According to Liebig's principle, N supply of soils required leguminous crop species which not only supplied crops directly required for men but a substantial part of leguminous crops was required for animal forage. Hence men and animals competed for fertile soils. This situation changed drastically when the chemical production of N fertilizers was invented (Haber-Bosch technique). Now farming was possible without animals, which was the preposition for the separation of crop farming from animal farming. This had an enormous impact on the economics of farming and a disastrous one on ecology. In many animal farming systems land is overdressed with farmyard manure or slurry. In the Netherlands, the average surplus of mineral N, mainly nitrate, for all dairy farms is in the range of 200-400 kgNha~ . This quantity of plant-available N would suffice to produce 6000-12 000 kg wheat grain per ha per year. This surplus of N leads to an enormous nitrate leaching with a risk for the groundwater and also for denitrification by which N2O, a greenhouse gas, is produced.
Australian sheep farmers did not follow Liebig's recommendation as they did not lime their pastures. Consequently, during a period of 50 years, soil pH gradually decreased from about 6 to 4.8. At this low pH, soluble and exchangeable Al and Mn oxides/hydroxides are formed which are toxic to plant roots, particularly to leguminous species and affect their growth, particularly that of Trifolium subterraneum which is the most important symbiotic N2 fixer in Australian pastures. Thus the soils were rendered infertile. By this one of the most important worldwide resource, limiting resource, the fertile soil, is diminished. There are other soil processes which are less spectacular but still have a serious impact on soil fertility.
Potassium is an indispensable plant nutrient. In addition, it is indispensable in most fertile soils as an essential element of the structure of 2:1 clay minerals. Tributh et al. found that cropping fertile soils containing 2:1 clay minerals without K fertilizer application lose gradually their fertility because plant roots take up K+ from the 2:1 silicate structure and thus finally destroy the mineral structure which means a loss of fertility. These minerals are capable to store K+ and NH| and protect these important plant nutrients against leaching. Plant roots feed from thus-bound K+ and NH|. The nutrients thus taken up by the crop must be replenished by fertilizer application in order to maintain soil fertility as according to Liebig's principle what was taken from the soil should be given back. This degradation of a 'fertility mechanism' is a slow but irreversible process which renders eventually fertile soils infertile.
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