Manures and Fertilizers

Kiran Yadav

GBPUAT, Pantnagar

Organic manures and chemical fertilizers are both important for rice cultivation. Application of bulky organic manures is desirable particularly under upland conditions, to maintain the soil in good physical condition and to increase the water holding capacity of the soil for maximum utilization of rain water. About 10-15 tones of well rotted farm yard manure or compost per hectare should be applied 4 to 6 weeks before sowing.

Application of chemical fertilizers depends basically upon the fertility status of the field, previous crop grown and amount of organic matter added. Soil testing is the simplest and easiest scientific method to know the quantity of nitrogen, phosphorous and potassic fertilizers that are needed to get optimum rice yielded in a field.  

Soil fertility is a very variable factor. It varies in different agro-climatic zones. A generalized fertilizer recommendation does not take into account the variability in soil fertility and can not assure optimum yield levels from the crop. Most Agriculture Departments and Agricultural Universities in India have formulated fertilizer recommendations for rice appropriate to local conditions.

Fertilizer management in transplanted paddy field id different from upland rice because continuous submergence of the field creates conditions which are not obtained in upland well drained fields. Presence of excess water in paddy fields sets in series of reactions - physical, chemical and biological in the soil. The root zone is converted from aerobic to anaerobic environment in the paddy soil which is responsible for gaseous loss of fertilizer nitrogen by phosphorous as well as micro-nutrient especially iron and manganese. 

The low land puddle soils remain waterlogged during the entire growth period of the crop. The important change in the soil due to water logging conditions is the conversion of the root zone of the soil from an aerobic to anaerobic environment due to depletion of oxygen in the soil profile. The flooded or water logged puddled soil develops two zones. The upper zone ( 1 to 10 millimeter thick) reccieves oxygen periodically from fresh supplies of irrigation water turns brown in color called 'oxidised zone' and reacts like an unflooded upland soil. The remaining lower portion of puddle soil without oxygen is called 'reduced zone'. Ammoniacal nitrogen fertilizers when applied in such a soil system get oxidized to nitrate form (NO₃^­­) in the oxidized zone (surface layer of the soil). This nitrate nitrogen in turn leaches down to the reduced zone (sub surface layer) and gets denitrified to gaseous nitrogen. This loss of nitrogen can be prevented by incorporating ammoniacal nitrogen into the reduced zone of the soil where it is held.

Fertilizers containing nitrate such as calcium ammonium nitrate (CAN), ammonium sulphate nitrate (ASN) and certain NPK complex fertilizers or mixtures are more susceptible to loss of nitrogen through leaching and denitrification then are urea, ammonium sulphate, ammonium chloride or NPK complex fertilizers containing no nitrate. Use of nitrate fertilizers should, therefore, be avoided in paddy. Diammonium phosphate (DAP) is an excellent fertilizer for basal dressing in paddy fields.

Sulphur coated urea has been found better than normal urea for paddy fields where flooding is delayed or intermittent. In acid soils, rock phosphate in finally powered form can be used as phosphorous source with advantage but it should not be applied in neutral or alkaline soils.