil types was significant in twelve soil types such as Paippara, Alappuzha, Pampadumpara, Mannur, Thirunavayapuram, Koratti, Manathala, Kozhykully, Pulpally, Battuvady, Bevinahalli and Periyanaikenpalayam (P < 0.05). Among the soil types, the Paipara series shows the highest fluctuation (763.33 ± 514.52 kg ha^− 1) in monsoon and summer seasons (902 ± 651.01kg ha^− 1). It may be noted that 98.82% of organic fields showed a high SAK (> 280 kg ha^− 1) in both seasons and the rest, 1.18%, showed medium (120–280 kg ha^− 1) SAK. However, all the chemicalized farms showed high SAK content in the fields.

To the best of our knowledge, the current study represents the first detailed investigations on soil fertility parameters of organic fields concerning the diversity of soil types, different kinds of organic farming practices, and vegetable crops in south India. The systematic survey of organic fields in south India revealed that organic farming has not yet become a standardized agriculture program in the region. Most organic fields are pretty recent in the area; farming is carried out comparatively at small scales, usually in plots among chemicalized fields, and farmers are not very strict in specific organic practices. For most farmers, even when different names carry out farming, organic farming is just a kind of farming using organic fertilizers. The impact of chemicalized practices in the nearby fields can also influence organic fields. Accordingly, except for specific differences in soil quality parameters over organic and chemicalized farms in the area, the soil quality parameters of such fields are not yet distinctive in the significant soil fertility parameters. The fields cultivated under organic practices do not show significant differences in most soil quality parameters, especially in NPK. Although diverse vegetable fields are under different organic practices and soil types, no significant differences in soil parameters were observed over various such vegetable fields. The positive effects of long-term organic manure applications on paddy field soils' overall soil fertility status over that of chemicalized fields are known (Yu, 2023). However, such information is not available on tropical Indian soils. The current critical analysis of the comparative results of specific soil quality parameters reveals certain valuable observations discussed below.

Soil pH over organic and chemicalized fields

The soil's pH is one of the principal determinants of soil quality that profoundly influences many soil biogeochemical processes (Neina, 2019) and is crucial to plant growth and biomass productivity (Ejigu et al., 2023). Perhaps the current study is the first comparative analysis of pH variations over organic and chemicalized fields over the broad South Indian tropical climatic zone. Although the majority of fields (about 70%) showed a pH of medium soil quality range of 6.5 to 7.5 (Todorova & Zhiyanski, 2023), statistical analysis reveals that the variation in pH over seven specific soil types in the region is significant and varies widely from 5.43 (Pampadumpara soil series) to 8.88 (Koratty soil series). Naturally, soil series and orders may be distinct in pH (Bonfante & Bouma, 2015). However, in general, the range of pH over vegetable fields currently observed in the study falls within the recommendation for vegetable cultivation (TNAU Guide, 2020; Tashi et al., 2023). The absence of variations in soil pH over different vegetable fields under diverse organic practices reveals that such variations in practices have no influence on soil pH in the South Indian tropical region.

A significant observation in the present study was that the variations in soil pH over organic and chemicalized fields were insignificant except for seven distinct soil series. However, certain studies previously observed significant differences in soil pH between organic and conventional farms (Maucieri et al., 2022). Chemicalized practices are expected to cause soil acidification and low pH values (Pahalvi et al., 2021; Dikir, 2023). Moreover, soil pH decreases with the intensity of soil management (Feeney et al., 2023), and usually, soils are most intensely manipulated in chemicalized practices. However, no such influences on soil pH were observed over the currently investigated chemicalized and organic fields of South India. It may be because organic fields in the regions have recently converted to chemicalized fields or because of the climate and soil kinds in the area.

Moreover, significant seasonal fluctuations in soil quality parameters are not observed in the organic fields of south India (p > 0.05), unlike such significant seasonal changes observed in some previously examined specific soil types (Semy et al., 2023) elsewhere. It may be because vegetables are often cultivated in South India in the rainy season. Since rain overlaps over monsoon and post-monsoon seasons, the seasonal categorization of samples might remain insignificant in the present study.

