No matter how brightly the sun shines and how often the rain falls, and no matter how equable the temperature is, productivity must be low if there is no soil in a terrestrial community, or if the soil is deficient in essential mineral nutrients. The geological conditions that determine slope and aspect also determine whether a soil forms, and they have a large, though not wholly dominant, influence on the mineral content of the soil. For this reason, a mosaic of different levels of community productivity develops within a particular climatic regime. Of all the mineral nutrients, the one that has the most pervasive influence on community productivity is fixed nitrogen (and this is invariably partly or mainly biological, not geological, in origin, as a result of nitrogen fixation by microorganisms). There is probably no agricultural system that does not respond to applied nitrogen by increased primary productivity, and this may well be true of natural vegetation as well. Nitrogen fertilizers added to forest soils almost always stimulate forest growth.
The deficiency of other elements can also hold the productivity of a community far below that of which it is theoretically capable. A classic example is deficiency of phosphate and zinc in South Australia, where the growth of commercial forest (Monterey pine, Pinus radiata) is made possible only when these nutrients are supplied artificially. In addition, many tropical systems are primarily limited by phosphorus.
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