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Nutrient Management in Organic Agriculture

Sagar Paudel

IAAS Lamjung

Organic agriculture is a holistic production system that relies on ecological processes, biodiversity and cycles adapted to local conditions, rather than the use of external inputs as chemical fertilizers and pesticides with adverse effects. Organic Agriculture combines tradition, innovation, and science to benefit the health of soil, environment and people. It emphasizes the use of management practices in preference to the use of off-farm inputs which is accomplished by using, where possible, agronomic, biological, and mechanical methods, as opposed to using synthetic materials, to fulfil any specific function within the system.

Maintaining or enhancing long-term soil productivity is a key provision of the National Organic Program (which states that the producer must (a) select and implement practices “that maintain or improve the physical, chemical, and biological condition of soil and minimize soil erosion,” (b) “manage crop nutrients and soil fertility through rotations, cover crops, and the application of plant and animal materials,” and (c) “man-age plant and animal material applications to maintain or improve soil organic matter,” but minimize “contamination of crops, soil, or water by nutrients, pathogenic organisms, heavy metals, or residues of prohibited substances.” Any soil-applied product, including composts or manures, must be produced in compliance with organic program regulations.

Nutrient management on organic farms should economically meet crop nutrient needs and avoid soil nutrient depletion, while maintaining or improving soil productivity without excessive nutrient losses. Soil nutrient availability is dependent on diverse soil chemical, physical, and biological properties, their interactions, and their interaction with the cropping system. While measurements can be made for many soil properties, crop performance is the best indicator of soil productivity. Farmers typically manage to minimize soil physical and chemical constraints to sustainable productivity through practices such as:

  • Applying organic materials such as manure, compost, and biofertilizers to supply nutrients and maintain soil organic matter
  • Growing cover crops to cycle soil nutrients and biologically fix atmospheric nitrogen
  • Diversifying crop rotations

Some of the techniques of Nutrients Sources are:

Manure:

Manure application is often valuable to organic production. However, applying manure to meet all of the crop nitrogen demand can lead to excessive soil phosphorus because crops remove more nitrogen than phosphorus. The excessive soil phosphorus is not likely to be harmful to crops but contributes to phosphorus loss in runoff and erosion and contamination of water bodies. Therefore, manure nitrogen needs to be complemented by biological nitrogen fixation or other nitrogen sources in organic systems.

The supply of nitrogen (N) in organic farming is usually provided by legumes. Through a symbiosis with nodule bacteria, these plants are capable of fixing atmospheric nitrogen and making it available to plants. There are other bacteria that can also fix nitrogen (Actinomycete which are present in dead wood, soil-bound Azotobacter bacteria, which live in association with the tropical fodder-grass Paspalum notatum and other Gramineae). In paddy rice, the bacteria Anabaena azollae are used, which forms a symbiosis with the water fern Azolla, and CAN, under tropical conditions, fix up to 400 kg N/ha/year, and which is very often used as a green manure for rice crops. In crop rotation systems, at least 20% of entire cultivated area should be planted with legumes.

 

Compost:

Composting processes organic waste into material of higher   nutrients   concentration, and reduces the bulk of organic materials through carbohydrate and water loss during decomposition. Compost is often easier to handle than the bulk organic material, and the composting process kills some pathogens and weed seeds. Compost has less odor and fewer microbial pathogens, with less risk of microbial contamination of fresh produce than with raw manure. There are fewer restrictions on the use of compost in edible crops, but composted animal manures must be applied to soil prior to planting to mitigate pathogen risks Nitrogen mineralization for finished composts with a carbon: nitrogen ratio (C: N) less than 20:1 occurs readily with much of the organic nitrogen released to the first crop following application.

Composts with C: N greater than 30:1 will result in net immobilization of soil nitrogen with reduced nitrogen availability to the first crop following application. Nearly all of the nitrate and ammonium in the compost will be available to the plants the first season after application, but only 15 percent of the organic nitrogen in the compost may be available during that season if the C: N is high. Organic N availability estimates for years two through four are the same as for manure (15, 7, and 4 per- cent, respectively).In addition to supplying nitrogen, compost is an important source of other macro and micronutrients. As with other organic materials, compost application can improve soil organic matter content, cation exchange capacity, soil porosity, aggregate stability, and water holding capacity, although the magnitude of improvements will depend on current soil organic matter levels. 

Biofertilizers:

Biofertilizers are products designed to provide enhanced nutrient availability and uptake, stimulation of crop growth, biological nitrogen fixation, and protection against insect pests and disease. Depending on the purpose, biofertilizer products can be applied to soil, seeds, or foliar tissue. Evidence to support product claims is often limited or mixed. Recent research suggests biofertilizers may be most beneficial in soils of low to moderate soil organic matter and nutrient availability or with foliar application. In contrast, biofertilizer applications to soils of >3 percent organic matter rarely results in measurable crop or soil benefits. Introduced microorganisms often fail to compete and survive with already well- established and resilient microbial communities. Several common categories of biofertilizers include nitrogen-fixers, phosphorus solubilizers, phosphorus absorbers, and humic acid. Nitrogen fixers such as Rhizobium (in symbiosis with legumes), Azospirillum, and Azotobacter convert atmospheric nitrogen into ammonia. Bacillus and Pseudomonas are examples of microbes found in phosphorus-solubilizing biofertilizers that lower the soil pH to dissolve soil-bound phosphate for plant availability and may be most effective for calcareous soils. 

 

Cover Crops:

Nebraska Farmers Share Cover Crop Why's and How's | CropWatch | University of Nebraska–LincolnCover crops can improve soil physical properties, nutrient cycling, and soil microbial activity. In addition, cover crops can scavenge residual nitrogen mineralized from soil and organic amendments before it is lost to volatilization, runoff, or leaching. However, like other sources of organic nitrogen, nitrogen contained in cover crop biomass is not entirely available to the next crop. It is important to consider the C:N of the cover crop residue. Species with high C:N (>20:1; e.g., grasses) result in net immobilization of soil nitrogen in the short term, whereas nitrogen will be more readily available following decomposition of species with low C:N (<20:1; e.g., legumes).

 

 

 Crop Rotation:

The Benefits of Crop Rotation and Diversity – U.S. Farmers and Ranchers In ActionCrop rotation can contribute to improved soil physical properties, pest management, nutrient availability, nutrient use efficiency, and crop yield. Legumes in the rotation (e.g., beans, alfalfa, or clover) can result in a nitrogen credit for subsequent crops due to biological nitrogen fixation and less nitrogen immobilization compared with a non-legume as the preceding crop. However, the nitrogen credit for forage legumes is greater than for grain or vegetable legumes because most of the nitrogen (whether scavenged from the soil or biologically fixed) is removed during harvest of the grain. In contrast, forage legumes contribute nitrogen from vegetative biomass, which can be comparable to cover crop nitrogen contribution if the forage has time to regrow between the last cutting and termination. Cereal and other non-legume crops in rotation can be useful for building soil organic matter–an important reservoir of nutrient in organic farming. Root architecture is important to sustainable crop rotation. Deep-rooted crops like alfalfa can scavenge immobile nutrients like phosphorus and leached nutrients like nitrate-nitrogen from deep soil layers, which may be released near the soil surface with decomposition of the crop residue for availability to subsequent crops in rotation.

 

Nutrient management on organic farms requires long-term planning and a diverse combination of cultural practices and inputs. There are an increasing number of commercially-available organic fertilizers and biofertilizers, but the most profitable organic farms typically source nutrients on or very near the farm by using organic wastes, scavenging residual soil nutrients, and biological fixation of nitrogen.

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