Adverse pH affects the soil biota negatively (Bisht & Singh Chauhan, 2021), and organic farming causes soil to stabilize soil physicochemical properties (Gopinath et al., 2023; Kaur & Singh, 2023)) and improvement in soil biota (Babu et al., 2023). Since an increase in organic content may cause a decrease in soil pH (Sun et al., 2023), regular monitoring of soil pH has become imperative in organic farms within specific soil types in the region. However, monitoring soil pH over regular intervals (Müller et al., 2022) as a usual agricultural practice is absent in organic fields in South India. Overall, any negative influence of conventional chemicalized farming on soil pH (Duddigan et al., 2023) or any advantage or disadvantage of organic farming on soil pH could be established in the current search of soil pH over organic and chemicalized fields of the broad tropical climatic zone of south India.

TOC over organic and chemicalized fields

Just like soil pH, the influence of organic farming on the TOC of the organic and chemicalized vegetable fields of south India is not very distinctive. In general, organic farming is considered a means to improve carbon sequestration in soil (EU Report, 2023; Holka et al., 2022; Shade et al., 2021) because of an increased addition of organic manure into such fields and a consequent improvement in soil biotic activities (Babu et al., 2023). Increasing soil microbial activity without agrochemicals enhances the conservation of carbon sources in soils, including carbohydrates and amino acids (Liu et al., 2023). Therefore, substituting chemicalized to organic management is often considered a good option for increasing the carbon sequestration ratio (Hayatu et al., 2022; Jat et al., 2022) in soils. Moreover, many studies show that changing fertilizer inputs from chemical to organic manure has increased the quantity and quality of carbon in soils (Oliveira et al., 2023; Pacheco et al., 2023; Zhang et al., 2022). However, the absence of significant differences in TOC over chemicalized and organic fields in any of the seasons in the present study may not be considered a counter to the known truth because most organic fields in the area are recently converted farms. Such fields might not have received sufficient time for a significant difference in soil quality parameters.

Usually, a positive influence of organic farming on TOC occurs over a long period of organic practices in the fields. Moreover, most of the organic and chemicalized fields included in the current study were nearby plots in the same villages, and the transition to organic means is relatively recent. Moreover, in Indian farms, farmers usually apply organic fertilizers in both chemicalized and organic farms and may add chemical fertilizers such as K₂O into organic fields. However, they are often strict in avoiding toxic agrochemicals in organic fields. The impact of market fluctuation on organic practices observed elsewhere (Brady et al., 2023) applies to Indian fields as well. Overall, a positive influence of organic farming on TOC is not observable in the south Indian fields.

SAN over organic and chemicalized fields

Nitrogen is a major plant nutrient and constitutes approximately 56% of the total fertilizer requirement in agriculture (FAO, 2022) worldwide. Since the production and consumption of synthetic N fertilizer in agriculture contributes to 10.6% of total agriculture emission of greenhouse gases (Menegat et al., 2022), the conservation of N in agricultural fields is significant to the development of sustainable agriculture and control of global warming and the consequent climate change. Therefore, monitoring the N regime in soils over different agricultural fields in diverse soil types, cultivated of other crops, and managed by organic cultivation practices from different countries is significant to sustainable agriculture globally.

Measuring the field N content is the most critical process (Liu et al., 2023). Along with field monitoring, the success in control and conservation of N in agriculture depends on employing diverse means of N conservation methods in agriculture, such as improving the N accumulation efficiency of crops through manipulation of N-responsive genes or gene editing of crops for improving nitrogen use efficiency, precise methods of N application in fields, and practicing of organic agriculture. In organic agriculture, N conservation is achieved through adopting conservation tillage practices, retaining crop residues, and adding green biomass through the cultivation of such crops to reduce the consumption of synthetic N fertilizers. N conservation approaches aim to maximize the utilization of nitrogen resources while minimizing losses and avoiding environmental impacts (Govindasamy et al., 2023). However, none of such procedures can be fruitful without adequately assessing and monitoring diverse agricultural fields. According to (Zhu et al., 2023), nitrogen use efficiency (NUE) is the lowest in cases where crops are solely fertilized with inorganic nitrogen fertilizers. As alternatives to chemicalized farming, diverse organic agricultural practices have become significant in controlling global climate change.

