In developed countries, the consumption of meat and meat products has risen markedly. Animals convert plant protein to animal protein at low efficiency, and hence there has been a parallel increase in the demand for grain and livestock feed concentrates. The change in human dietary habits has therefore led to a substantial rise in fertilizer use. The increased application of chemical fertilizers supplying the plant nutrients (nitrogen, phosphorus, and potassium) is an essential component of modern agriculture. Plants rarely use more than 50-60% of the nitrogen in fertilizers or 30% of that in animal manure. The residual nitrogen (nitrate) is liable to pollute ground and surface waters, causing over-enrichment (eutrophication), while some may be converted to nitrogen oxides.
The use of fertilizers changes both the quantity and the quality of crops. For instance, nitrogen fertilizers increase the protein content in maize and wheat grains, just as the phosphorus content of crops increases with the application of phosphate fertilizers. However, added nutrients may be detrimental in the case of legumes. If the plant takes up an excessive dose of nitrogen and cannot convert it into protein, the nitrogen remains in ionic form in the plant and might cause harm to the human body, particularly if it is transformed into a nitrate compound, and consumed as such. Furthermore, the interaction of nutrients can considerably change the composition of food. In certain vine experiments in 1982, the excessive use of potassic fertilizers resulted in a magnesium deficiency. This proves that food quality is greatly influenced by the primary and additional nutrient content of fertilizers. The general practice is to increase the nutrient content of fertilizers so as to reduce costs and to improve the efficiency of transport.
The micro-elements contained in fertilizers are no less important. Depending on the origin of raw materials and on how far they have been processed, different quantities of micro-elements can be found both in phosphorus and potassic fertilizers. Because of overdoses of phosphate fertilizers, plants take up zinc in insufficient quantities. The lack of zinc results in deficiency symptoms, and can cause illness in animals fed with such fodder.
Nitrogen is the most limiting element for crop production. Traditionally, expensive commercial fertilizers are used to correct soil nitrogen deficiencies. 50% of the increase in rice yields after WW II can be attributed to increased fertilizer nitrogen use.
Excessive reliance on mineral fertilizers has resulted in pollution of surface and subsurface waters and tends to lead to loss of soil structure. FAO estimates that world consumption will increase to about 84 million tonnes in the 1985/1986 period. The developing countries (70% of the human population) use only about 15% of the world's fertilizers today, but this proportion is certain to change in the near future.
Nitrous oxide generated by applications of fertilizer, notably low-grade and highly volatile ammonium bicarbonate fertilizers commonly used in countries like China.