The concept of two global commonalities–biological diversity and nutrient cycling among agro ecosystems is supported by the literature on ecosystems and their management anecdotal account of indigenous practices, and the rapidly emerging literature on agro ecology. Organic matter is the basis of all bio-geo chemical cycles.
The fundamental issues concerning efficient use of organic matter are leakage of nutrients from agro ecosystems and the rates of decomposition. Organic matter and the nutrients if contains are lost from soils by run off and mineralization (Tiuy, 1990), both of which can be controlled by appropriate tillage practices (Campbell et. al., 1995); Lal et. al., 1994).
Loss of nutrients to mineralization is also controlled by assuring sufficient inputs of plant or animal material to maintain the soil organic matter (SOM) reserves (Woodmansee, 1984). Legumes are important in maintaining SOM and increasing soil N suffer. In addition, they prefect the soil from run off water and wind erosion and improve infiltration, agro forestry systems use leguminous and other trees to provide alternative crops (Steppler and Lundgren 1988), produce animal forage and fuel, recycle nutrients for crop use and project soil from wind and water erosion (Altieri, 1987).
Plant biodiversity plays an important role in pest, disease, and weed management. Crop rotations are effective in controlling pests, diseases and weeds (Altieri, 1987). Living mulches control weeds and minimize the need for herbicides (Regnion and Jahnke, 1990); Increases in structural diversity within the crop canopy leads to greater diversity in insects and less damage from insect pests (Stinner and Blair, 1990).
Integration of animals into Agro ecosystems offers further diversity and stability. McInfire and Cryseels (1987) summarized the potential benefits of integration of crops and animals. Integration of animals facilitates nutrients movement and increases the opportunities for efficient nutrient management across the whole farm system. Animals increase overall net productivity of the farm and reduce environmental degradation by serving as alternatives to crops on the marginal areas of farms by utilizing crop residues as feed.
Optimizing Nutrient Availability In Sustainable Agricultural System
A very important condition for good plant growth and health and, indirectly, for good animal and human health is the timely provision of sufficient and balanced quantities of nutrients that can be taken up by the plant roots. Nutrient deficiencies and imbalances are main constraints to crop production, especially in regions with poor and very poor or alkaline soils. There is a constant flow of nutrients through the farm.
Some of the nutrients are lost by export of products, erosion, leaching and volatilization. For example, it has been estimated that in Africa nutrient losses through soil erosions and other processes exceed application of artificial fertilizers (Stocking, 1986). If the farm is to remain productive it must be ensured that the amount of nutrients leaving the farm does not exceed the amount returned to it. In other words, over time, there must be a positive nutrient balance.
Micronutrient Deficiencies In Sustainable Agricultural System
Due to intensive cropping the micronutrients are removed to a considerable extent, which control various aspects of plant growth. A study at Ranchi, India revealed that applying looks NPK (10:25:25) per ha. Led to depletion of Zinc by 0.619/ha and copper by 0.49/ha. this can depress yields by up to 4t/ha in rice, 2 t/ha in wheat and 3.4 t/ha in maize. Also iron is a limiting factor in rice production in the new rice-wheat rotation evolved in the non-traditional rice growing areas of Punjab.
One of the solutions to correct this micronutrient deficiencies is greater use of organic manures and multiple cropping with legumes. At Punjab Agricultural University, Ludhiana field experimental results proved that application of poultry manure, pig manure and farmyard manure were effective in meeting zinc requirements in a maize-wheat rotation. Also cultural practices such as prolonged submergence of the field can be used to tackle iron and manganese deficiencies (Sharma, 1985).
Limiting Nutrient Losses In Sustainable Agricultural System
Nutrient losses can be limited by:
• Recycling organic wastes by returning them to the field, either directly or treated (composted, fermented etc.).
• Applying organic and artificial fertilizers in such a way that nutrients are not leached by excessive rain or volatilized by high temperature or solar radiation.
• Reducing losses due to run-off and soil erosion.
• Minimizing nutrient losses due to biomass bussing.
• Reducing volatilization of nitrogen by denitrification under wet soil conditions.
• Avoiding leaching by using organic and artificial fertilizers, which release nutrients slowly, maintaining high humus content in the soil and intercropping plant species with different rooting depth.
• Limiting nutrient export in products by producing crops with relatively high economic value relative to nutrient content.
Use of Chemical Fertilizers In Sustainable Agricultural System
The use of chemical fertilizers is essential for obtaining high crop yields. However, many small landholders and resource-poor farmers cannot offer costly fertilizers. Most soils in the tropics are so deficient in primary nutrients that it is imperative that strategies be developed for adding them from outside the ecosystem.
There is some potential for enhancing N supply by biological N fixation. Additional N and other nutrients must be supplied. The requirements for chemical fertilizers, however, can be reduced considerably by decreasing losses, recycling nutrients and through biological N fixation.
Nutrient Recycling In Sustainable Agricultural System
Nutrient recycling or regime is an important strategy for sustainable crop production. It involves returning nutrients removed by crops to the soil for further use. In addition, soil fauna (e.g., earth worms, termites) also play an important role in recycling of plant nutrients.
Growing deep-rooted crops is important in order to recycle nutrients from the sub soil by returning them through crop residue to the surface where the succeeding shallow rooted crops can use them. Use of mulches, incorporation of crop residues and animal waste, growing legumes as intercrops in cereals etc., can substantially reduce chemical fertilizer requirements.
Use of Crop Residues In Sustainable Agricultural System
Crop residues contain substantial quantities of plant nutrients. The beneficial effects of returning crop residues as mulch on crop yields are well known (Akimbo and Lal, 1980 and Kang, 1993). These benefits are not only to the recycling of plant nutrients but also to improvements in soil moisture and temperature, enhancement of soil structure and soil erosion control.
Nutrient Composition (%) of Crop residues of Major Crops grown in the tropics
Biological Nitrogen In Sustainable Agricultural System
Fixation Augmenting the nitrogen supply to crops through biological nitrogen fixation is a viable officer for resource– poor farmers of the tropics. The amount of N fixed by legumes can range from 20–250 kg/ha/yr depending on the species, soil type, climate and agro-eco-region.
Quantities of N field by various Legume Crops
Use of Biofertilizers In Sustainable Agricultural System
Biofertilizers have been recognized as important inputs in integrated plant nutrition systems. The use of legume green manure, blue green algae and Azolla for rice: Azotobacter and Azospirillum for wheat, millets and vegetable crops; Rhizobium for pulses and oil legume crops, Phosphate solubilizers Vesicular Arbuscular Mycorrhizae) for various crops is well reported, on an average these biofertilizers can minimize the use of inorganic N by 25–50 kg/ha.
Green Manuring In Sustainable Agricultural System
The green manure crops when applied improve the physical and chemical properties of the soil. Green manures also increase the fertilizer use efficiency of crops when applied in combination with inorganic fertilizers. Among the green manure crops, special attention is being given to Sesbania rostrata, which bears stem nodules in addition to the root nodules.
The amount of N contributed in terms of fertilizer N equivalence ranges from 80–120 kg/ha. In a field trial comprising different green manure crops, it was found out that Sesbania rostrata produced the highest biomass (20–25 tons/ha) and accumulated a maximum of 150–220 kg N/ha.
Common Green Manure Crops and their N content
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