In the above contexts, the current assessment of N regimes of organic and chemicalized vegetable fields is perhaps the first attempt to compare nitrogen regimes over such differently managed organic fields compared to chemicalized fields in south India. Generally, vegetables form one of the crops that contribute to significant emissions of greenhouse gases (Chataut et al., 2023)d content up to a critical level of 420 kg N ha^− 1 in the fields is considered safe to prevent excessive GHG emissions from crop fields (Wang et al., 2023). However, in the current investigation, although most organic and chemicalized fields had N content below the critical level, some of the fields in both cases had N content above the required level. Organic farming focuses explicitly on optimizing nitrogen availability during distinct crop growth stages while concurrently minimizing detrimental environmental impacts (Valenzuela, 2023), and synthetic N fertilizers are avoided altogether.

The nitrogen content of fields above 180–190 kg ha^− 1 usually contributes to excessive N₂O emission from crop fields (van Groenigen et al., 2010). However, the current study reveals that about 38% of organic fields in the region have N content above 190 kg ha^− 1, whereas 56.25% of chemicalized fields show N content above that critical level. However, the chemicalized fields show a significantly high percentage of global warming potential; the data indicate that some of the organic fields are similar to the chemicalized fields in N content, especially the 3.52% fields showing N > 560 kg ha^− 1 because the natural means to control freely available N in the field soil (Yao et al., 2023) per organic agricultural norms are not found in such fields. Another reason for an excessive N in organic fields is the application of poultry manure rich in N content (Zhang et al., 2023) by farmers in organic areas without a limit. Whether organic or chemicalized farming, excessive use of N sources in agriculture significantly impacts global environmental integrity and human health (Pei et al., 2023). A significant portion of the unutilized synthetic nitrogen (N) in chemicalized agricultural fields reaches natural environments that degrade the quality of soil, water, and air, disrupting various ecosystem functions (Alam et al., 2023). Applying particular organic wastes, such as poultry manure rich in NPK, also has environmental concerns, such as air, soil, and water (Rahman et al., 2022). Overall, the organic farming practices in the broad tropical south India need close monitoring to get standardized, at least in N consumption, as evidenced by a random comparative assessment of SAN in the current study.

SAP over organic and chemicalized fields

Phosphorus is an essential nutrient for plant growth and is a critical component of chemical and organic fertilizers used in agriculture. In the current study, over 70% of organic and chemicalized fields in south India are showing a high SAP (> 25kg ha^− 1) per agricultural norms. However, compared to the P content of chemicalized fields elsewhere in the world (J. Chen et al., 2022), the P content of both organic and chemicalized fields in South India is not that high. Moreover, an excessive amount of SAP in soils above 100 kg ha-1 is often considered an increased P transformation efficiency, which depends on climate, soil factors, and agronomic practices (Chen et al., 2023). In general, the excessive usage and mismanagement of phosphorus can lead to adverse environmental consequences. Excessive P usage threatens soil health and aquatic ecosystems despite its crucial role in plant growth and food security (Panagos et al., 2022). In general, it is believed that the excessive use of P in chemicalized farms leads to a high amount of P in soil solution (Meyer et al., 2023), which can be prevented using organic production (Dikir, 2023) because the latter focuses on the biological recycling of minerals in soils by improving the microbial means to enhance nutrient uptake (George & Ray, 2023) by plants. However, the current study reveals that organic and chemicalized fields show high P in South Indian soils. Excessive P in soils can be due to the uncontrolled use of organic fertilizers such as poultry manure (Waldrip et al., 2011; Zhang et al., 2023) or cattle manure and bone meal (Ullah et al., 2023) into fields, thinking that the addition of organic fertilizers into fields is safe, especially in the absence of any controlling norms of limits in this regard. A short-term incubation study in typical acidic red soil has discovered that cattle manure, a mainstay addition in organic farms, tends to act as a superior phosphorous source to other additives (Wang et al., 2023). To avoid excessive amounts of soil-available P occurring in the fields, which may leach into waters, biochar-blended organic fertilizers are now used and are an effective agronomic way to improve sustainable phosphorous availability (Jin et al., 2022) throughout the growth cycle of the crops. In general, it is believed that long-term utilization of phosphorus resources for agricultural sustainability, limiting the utilization of mineral P fertilizers and biological means to enhance the utilization of residual P in the field by plants is highly significant (Withers et al., 2014; Solangi et al., 2023). However, certain studies show that soils of organic production systems sometimes have high phosphorous content (Gopinath et al., 2023), which can be an impact of the increased addition of P-rich organic fertilizers into fields. The current study also endorses the same.

