Awareness and interest in improved Nutrient Use Efficiency (NUE) has never been greater.
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Awareness and interest in improved Nutrient Use Efficiency (NUE) has never been greater. Driven by a growing public belief that crop nutrients are excessive in the environment and farmer concerns about rising fertilizer prices, energy crisis and stagnant crop prices, the fertilizer industry is under increasing pressure to improve nutrient use efficiency. The mineral nutrients applied but not taken by the crop are vulnerable to losses from leaching, erosion, denitrification, volatilization, and temporary fixation. Low nutrient recovery efficiency not only increases cost of crop production but also causes environmental pollution. Agronomic nutrient use efficiency (kg crop yield increase per kg nutrient applied) is the basis for both economic and environmental efficiency. The use efficiency of Nitrogen (N) ranges from 20-30% under rainfed conditions and 30-40% under irrigated conditions, 10-30% for Phosphorous (P), 50-60% for Potassium (K), 8-12% for Sulphur (S), and hardly 4-5% for the micronutrients. It indicates that, roughly 50% of nitrogen and 90% of phosphorus applied through chemical fertilizers are lost to water and the atmosphere, which causes water eutrophication, Green House Gases Emission (GHGE), and other associated environmental issues. Application of organic manures is recognized as a key strategy for smart farming, because it optimizes the nutrient use efficiency, by balancing nutrient supply with crop demand, reducing leaching, and improving soil health.
Optimising Nutrient Use Efficiency through Organic Manures
Organic manures are the natural materials from plants and animal sources that directly and indirectly affect the soil’s physiochemical and biological properties. A bio-fertilizer, a misnomer, is also a type of organic carrier that contains beneficial microbes (algal, fungal, bacteria) that improves plant growth by mobilizing the soil available nutrients through their biological activities. These microorganisms enhance nutrient use efficiency (NUE) by fixing atmospheric N, solubilising insoluble P and K, production of phytohormones, siderophores, and improving plant nutrient uptake through enhancement of root architecture and rhizosphere interactions. Apart from nutrient supply, bio-fertilisers enhance soil enzymatic activity, increase soil organic carbon, promote soil carbon stabilisation and restore microbial diversity. Moreover, they contribute to climate-resilient agriculture by improving plant tolerance to drought, salinity, and temperature stress, and reducing greenhouse gas (GHG) emissions. Animal excreta are the greatest source of organic manure around the globe, followed by poultry and pig manures. Animal manures are a good and sustainable source of NPK, and the total N excreted in animal manure globally ranges from 81.5 to128.3 Tg per year, though the type and amount of N in animal manure vary significantly. The fertilizer industry supports the application of nutrients at the right source, right rate, right time, and in the right place (4R Stewardship) as a best management practice (BMP) for achieving the optimum nutrient efficiency.
Right source
Organic sources of nutrients are considered as the most sustainable and environment friendly than synthetic inorganic fertilizers because they release nutrients slowly; ensuring a steady supply of nutrients to the plants, reducing the risk of nutrient wastage, water pollution and harming beneficial soil organisms. In addition to increasing soil fertility, organic manures improve soil organic carbon (SOC) content, soil microbial activity for crucial nutrients recycling, water retention and soil aeration, aggregates stability, maintain soil pH, and minimize risk of groundwater contamination. Due to their slow-release nature, organic manures reduce volatilization of nitrogen species, and their ability to prevent acidification, while increasing organic matter content of soils, promotes carbon sequestration, further preventing greenhouse gases emission. Since organic manures are derived mainly from waste, they are renewable, and do not compete with humans and animals for food. Organic manures are therefore a sustainable alternative to inorganic fertilizers.
Most crops are location and season specific-depending on cultivar, management practices, climate, etc. Key strategies for enhancing NUE via organic sources include:
Organic Amendments: Compost, animal manure, de-oiled cakes, and crop residues add nutrients to soil, while enhancing soil organic matter and microbial activity, which helps nutrient cycling.
Cover Crops and Rotations: Planting cover crops, especially legumes, fixes nitrogen and reduces nutrient leaching, with some holding onto nitrogen that would otherwise be lost.
Precision Application: UtiliSing precision agriculture techniques, such as Variable Rate Technology (VRT), enables applying organic amendments exactly when and where needed.
Right rate
Adequate and balanced application of mineral nutrients is one of the most common practices for improving the efficiency of applied N. Both over- and under-application of plant nutrients result in reduced nutrient use efficiency or losses in yield and crop quality. Soil testing is the most powerful tool available for determining the nutrient supplying capacity of the soil, thereby enabling the farmers to make appropriate decision on right rate of nutrient application, through organic manures. Unfortunately, soil testing is not available in all regions because reliable laboratories using methodology appropriate to local soils and crops are inaccessible or calibration data relevant to current cropping systems and yields are lacking.
Right time
Greater synchrony between crop demand and nutrient supply, is highly necessary to improve the nutrient use efficiency, especially for N. Split application of N during the growing season is effective in increasing N use efficiency. Tissue testing is a well-known method to assess N status of growing crops, but other diagnostic tools are also available. Chlorophyll meters are useful in fine-tuning in-season-N management and Leaf Colour Charts (LCC) have been highly successful in guiding split N applications in rice and maize. Precision farming technologies have introduced, and now commercialized, on-the-go-N sensors that can be coupled with variable rate fertilizer applicators to automatically correct crop N deficiencies on a site-specific basis. Use of N stabilizers and controlled release fertilizers synchronize the release of N with crop need. Similarly, nitrogen stabilizers {Nitrapyrin, DCD (dicyandiamide], NBPT [N-(n-butyl) thiophosphoric triamide]} inhibit the nitrification or urease activity, thereby slowing the conversion of the applied nitrogen forms into nitrate. The most promising coated-fertilizers like neem coated urea (NCU), sulphur coated urea (SCU) and lac coated urea (LCU) are the slow release fertilizers that release the nitrogen in a controlled manner, thus enhance the nutrient use efficiency.
Right place
Application method has always been critical in increasing nutrient use efficiency. Numerous placements are available, but most of these involve surface or sub-surface applications before or after planting. Prior to planting, nutrients can be broadcast, applied as a band on the surface, or applied as a subsurface band, usually 5 to 20 cm deep. At planting, nutrients can be banded with the seed, below the seed, or below and to the side of the seed. The recommended dose of phosphorus must be applied through basal application, preferably in root zone at planting. After planting, the application is usually restricted to N, and in some cases for K. Placement can be as a top-dress or a subsurface side-dress. In general, nutrient recovery efficiency tends to be higher with banded applications because less contact with the soil lessens the opportunity for nutrient loss due to leaching or fixation reactions. Placement decisions depend on the crop and soil conditions, which interact to influence the nutrient uptake and availability.
A.S. Mailappa
(The writer is Professor and Head, Department of NRM College of Horticulture and Forestry, Pasighat, Arunachal Pradesh; Central Agricultural University, Imphal, Manipur)