Untitled - IRRI books - International Rice Research Institute

REMOVING SOIL CONSTRAINTS TO CROP PRODUCTION 15processes determine the amounts of trace elements present in soils. The need forresearch on the most efficient methods and materials to supply micronutrients isalso urgent. There are large differences in the amounts and costs of theseelements needed to correct deficiencies by 1) applications to soil, and 2) foliarsprays. We know little of the value to later crops of the large residues left bysoil applications. There is controversy on the rival merits of applying solublesalts or chelates of the elements to both soil and leaves.Efficiencies of NPK FertilizersThe efficiency of fertilizers has received far too little attention in thepast. The objective of all scientific work on the subject must be to secure 100percent uptake of the applied nutrients by the crop, or series of crops, for whichthe fertilizers have been applied.Nitrogen: Often no more than 30% of the fertilizer nitrogen applied is recoveredby a crop. Much of the remainder is lost by leaching or dentrification in wetperiods, volatilisation of ammonia also occurs; but most early investigationsfailed to account for a considerable proportion of the N applied. In developedcountries little interest was taken in this serious inefficiency of an input,either by fertilizer manufacturers (whose sales would logically be diminished ifthe N sold was more efficient), or by farmers who were more concerned with buyingfertilizer cheaply than with using it efficiently. Discussions on environmentalpollution began to change these attitudes in the late 1960's. Leached nitratedamages the quality of natural waters by eutrophication, and it may pose a problemto health when much is present in drinking water. Nitrous oxide, evolved bydentrification, may interfere with the ozone layer of the stratosphere. Theseconcerns, reinforced by the large increases in the price of oil, and hence in thecost of fixed nitrogen, have given high priority to research on the efficiency ofN-fertilizers.Much success has been achieved. Work with 15 N has made it possible toaccount for about 90% of the N-fertilizer applied (Jenkinson 1982a, 1982b).Modelling of the transformations and movement of forms of N in soil, in relationto soil type, weather and crop, has lead to more precise recommendations forapplying fertilizer (Greenwood, 1982). Growing a larger crop by the use of theirinputs to high-yielding varieties can double the amounts of N recovered by thecrop. Research on the production of forms of N-fertilizers, and on methods ofapplication, that lead to higher efficiency have been successful, particularly forflooded rice culture. IFDC (1982) has taken a leading role in this research toimprove the efficiency of N. McCune (1981, 1982) has described research on theproduction and use of large granules placed deeply in soil, on coating of granulesto delay their solution, and on nitrification and urease inhibitors; all of thesetechniques have a place in raising the efficiency of nitrogen.Phosphate: The reactions by which soluble phosphates are precipitated in soil('fixation' of phosphate) result commonly in not more than a quarter of theapplied phosphate being recovered by the first crop. While there has beenconsiderable advance in understanding of the processes involved, the onlypractical advance has come from placing the phosphate close to seeds or roots ofthe crop which has increased short-term efficiency. The residues of phosphatefertilizers in soil retain some solubility and can benefit later crops, and theyenhance soil fertility. When account is taken of recoveries over very longperiods, phosphate fertilizers can have very high efficiencies. More research isneeded on phosphate economy in developing countries to determine whether nearcompleterecoveries can be expected in the long-term on most soils where residueshave built up the level of soil phosphate. Research, such as that done by IFDC(1982), is required on forms of phosphate, and particularly on the use of