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I or fall over with the slightest wind. Also, crop maturity is delayed and the plants are more susceptible to diseases and insect pests. An oversupply of nitrogen may adversely affect grain and fruit quality, as in barley, apples, and peaches. It also adversely affects sugar levels in sugar cane or sugar beets. ejected to It must not be inferred, however, that all plants are detrimentally affected ery good by large amounts of nitrogen. Many crops, such as the grasses and vegetables, is small, ; should have plenty of this element for their best and most normal development. large. At Detrimental effects are not to be expected with such crops unless exceedingly is lost in large quantities of nitrogen are applied. Nitrogenous fertilizers may be used it is lost freely in such cases, the cost of the materials in respect to the value of the unavail crop increases being the major consideration. y marked !s occurs. , roots soil 9.2 Forms of Soil Nitrogen e. Others, >r through. There are three major forms of nitrogen in mineral soils: (a) organic nitrogen -ombined associated with the soil humus, (b) ammonium nitrogen fixed by certain clay at should minerals, and (c) soluble inorganic ammonium and nitrate compounds. I Most of the nitrogen in surface soils is associated with the organic matter. In this form it is protected from rapid microbial release, only 2-3% a year being mineralized under normal conditions. About half the organic nitrogen is in the form of amino compounds. The form of the remainder is uncertain. Some of the clay minerals (e.g., vermiculites and some smectites) have 'mpoun the ability to fix ammonium nitrogen between their crystal units. The amount I enzyme fixed varies depending on the nature and amount of clay present. Up to 8% I w' of the total nitrogen in surface soils and 40% of that in subsoils may be in ( I' ell nuc e ' the "clay-fixed" form. In most cases, however, both these figures would be ntial f considerably lower. Even so, the nitrogen so fixed is only slowly available develo to plants and microorganisms. I s usu kThe amount of nitrogen in the form of soluble ammonium and nitrate com- I I est an pounds is seldom more than 1-2% of the total present, except where large i and ' i, applications of inorganic nitrogen fertilizers have been made. This is fortunate f fumpn I�II since inorganic nitrogen is subject to loss from soils by leaching and volatiliza- pho tion. Only enough is needed to supply the daily requirements of the growing I� �spho � ccul crops. •I po, 8.3 The Nitrogen Cycle' and' ch In all soils the considerable intake and loss of nitrogen in the course of a year are accompanied by many complex transformations. Some of these changes may be controlled more or less by man, whereas others are beyond re ho tey) For this and subsequent sections on nitrogen, the reader is referred to the monograph edited by Stevenson (1982). 9.3 The Nitrogen Cycle 2$5 a