AGRONOMIC MEASURES FOR BETTER UTILIZATION OF SOIL AND FERTILIZER PHOSPHATES

Global known phosphate deposits are a finite resource which will run o ut in about four centuries at the present consumption rate. Since abou t 90% of the phosphate mined is used for fertilizer, soil and fertiliz er phosphate should be efficiently used. Various agronomic measures ar e discussed relevant for saving phosphate and avoiding losses. Phospha te fertilizer rates should be adjusted to measured requirements for ph osphate using soil tests. Particularly in areas with high livestock in tensities soils frequently are much enriched in available phosphate an d do not need further phosphate application whether in organic or in i norganic form. Excessively high levels of available soil phosphate, mu ch higher than required for optimum crop production increase the hazar d of phosphate loss by wind and water erosion and even leaching. Loss of plant available phosphate in soils occurs by phosphate fixation whi ch is especially strong in acid mineral soils. Such losses can be dram atically reduced by liming soils to a pH of 6-7. In tropical areas whe re lime frequently is not available row placement of phosphate fertili zer is recommended. Oxisols with a very low pH liming, however, may pr omote phosphate fixation due to the formation of phosphate adsorbing A l complexes. Biological assimilation of phosphate may prevent inorgani c phosphate from fixation by soil particles. Organic anions produced d uring the decomposition of organic matter in soils as well as the excr etion of anions by plant roots depress phosphate adsorption by competi ng with phosphate for binding sites at the adsorbing surface. Hence fa rming systems and rotations which bring much organic matter into soils contribute to a better use of soil and fertilizer phosphate. Mycorrhi zation of plant roots with appropriate fungi ecotypes may essentially improve the exploitation of soil phosphates. The choice of the appropr iate phosphate fertilizer type is crucial for its efficient use. This applies particularly for apatitic fertilizers of which the availabilit y is poor in weakly acid to neutral and calcareous soils. (C) 1997 Els evier Science B.V.