284 Nutrients 



should again be placed on the fact that these factors are often related 

 and then- effects mutually interdependent to a considerable extent. 



The most commonly deficient nutrients in the soil are phosphate, 

 nitrate, and potassium. A general discussion of the factors tending 

 to conserve or to deplete these materials in the terrestrial substratum 

 was presented in Chapter 3. The existing concentration of these 

 nutrients depends upon the composition of the parent rock material 

 and the extent of the modification of it by leaching and upon the 

 decomposition of biological products. Our information on the limita- 

 tion of plant growth by lack of phosphate, nitrate, or potassium in the 

 soil is derived chiefly from agricultural research. A consideration 

 of this vast body of knowledge is obviously beyond our present scope, 

 but the reader will find further discussions in the Year Books of the 

 U.S. Dept. of Agriculture, in Meyer and Anderson (1952), and in 

 Lyon, Buckman, and Brady ( 1952 ) . 



Under natural conditions the phosphate removed from the soil by 

 plant growth is in part replaced by the breakdown of the parent rock, 

 although this process is extremely slow and some rocks contain little 

 or no phosphorus. Of much greater significance in the restoration of 

 phosphate to most soils is the decomposition of organic matter. Since 

 this material is derived from the bodies of dead organisms and from 

 animal excreta (Hutchinson, 1950), the process represents a critical 

 instance of the reciprocal action of organisms on their environment. 



Replenishment of nitrate and of potassium in the soil is similarly 

 dependent primarily on the decomposition of organic material, but 

 the latter element is also derived from potassiferous silicates found in 

 most rocks. Supplementary supplies of nitrate are derived from am- 

 monia formed in rain water by electric discharges in the upper air 

 (Hutchinson, 1944) and from the fixation of free nitrogen by certain 

 soil bacteria. 



Not only must the essential nutrients occur in the soil in sufficient 

 quantity, but, even more important, they must be present in available 

 form. Iron, manganese, magnesium, zinc, and sometimes phos- 

 phorus remain in essentially unavailable states in soils that are too 

 alkaline. On the other hand, under strongly acid conditions phos- 

 phorus forms insoluble phosphates with iron and aluminum, nitrates 

 cannot be readily formed from ammonia, and certain elements may 

 become so soluble as to attain toxic concentrations. One of the chief 

 reasons for adding lime to agricultural soils is to correct unfavorable 

 acidity and thus to render nutrients available for absorption by the 

 plants. 



Calcium has important effects as a nutrient in addition to the part 



