Effect of Fourteen Years of Long Term Organic and Inorganic Fertilization on Productivity, Soil Quality and Grain Quality of Rice (Oryza sativa L.)

Soil health and fertility decline significantly due to intensive use of inorganic and agrochemical fertilizers (Souri 2016; Ge et al., 2018). Maintaining soil health and restoring degraded gricultural land are the major strategies that need to be adapted. Rice stands second in the world after wheat in area and production. Organic farming can enhance soil biodiversity, alleviate environmental concerns and improve food safety through eliminating the applications of synthetic chemicals. However, yield reduction due to nutrient limitation and pest incidence in the early stages of transition from conventional to organic systems is a major concern for organic farmers and is thus a barrier to implementing the practice of organic farming. Therefore, identifying transition strategies that minimize yield loss is critical for facilitating the implementation of organic practices. Soil microor ganisms play a dominant role in nutrient cycling and pest control in organic farming systems and their responses to changes in soil management practices may critically impact crop growth and yield (Tu Cong et al., 2006).

Studies on the influence of organic farming practice on the yield and quality parameters of rice grain and on soil physico-chemical properties in vertisols of K.C. Canal ayacut is meager. Also, since the consumption of rice is more, it is highly essential to study the influence of organic farming in rice keeping in view the human health and soil health for achieving sustainability in quality rice yields and improving the productivity of soils.

Experimental site characteristics
 
A field experiment was initiated during Kharif 2007 and was conducted for 14 kharif seasons of 14 crop years, from 2007 to 2020 at Regional Agricultural Research Station, Nandyala Andhra Pradesh. The soil of experimental site was medium deep black, clay loam in texture, low in organic carbon (0.25%), low in Nitrogen (116 kg ha^-1) high in available P₂O₅ (69.5 kg ha^-1 ) and available K₂O(536 kg ha^-1 ).
 
Treatments details
 
The experiment was laid out in non replicated two block design with each block of an area of 2000 m² each. The details of nutrient composition of different organic sources used in experiment was given in Table 1. The organic plot was green manured with green manure crop, Dhaincha (Sesbania aculeata) before one month of paddy crop transplanting. Dhaincha was grown for 45 days and at flowering stage of Dhaincha it was incorporated in the same organic plot. FYM @ 10 t ha^-1, castor cake and neem cake were applied by broadcasting method @ 500 kg ha^-1 as basal dose of manure and topdressing of vermicompost @ 500 kg ha^-1 was added in two splits at panicle initiation time and at max tillering stage as a nutrient sources. In in-organic plot, 100% RDF (240:80:80 kg NPK ha^-1) through urea, single superphosphate and muriate of potash, was applied. Nitrogen was given in three equal splits at basal (before transplanting), maximum tillering (30 days after transplanting) and panicle initiation (60 days after transplanting) stages, whereas P and K were given as basal doses only. Through organic fertilizers, the nitrogen dose was adjusted to the recommended level based on the moisture content and total nitrogen concentration on dry-weight basis was applied. Nutrient composition of different organic sources was presented and explained in Table 1.



Soil pH was measured in 1:2 soil water suspensions, with pH meter. The electrical conductivity was measured in 1:2 soil water suspension using conductivity meter. Organic C was determined by the modified Walkley-Black wet digestion method. Available nitrogen was estimated by alkaline- KMnO⁴ method. Available phosphorous was extracted with 0.5 M sodium bicarbonate (pH of 8.5) and was determined colorimetrically. Available Potassium (K) was extracted with neutral normal ammonium acetate solution and the extract was analyzed for potassium by flame photometer.

Bulk density, Total porosity, water holding capacity was measured by Keen box method. Mean weight diameter and the distribution of water stable aggregates was determined by wet sieving technique.

Soil quality parameters
 
Application of organic manures and inorganic NPK fertilizers showed marked difference on physico-chemical properties of soil (pH and EC) at tillering, panicle initiation and harvest stage of rice. At all growth stages of rice, the highest available nitrogen, phosphorus, potassium, micronutrients (Fe, Mn, Zn and Cu), urease, dehydrogenase and microbial population (bacteria, fungi and actinomycetes) in soil were recorded with application of In-organics+Dhaincha @ 10 t ha^-1. Changes in soil physical, chemical and biological parameters were monitored at the end of every year and results at the end of fifth year are presented in Tables 1 to 4. There was a significant improvement in soil physical (bulk density and water stable aggregates), fertility (organic carbon and available N, P₂O₅ and K₂O) and biological properties (soil microbial populations and enzyme activities viz. Urease, glucosidase, phosphatase and dehydrogenase), with organics compared to inorganic fertilizers. Compared to inorganics, there was an increase in soil organic carbon (SOC), available N, P and K by 59-65, 3-10, 10-27 and 8-25% with organics, respectively, at the end of 14 kharif years. Application of organic manures and inorganic NPK fertilizers showed marked difference on physico-chemical properties of soil (pH and EC) at tillering, panicle initiation and harvest stage of rice. At all growth stages of rice, the highest available nitrogen, phosphorus, potassium, micronutrients (Fe, Mn, Zn and Cu), urease, dehydrogenase and microbial population (bacteria, fungi and actinomycetes) in soil were recorded with application of RDNK+Dhaincha @ 10 t ha^-1.

