Nitrogen, Phosphorus, and Potash in Agriculture

Agriculture therefore depends on there being a sufficient supply of inorganic nutrients to plants. Cereals, vegetables, fruit-bearing trees or plants, and animal fodder require bioavailable nutrients, that is, nutrients in forms that they can use. Since intensive agriculture depletes many natural nutrients, synthetic nutrients (fertilizers) must be supplied.1-7 In particular, we need to fix the inert N2 of the atmosphere as soluble, reactive compounds such as nitrates, ammonia, and ammonium salts. Other major fertilizer components are sulfate, potassium, and phosphate ions. It may also be necessary to provide trace nutrients, such as cobalt compounds, or to remove excess soil acidity by treatment with lime (CaO). World fertilizer demand in the year 2001 is expected to be about 1.5 x 10s metric tons N, 7.6 x 107 metric tons P2O5, and 6.7 x 107 metric tons K2O these projections represent an 

Apart from minor sources such as thunderstorms (Section 9.2), atmospheric nitrogen is fixed in Nature by certain soil bacteria, blue-green algae, and microorganisms in the root nodules of legumes. This is accomplished either by oxidation to nitrate 

Reaction 9.1 might seem to be thermodynamically favored, but in fact no kinetically easy route from triply bonded N2 to N03 exists, since the endothermic intermediate NO (Section 8.4.2) is likely to be involved. As written, reaction 9.2 has prohibitive energetics, but in practice the process is more complex than this. For example, the fact that free 02 is not formed, but is in effect consumed in other biochemical reactions, makes for a favorable energy balance. The limiting factor is again kinetics, as plausible intermediates such as hydrazine (H2N—NH2) are endergonic compounds. 

The prospect of mimicking the ability of nitrogenases to fix N2 in mild aqueous conditions is attractive in view of the high energy and capital 

As noted in Section 8.4, atmospheric N2 and 02 combine endothermically in small but significant yield and at a sufficiently rapid rate above about 2000 K, but the gases must be quenched rapidly if the high-temperature yield of NO is to be recovered for subsequent conversion to nitric acid  

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