CONTROLLED-RELEASE FERTILIZERS TO INCREASE EFFICIENCY OF NUTRIENT USEAND MINIMIZE ENVIRONMENTAL DEGRADATION - A REVIEW

Total world consumption of fertilizer N, P2O5, and K2O in 1990/1991 wa s 78, 37, and 26 million tons per annum, respectively, with a projecte d yearly increase of demand of about 2 to 3%. Trends in crop productio n (maize and wheat) in the last four decades show that N application r ates increased about 15 times whereas its accumulation in grain increa sed only 3 to 4 times. At the same time nutrient recovery by crops rem ained relatively low (e.g. about 50% for N). This represents a potenti ally alarming situation from environmental, economic and resource cons ervation points of view and indicates an urgent need for improving eff iciency of fertilizer use. Anticipated benefits from slow/controlled r elease fertilizers (SRF/CRF) are addressed through two main processes: a. nutrient availability in the plant-soil system as affected by the interaction/competition between: plant roots, soil microorganisms, che mical reactions and pathways for loss; and b. matching nutrient releas e with plant demand. The various aspects of fertilization and environm ental hazards associated with SRF/CRF and factors affecting nutrient u se efficiency (NUE) are discussed in the light of these controlling pr ocesses. Environmental aspects include: pollution by nitrate, phosphat e, and emission/volatilization of N2O or NH3; quality of food and fibe rs; and factors affecting soil degradation. Agronomic or physiologic a spects include: reduced losses of nutrients, labour saving, reduction of specific stress or toxicity, increased availability of nutrients an d induction of synergistic effects between specific chemical forms of nutrients (e.g. interaction of mixed NH4/NO3 nutrition with K, effects of physiological acidification of the rhizosphere on P and Fe availab ility etc.). Despite the environmental and agronomic benefits offered by SRF/CRF their practical use in agriculture is still very limited. P ossible measures which may encourage their use in practice are: a bett er assessment of expected benefits; attainment of improved technologie s or concepts for producing more efficient and less expensive SRF/CRF; optimal design of fertilizer compositions to induce synergistic effec ts; better understanding of the mechanisms which control nutrient rele ase; construction of conceptual and mathematical models for predicting release rates and patterns under both laboratory and field conditions , for supporting the technologist, farmer and environmentalist in thei r decision making.