SAK over organic and chemicalized fields

The amount of SAK in soils is crucial to plant growth and productivity. Insufficient application of K into soils and lack of proper monitoring of K in agricultural soils have been causing detrimental effects on both soil fertility and long-term crop productivity in the Indian fields (Das et al., 2022). The high price of chemical fertilizers and the low K use efficiency in tropical soils remain significant hurdles to the optimum supply of K in such fields (Basak et al., 2023). However, the current study reveals that more than 98% of organic and 100% of chemicalized fields in South India show an agriculturally high amount of SAK (> 280 kg ha^− 1) in soils. Farmers generally use different kinds of organic fertilizers and green manures to enhance K in organic fields (Gabhane et al., 2020) against mineral fertilizer applications in chemicalized fields. According to some reports, although there have been many organic resources to improve K in soils recently, the required level of SAK is not observed in soils of many areas worldwide (Song et al., 2019). However, proper monitoring of soil nutrient regimes significantly reduces the repeated application of costly fertilizer applications into soils (Schnitkey et al., 2023). The current study reveals that in the amount of SAK, the organic soils of South India are at par with the chemicalized fields. The positive impact of repeated use of organic manure on SAK in soils is well known (Reimer et al., 2023). However, the exact source of high SAK in organic and chemicalized soils in South India needs further investigation.

In general, chemicalized agricultural production is considered one of the major causes of environmental degradation and biodiversity erosion worldwide. Moreover, adopting organic means of cultivation is considered inadequate to maintain sufficient nutrient regimes in soils for acquiring the required food security in a food-deficient world, especially for a growing population in the future. On the contrary, organic agricultural practices are considered safe for the environment, control climate change, improve soil health, and are crucial to sustainable agriculture. However, insufficient comparative data on nutrient regimes of organic and chemicalized fields of different agricultural belts worldwide cannot substantiate such generalizations. The current systematic survey of the major soil nutrient parameters of organically managed vs. chemicalized fields of South India is a significant contribution to the same. It reveals that soil pH, TOC, SAN, SAP, and SAK of most organic fields over the broad tropical South India remain at par with chemicalized fields in the same region. It also reveals that organic farming is carried out in different labels and soil types without standard norms, especially in tropical South India. Although private and government agencies certify that chemicals are not applied in the fields, the nutrient regimes of the fields are not monitored per standard norms to ensure soil health, food, and environment safety, or organic farming in the region. The preliminary data generated in this regard remains an eye-opener for agricultural researchers worldwide to intensify research on the standardization of organic farm practices concerning the sustainability of physicochemical soil nutrient regimes, specific manure applications, farming practices, soil types, and climate for ensuring sustainable agriculture for a safe and secure future world.

Acknowledgment

The first author wishes to acknowledge the junior research fellowship provided to him by Mahatma Gandhi University during the tenure of the study.

Funding

The authors declare that no funds, grants, or other support were received during this study.

Competing interests

The authors have no relevant financial or non-financial interests to disclose

Author Contributions

The first author carried out the field and laboratory studies and prepared the primary draft of the article per the plan and strict guidance of the corresponding author. The corresponding author critically analyzed and evaluated the data and edited the article to finalize it for submission. Both authors approved the final proposal of the report to the Journal' Plant and Soil'.

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Sustainability of Organic Farming from a Global Perspective: A Critical Analysis of Soil Types and Fertility Status of Organically Managed vs. Chemicalized Vegetable Fields of South India

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