Sustainable yield index: An approach to evaluate long term sustainability of finger millet and maize under finger millet-maize cropping system in Alfisols

Introduction

Soil is a non-renewable resource that has the ability to degrade quickly and has extremely slow in formation and regeneration processes [18] . Due to indiscriminate use of agrochemicals, deep ploughing, luxury irrigation, intense cropping with high yielding varieties, little or no use of organic materials, and incorrect soil management practises, the majority of cultivable lands are degrading day by day [9] . In peninsular India, finger millet and maize are important premier crops. In Karnataka, these crops occupy 6.63 and 13.20 lakh hectares area, respectively with an average productivity of 1645 and 3470 kg ha -1 , respectively in the year 2020-21 as against of respective productivity of 1709 and 2559 kg ha -1 from total area of 10.13 and 92 lakh hectares in the country. In agriculture, fertilizers consumption is increasing rapidly due to their importance in raising agricultural production. Intensive cropping of finger millet-maize system with continuous application of inorganic fertilizers results in loss of soil organic matter and may have some adverse deleterious effect on soil fertility as well as production and that system is not sustainable [16] . The term "sustainability" refers to the long-term maintenance and development of productivity through integrated land management [14] . No agricultural system can be sustained unless its soil productivity and quality are consistently maintained [8] . The sustainable yield index (SYI) as a quantitative measure to assess the long-term viability of an agricultural practice in this direction [17] . In the SYI concept, a low standard deviation (SD) indicates the system's long-term viability. If SD is high, however, SYI will be low, indicating an unsustainable management practice [17] .

The index takes values between zero and unity. The ideal treatment is one in which SD = 0 and mean = highest observed yield (y m ), implying SYI = 1. This treatment consistently produces maximum yield in all years. But invariably SD is always greater than zero in turn SYI will be less indicating improper management practices [15] . Long-term trials are critical for comprehending the intricate interactions between plants, soils, climate, and management approaches, as well as their effects on crop productivity. Longterm trails have been acknowledged by agricultural scientists as significant tools in the study of agro ecosystem dynamics. A long-term evaluation of the application of organic manure for preserving soil fertility to improve the productivity of the ricewheat system in Bangladesh [1] . The combined application of mineral fertilizers and organic manure has generally provided the optimum crop output and soil quality in most long-term trails [19] and [2] . As a result, the current study was conducted to analyse the sustainable yield index of finger millet-maize cropping system using an integrated application of organic manure and chemical fertilizer. To achieve this goal, yield data generated from initiation of experiment (1986) up to 2019 from long term fertilizer experiment were considered.

Material and Methods

Experimental Site and Treatment Details

The long-term fertilizer experiment was initiated in 1986 at Zonal Agricultural Research Station of University of Agricultural Sciences, GKVK, Bengaluru located in Eastern Dry Zone of Karnataka at an altitude of 930 meters above MSL, latitude of 13 • north, longitude of 77 • 33" east with finger milletmaize cropping system. The experimental soil was lateritic and sandy clay loam of texture with red colour. It belongs to the Isohyperthermic family of the sub group typic kandic paleustalfs of Vijayapura series. The pH of the soil is 6.17 (slightly acidic) with sandy clay loam in texture, low in organic carbon content (4.6 g kg -1 ). The available nitrogen (256.7 kg ha -1 ) was low, available phosphorus (34.3 kg ha -1 ) was medium and available potassium content (123.1 kg ha -1 ) was low. The physicochemical properties of initial soil sample (1986) of the experiment site were given in Table 1. Finger millet and hybrid maize crops were grown in sequence during Kharif and Rabi seasons, respectively. The details of experiment, treatments, recommended dose of fertilizers for the study crops, sources of fertilizers etc. are as follows. The normal meteorological data of annual rainfall for the last 33 years (from 1986 to 2019) was ranged from 361 to 1363 mm, the average being 908. 2. However, if the soil pH exceeds 6.00 then lime was applied @ 500 kg ha -1 . After 25 to 30 days of sowing/transplanting of crops, 50 percent of the recommended nitrogen dose, 100 percent of the recommended P and K doses were applied as a basal, and the remaining 50 percent nitrogen dose was administered in two equal splits as a top dress.

Sustainable Yield Index (SYI)

Sustainable yield index of finger millet and maize were analysed to determine the yield sustainability of finger millet and maize. SYI of were calculated finger millet and maize using the equation [17] . SYI = (y-s) / Y max Where, y = Estimated average yield s = Standard deviation of yield across years Y max = Maximum observed yield SYI nearness to 1 implies the closeness to an ideal condition that can sustain maximum crop yields over year.

Statistical analysis

For the interpretation of results, the data obtained in the laboratory studies will be subjected to statistical analysis using the technique of analysis of variance for randomised block design [5] . The level of significance used in "F" and "t" test was P=0.05. Critical difference (CD) values were calculated for the P=0.05 whenever "F" test was found significant.

