Total world consumption of fertilizer N, P2 O5, and K2O in 1990 / 1991 was 78. 37. and 26 million tons per annum respectively, with a projected yearly increase of demand of about 2 to 3% . Trends in crop production (maize and wh eat) in the last four decades s how that N application rates increased about 15 times whereas its accumulation in grain increased only 3 to 4 times. At the same time nutrient recovery by crops remained relatively low (e .g. a bout 50% for N). This represents a potentially alarming situation from environmental, economic and resource conservation points of view and indicates an urgent need for improving efficiency of fertilizer use . Anticipated benefits from slow/controlled release fertilizers ( SRF / CRF) are addressed through two m a in processes: a. nutrient availability in the plant-soil system as affected by the interaction/competit ion between: plant roots. soil microorganisms. chemical reactions and pathways for loss; and b. matching nutrient r e lease with plant demand. The various aspects of fertilization and environmental hazards associated with SRFI CRF and factors affecting nutrient use efficiency (NUE) are discussed in the light of these controlling processes. Environmental aspects include: pollution by nitrate , phosphate, and e mission / volatilization of N 2 0 or NH 3 ; quality of food and fibers; and factors affecting soil degra dation. Agronomic or physiologic aspects include: reduced losses of nutrients, labour saving, reduction of specific stress or toxicity , increased availability of nutrients and induction of synergistic e ffects between specific chemical forms of nutrients (e.g. interaction of mixed NH4 /N0 3 nutrition with K e ffects of physiological acidification of the rhizosphere on P and Fe availability etc.). Despite the environmental and agronomic benefits offered by SRF / CRF their practical use in agriculture is still very limited. Possible measures which may encourage their use in practice are: a bett e r assessment of expected benefits; attainment of improved technologies or concepts for producing more efficient and less expensive SRF / CRF; optimal design of fertilizer compositions to induce synergistic effects ; better understanding of the mechanisms which control nutrient release; 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 their decision making.
Available at: http://works.bepress.com/robert_mikkelsen/22/