Integrated Nutrient Management in Rice

Intensive rice cropping with short-duration high-yielding varieties along with increased use of mineral fertilisers and improved irrigation facilities have resulted in spectacular increases in crop productivity.  This has, however, led to gradual replacement of organic manures as sources of plant nutrients.  There has been a sharp increase in the prices of P and K fertilisers following withdrawal of subsidy, which has led to their decreased consumption by the farmers.  The low purchasing power of the farming community and the issue of soil health have again renewed interest in organic recycling.  Organic sources available for use in rice production include the bulky organic manures like FYM, quick growing leguminous shrubs grown in the cropping sequence, leguminous trees grown in alley formations and using their loppings as mulch materials, forage or food legumes properly inoculated with Rhizobia and grown in the sequence, blue green algae and Azolla.  Yield potential of both the crops in rice-based cropping systems can be realized by organic manuring of Kharif rice with the available sources along with mineral fertilization of both the crops in the acid lateritic soil conditions of eastern India.  Highest yields of Kharif and Rabi rice were obtained with application of green manure and N application.

Long-term experiments have shown that neither organic sources nor mineral fertilisers alone can achieve sustainability in crop production.  Continuous use of FYM is effective in stabilizing rice productivity under low to medium cropping intensity where the nutrient demand is relatively small.  Nonetheless, integrated use of organic and mineral fertilisers has been found to be more effective in maintaining higher productivity and stability through correction of deficiencies of secondary and micronutrients in the course of mineralisation on one hand and favourable physical and soil ecological conditions on the other. Organic manuring also improves the physical and microbial conditions of soil and enhance fertiliser use efficiency when applied in conjunction with mineral fertilisers.  Thus, all the major sources of plant nutrients such as soil, mineral, organic and biological should be utilised in an efficient and judicious manner for sustainable crop production in rice-rice cropping system.

Several studies have been conducted on the complementary use of organic and mineral fertilisers in cropping systems involving rice.  Extensive field investigations in the acid lateritic soils at Kharagpur revealed that locally available organic materials such as chopped straw, FYM, water hyacinth compost, Azolla and green manure in situ with sunnhemp and dhaincha can substitute N fertilizer up to 50 per cent of the total crop requirement.  In a study on fertilizer economy through organic manures in rice-rice cropping system, it was found that total produce with application of 90 kg N + 12 t FYM/ha to Kharif rice and 60 kg N/ha to Rabi rice was close to 120 kg N/ha +60 kg P2O5/ha to Kharif rice and 60 kg N/ha to Rabi rice.  Annual dressing of 10-15t FYM/ha along with optimal NPK fertilisers enhanced rice yield over the optimal NPK dose by 25 and 20 percent in Kharif and Rabi season respectively under the lateritic soils of Bhubaneshwar.  It was however more beneficial to apply the organic manure in the Kharif season because of its better decomposition and mineralisation of nutrients in soil.  Further, the effect of FYM application was found to be comparable or even superior to lime amendment on the acidic soils which seemed mainly due to the formation of A1-organo-chellates, resulting in the reduction of A1 concentration in soil solution to levels beneficial to plant growth.  Similarly, in the red soils of Hyderabad, the average increase in rice yield due to NPK + FYM was 17-21 percent in both Kharif and Rabi seasons.  These and other results suggest that substitution of 25-50 percent of the N through FYM in Kharif and application of 75-100 percent optimum level of NPK in the Rabi produced the highest yields of rice.  The beneficial effect of organic manures in increasing nitrogen-use-efficiency was also evident, particularly in the Rabi season.  Further application of NPK fertilisers alone did not influence the yield of succeeding rice crop but the residual effect was observed with the incorporation of organic manure.The grain yield of Kharif rice was increased by 21-22 percent and 10-13 percent with the application of Azolla or wheat straw and FYM or water hyacinth compost, respectively.  The residual effect of these sources on the succeeding Rabi rice showed a yield increase of 14-18 percent and 8-10 percent compared with the control.  Further, the residual effect was more pronounced with integrated used of NPK fertilisers and organic manures.  Therefore, integrated nutrient supply is important as a soil ameliorant in alleviating the adverse soil ecological conditions as in the case of acid lateritic soils and in improving soil fertility and productivity.

Green manure of legume shrubs or tree loppings has been known to be beneficial for sustaining rice productivity.  Sunnhemp and dhaincha are popular legumes for green manuring in rice and can accumulate up to 100 kg N/ha in 50-55 days.  Incorporation of these green manures in situ before transplanting rice supplies about 45-60 kg N/ha, besides providing a significant residual effect to the succeeding crops.  Fertilizer use efficiency is improved when a legume crop such as Sesbania cannabina  or Lathyrus sativus is introduced in rice-rice cropping system.  Adding loppings of leguminous trees like Leucaena leucocephalla and Glyricidia napus grown in alleys can also meet the crop N requirement substantially. The productivity of rice-rice cropping system can be increased by about 1 t/ha besides a net saving of 30 kg fertilizer N/ha by including a short-duration legume such as cowpea or greengram and incorporating its residues into the soil after  harvesting the grains. Similarly, blue green algae culture in the rice field can contribute about 25 kg N/ha to the rice crop.  Algae multiply and cover the field like a carpet which when incorporated into the soil, decomposes and releases N for rice crop.  Azolla can be grown in tanks or in rice fields and incorporated into the soil after 4-6 weeks. Nitrogen contribution through Azolla dual cropping with rice has been worked out to be about 25-30 kg N/ha.  Azolla growth is generally poor without P fertilisation but a substantial improvement in growth and yield of rice is achieved when dual cropped Azolla is fertilized with P and incorporated into the soil at a later stage.  An application of about 30 kg P₂O₅ /ha is adequate for optimum growth of Azolla.  The use of Azolla appears to be promising in the Rabi season when there is good water management and availability of other organic sources like FYM and green manures is limited.