Results and Discussion

Yield of finger millet and maize

The mean grain yields of maize for the period from 1986 to 2020 at eight year interval except 2003-04 due to crop failure are shown in Table 3. The results of the long-term study from 1986 indicated that, under finger millet and maize cropping system application of super optimal dose of NPK (150% NPK) had recorded highest grain yield of finger millet (43.18 q ha -1 ) when compared to application of optimal and sub optimal dose of NPK. Initially grain yield was higher, then gradually decreases to 33.27 q ha -1 (2012-2020). Application of FYM along with 100% NPK in the treatment T 8 (41.62 q ha -1 ) and T 10 (39.31 q ha -1 ) resulted in higher yield of finger millet compare to 100% NPK alone but lower than the 150% NPK. This may be due to beneficial effect of the organic matter accumulation due to the addition of FYM. The continuous growing of crop without any fertilizer application (control) causes decrease in crop yield compared to other treatments. Initially, crop yield maintained higher and it gradually decreased from 6.18 to 4.74 q ha -1 (2004-2012) and later it shows increasing trend with mean yield of 6.10 q ha -1 Application of N alone without P and K resulted in better grain yield than control during initial phase of the experiment, but later it was not able to sustain the yield afterwards. The mean grain yields of maize for the period from 1986 to 2020 at eleven-year interval are shown in Table 4. Application of super optimal dose of NPK (150% NPK) had record the superior grain yield of maize (33.56 q ha -1 ) over sub optimal dose of NPK and non-superior over optimal dose of NPK. Application of FYM and lime along with 100% NPK in treatments T 10 (36.09 q ha -1 ) and T 8 (34.21 q ha -1 ) resulted higher yield of maize than that of 150% NPK alone. This may be due to beneficial effect of the organic matter accumulation due to the addition of FYM. The continuous growing of crop without any fertilizer application (control) and imbalanced application of nutrients (100% NP and 100% N alone) recorded lowest maize yield compare to rest of the treatments. Application of lime increases the yield of maize i.e., T 10: 100% NPK + lime (36.09 q ha -1 ) and T 5: 100% NPK + Lime (30.66 q ha -1 ) compare to treatment without lime application over the years T 8: (34.21 q ha -1 ) and T 2: (29.28 q ha -1 ). Over time, the use of FYM and lime resulted in increased yield of the finger millet and maize. Higher crop yields were attributed to the release of nutrients in accessible forms from FYM during decomposition and absorption by the crops [7]. The crop yield was significantly lower in the treatment with unbalanced nutrient application (T 6 and T 7 ) and control (T 11 ). This might be due to decrease in soil organic matter and associated nutrients in an imbalanced fertilizer application [3] . Higher crop yields with 150% of NPK indicates that 100% NPK was inadequate to provide enough N or P or K to the crops [10] . The addition of FYM supplies significant amounts of crop accessible nitrogen, phosphate, potassium, and micronutrients while also buffering yield fluctuations, resulting in long-term productivity [4] and [6] . Manna et al. (2005) found a similar positive yield trend in a soybean-wheat system on Alfisols at Ranchi using higher doses of NPK and NPK + FYM treatments. This clearly shows that adding organic materials to the soil and maintaining or increasing soil organic matter content can help in sustain substantial crop yields in dry land finger millet production systems. Over a 20-year period, continuous mono cropping without the addition of organics or artificial fertilisers reduced crop output significantly.

Sustainability yield index

The data pertaining to SYI influenced by long term manuring and fertilization in finger millet-maize cropping sequence have been presented in Table 5 and Fig. 2. The data on SYI values of finger millet indicate that, 100% NPK + FYM + lime (T 10 ) was superior for finger-millet with maximum SYI of 0.64 followed by T 3: 150% NPK (0.56). Lowest SYI of finger-millet was noticed in 100% NP (0.16) and followed by 100% N (0.24) treatments. Whereas in maize maximum sustainable yield index was observed in 100% NPK + FYM + Lime (T 10 ) followed by T 8: 100% NPK + FYM, T 3: 150% NPK and T 5: 100% NPK + Lime with 0.22. Lowest SYI of maize was noticed in application of 100% N (0.01) and 100% NP (0.07) treatments. This further supports the notion that unbalanced fertilizer application will not be able to sustain the yields in finger millet and maize cropping sequence in the long run. Although 150 percent RDF had a higher SYI (next to 100 percent NPK + FYM), in the context of balanced diet, need-based micronutrient and sulphur consumption would be more acceptable. Imbalanced supply of N alone or NP or NPK (-S) led to unsustainability and a steady loss in production over period of cropping. This may be ascribed to the decline in soil fertility leading to nutritional deficiencies in crop plant. Imbalanced and continuous use of nitrogen alone resulted in the greatest yield decline and had a negative impact on long-term yield sustainability and soil quality, indicating that other major and micronutrients were becoming limiting factors and that adequate N response could not be achieved unless those factors limiting yield were addressed. The sustainability of crop production was obtained by the application of organic manures [13] . The integrated use of lime or FYM coupled with balanced NPK is essential for increasing as well as sustaining crop production in the acid soils of Bangalore under finger-millet maize cropping system [11] .

Conclusion

The results revealed that SYI was able to accurately predict a minimum feasible yield. The SYI of NP, N alone, and control (unfertilized and unmanured) treatments were all very low after 33 years of continuous cropping, implying that such imbalanced treatments degrade soil fertility and crop yield. Regardless of the crops the treatments having greatest SYI values was with 100 percent NPK + FYM + Lime, which is comparable to 100 percent NPK + FYM and 150 percent NPK. The FYM and lime served as a soil ameliorant, improving soil fertility and productivity by ameliorate unfavourable soil ecological conditions (in particular, soil acidity). Thus, by providing balanced nutrition and correcting marginal secondary and micronutrient deficiencies, integrated use of organic and inorganic manures has considerable potential in providing higher stability and sustainability in crop output under current intensive farming.

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