Farmers must tread warily to avoid setbacks
Milkha Singh Aulakh and Kabal Singh Gill
CROPS need essential nutrients for seed, fruit or fodder production. Under natural ecosystems such as forests, nutrients absorbed by crops are recycled by soil micro-organisms, establishing a sustainable equilibrium. In traditional agriculture, returning crop residue and animal faeces to the soil used to replenish nutrients removed by crops. Since the Green Revolution, however, the application of fertilisers, mainly nitrogen (N), phosphorus (P) and potassium (K), has become essential for high crop yields, especially under irrigation. Fertiliser (NPK) application in India averages 161 kg per hectare per year (kg/ha/yr), which widely differs from one area to another. Furthermore, 7-9% fertiliser use and 9-16% agricultural production in India happens in Punjab, which accounts for only 1.53% of the country’s geographical area.
Punjab Agricultural University’s (PAU) fertiliser recommendations for field crops are: apply N in two splits (at seeding and a few weeks later), P at seeding to winter-grown crops (summer crops make use of leftover P), and sulphur (S) to specified crops only. Excessive and inappropriate fertiliser use has led to a large buildup of N, P and S in soils, making them susceptible to leaching into groundwater and erosion into surface water bodies.
Nitrogen: A PAU study revealed that fertiliser N’s use in an intensively cultivated and irrigated region often exceeds the recommended quantity. Leaching of nitrate-N below the root zone was observed after heavily-fertilised shallow-rooted maize, rice, potato and several vegetables. With the application of 120 kg N/ha (recommended dose) to each crop in rice-wheat rotation, 19 kg nitrate-N/ha was found in 60-cm soil after rice but 59% of it was used by subsequent wheat having a deeper and extensive rooting system. The application of 120 kg N/ha to each crop for four years resulted in 35-kg nitrate-N/ha in 150-cm soil profile, whereas 20-tonne green manure (GM) plus 60-kg N/ha (50% less fertiliser N) left only 19-kg residual nitrate-N/ha (decreased potential for groundwater contamination) and produced greater crop yields. Similarly, the use of farmyard manure (FYM) and compost have been found to reduce fertiliser N need without sacrificing production.
A survey (3,200 samples) revealed 13% of groundwater of Ludhiana district and 7% of Ferozepur district exceeded the drinking water safe limit of 10 milligram nitrate-N per litre.
Phosphorus: Contrary to PAU recommendations, several farmers apply P to summer crops. A 25-year long-term study showed 33% more P application than recommended caused up to 480 kg P/ha accumulation in 150-cm soil, with 43-58% below 60-cm and unlikely to be recovered by crops. More P accumulation occurred when applied to summer than winter crops. Another study showed higher P accumulation in 0-30 cm (70-80%) than in 30-50 cm (20-30%) of a loamy soil, and the opposite in more permeable soil (36-41% in 0-30 cm and 59-64% in 30-150 cm). Thus, there is enormous deeper movement of P in a porous sandy soil having low nutrient retention capacity, especially when applied at higher than recommended rates.
Sulphur: Fertiliser S is recommended only for oilseeds (sarson, rapeseed, etc.), pulses and some other crops, which have better ability to absorb S, respond and reduce leaching. Deficiency of S appears widespread in soils of Punjab, Haryana and some other states. Reasons for S deficiency include fertiliser P-induced detachment of soluble sulphate-S from the colloidal soil surfaces (because phosphate has higher attachment strength) and leaching to subsoil. Sandy soils have greater potential for leaching.
Excessive and inappropriate use of fertilisers lowers the farmers’ income and impacts the ecosystem through soil-water-plant-animal-and/or human foodchain in several ways. It also causes unwanted greenhouse gas fluxes and global warming.
Mitigation options
Researchers have identified appropriate fertiliser types, rates and times to minimise harms and enhance efficiency. Nutrient supply synchronised with crop needs, early to middle growing period when rapid nutrient uptake happens, can minimise losses. Rooting patterns of plants influence nutrient mobility in the root zone and leaching to deeper soil. Deep-rooted crops should follow shallow-rooted crops; rice should not be grown on highly permeable sandy soils; and heavy irrigation should be avoided. Don’t apply P fertiliser to summer crops, except when the soil test shows need or may be in alkaline soils having excessive salts. Avoiding excessive P fertiliser and applying S fertiliser to specified crops only could help reduce leaching of S to groundwater and water bodies. Using appropriate amounts, times and methods to apply fertilisers in combination with GM, FYM, compost, crop residue, cattle urine, etc. can minimise the movement of nutrients to deeper soil and water bodies.
Farmers should be encouraged to use fertilisers with experts’ guidance, as done in most developed countries. In the USA, farmers have to consult/engage professionals to purchase and apply fertilisers using appropriate practices. In Canada, farmers are assisted by the government and industry to develop Environmental Farm Management Plans, and follow soil test-based fertiliser recommendations to optimise inputs, minimise environmental impact and get benefits. The Fertiliser Institute of Canada holds conferences, provides guidance and encourages farmers to go for the 4Rs — right source, right rate, right time, and right place.
The interplay of fertiliser type, rate and time, crop rotation, irrigation and soil texture largely controls the downward movement of nutrients in soils and emission of greenhouse gases. Integration of the use of fertilisers and on-farm-generated nutrient resources (GM, FYM, animal urine, crop residue, etc.) is the sustainable way forward to improve soil health, crop production, food safety and the ecosystem.
Aulakh is ex-Dean (Agriculture), PAU; Gill is former Research
Scientist, ICRISAT