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Pattern of organic carbon (%) status from 2007 was presented in Fig 1 and the organic carbon status was medium in organic plot (0.59%) when compared to the inorganic plot (0.34%). Comparable increases in organic carbon, available N, P and K through addition of organic materials was also reported (Carpenter-Boggs 2000 and Melero et al., 2008). Superior soil fertility status on organic farms compared to soils fertilized with chemical fertilizers was also reported by Mader et al., 2002 and Nataraja et al., 2021.



(Rasool et al., 2007) observed that the average mean weight diameter (MWD), total porosity and water holding capacity (WHC) were highest in FYM treated plots both in rice (0.237 mm) and wheat (0.249 mm). Application of organic manures significantly improved the soil organic carbon content and available nutrient status of soil compared to either the chemical fertilizers or the control. At the end of cropping cycle, soil organic carbon was the highest in treatment receiving vermicompost (0.69%) followed by cattle dung manure (0.68%) (Ramesh et al., 2008).

Increased organic carbon of soil due to application of manures was reported by (Babhulkar et al., 2000). Babu and Reddy (2000) recorded significant increase in the organic carbon content of sandy clay loam soil from 0.61 to 0.92% due to the addition of FYM and inorganic nitrogen @ 5 t ha^-1 and 50 kg ha^-1, respectively in rice. (Hemalatha et al., 2000) reported higher content of organic carbon in soil with the application of dhaincha, followed by sunhemp and FYM. Swarup and Yaduvanshi (2000) reported that soil organic carbon was significantly lower in treatments receiving inorganic fertilizers as compared to the treatments involving organics. According to (Kumar et al., 2001), inclusion of green manure crop in the rice cropping system increased the organic carbon and available N, P and K of soil. Continuous and adequate use of organics with proper management can increase organic carbon content and physical properties of soils (Atiyeh et al., 2002).

Soil physical properties like bulk density, porosity, void ratio, water permeability and hydraulic conductivity were significantly improved when FYM (10 t ha^-1) was applied in combination with chemical amendments of sodic soil (Hussain et al., 2001). The bulk density of Vertisol decreased considerably with application of FYM (1.00 Mg m^-3) or sunnhemp incorporation (1.04 Mg m^-3) or retention of stubbles on soil surface (1.02 Mg m^-3) compared to farmer practice (1.18 Mg m^-3) is use of inorganic fertilizer (Guled et al., 2002). Sheeba and Chellamuthu (2002) reported significant improvement in properties of VerticUstropept by continuous use of FYM @ 10 t ha^-1 over twenty two years span of time. They reported significant increase in aggregate stability (68.49%), per cent water stable aggregates (78.22) and total porosity (55.73%) with application of FYM @ 10t ha^-1 as compared to application of 100 per cent N, P₂O₅ and K₂O through inorganic fertilizers i.e. 64.30, 75.70 and 51.03 per cent, respectively. They also noticed decrease in bulk density with the application of FYM @ 10 t ha^-1.
 
Grain and straw yield trends in 14 years of organic farming experiment
 
The pooled mean grain yield and straw yield of paddy grown in vertisols under K.C. Canal ayacut were given in Table 4. Under organic farming treatment grain yield was 4.04 t/ha while it was 5.6 t/ha under control (Inorganic/ Conventional farming) and the straw yield also was 5.07 t/ha under organic farming and 7.96 t/ha under control. The average yield increase in in organic plot was 27% and for straw it is 35%. The data on no. of tillers per hill, panicle length (cm) and no. of grains per panicle were also highest or superior in the inorganic plot compared to organic plot.

During all the 14 kharif seasons, inorganic fertilizer applied plots were near stable ranging from 6.1 to 6.8 t/ha and superior to organics (3.4 to 4.8 t/ha). This could be due to mismatch of nutrient release from organic sources and crop demand as influenced by seasonal conditions in the initial years and once the soil fertility was built up sufficiently, organic system also produced equal yields as conventional system. Thus, slow and gradual release of nutrients from organics during the initial years of conversion to organic farming could not result in increased yields. But, repeated application of organics over the years built up sufficient soil fertility by improving soil biological activity.
 
Grain quality parameters
 
Grain quality parameters recorded at the end of 2020 was presented in Table 5. Most of the grain quality parameters were not influenced even after fourten years of study, though moderate improvement in nutritional quality was recorded with organics and polishing reduced the quality improvement. Nutritional grain quality parameters-total moisture (%), total Ash (%), total Protein (%), total sugars, reducing sugars and heavy metals like chromium, cadmium and lead (Pb), pestiscide residues were not influenced by the nutrient sources even after 14 years of study. However, in the fifth year, there was an improvement in Total protein by 9.5%, with organics over inorganics (Table 5). A significant improvement in nutritional quality (Fe and Mn), with combined application of 2 or more organic sources and with 3 or 4 organic sources, Improvement in HRR, kernel length, breadth and L/B ratio after cooking with the application of organic sources alone was also reported by Surekha and Satish Kumar 2014.

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From the present 14-year study of crop productivity, soil fertility and grain quality under organic and conventional rice production systems, it can be concluded that in the initial years of experimentation when the field was under transition, organic fertilizers did not result in increased yields and chemical fertilizers were found superior. However, repeated application of organics over the years resulted in sufficient buildup of soil fertility and improved the grain yield. Further, organic production systems improved the soil properties like pH, Bulk denity and Aggregate stability, soil organic carbon status, productivity, soil quality indices of the soil when compared to conventional production system.

The authors are extremely grateful to Acharya N G Ranga Agricultural University, Guntur Andhra Pradesh for generous assistance for the said project.

None.