304 Nutrients 



atmosphere, but this represents a reservoir that can be drawn upon 

 only slowly by certain physical and biological agents. Phosphorus, 

 on the other hand, is a rare material: only about V700 of the earth's 

 crust is composed of this element. The only ready sources of phos- 

 phorus are the products of decomposition of the bodies of organisms. 

 We have seen that the supply of phosphate in the rocks and in the 

 deep sea can be obtained at only a very slow rate. Therefore the 

 acceleration of the loss of this element from the land by soil erosion 

 is a critically serious matter for life in general, and for our agriculture 

 in particular, since our food supply is chiefly derived from the ter- 

 restrial environment. Wells, Huxley, and Wells (1939) have said: 

 "Phosphorus is the weak link in the vital chain on which man's civili- 

 zation is supported." 



Regeneration 



The return process in the cycle of materials in natural environ- 

 ments, by means of which nutrients once used are made available 

 again for the further growth of organisms, is known in ecology as 

 regeneration. There are two aspects of regeneration: first, nutrients 

 must be rendered available chemically through the processes of de- 

 composition and transformation that we have traced in the preceding 

 sections; second, nutrients must be rendered available spatially, that 

 is, restored to zones where green plants can grow and resynthesize 

 organic material. Critical minerals that have been carried below the 

 level of plant growth must be brought up again before they can be 

 used. 



Mention has been made earlier of the favorable situation in prairie 

 soils in which rainfall is not sufficient to cause serious leaching, and 

 in semiarid soils in which an upward movement of the ground water 

 takes place. These effects, augmented by the activity of the roots 

 of grasses, account to a considerable degree for the restoration of 

 nutrients to the surface layers in pedocal soils. Regeneration of 

 nutrients in the pedalfer soils is less complete. In the deciduous 

 forest region, however, some restoration of materials to the surface 

 is accomplished by the fall of leaves and other litter, and by the min- 

 ing activities of earthworms and other animals. In the podzol re- 

 gions the greater rainfall produces excessive leaching with the result 

 that nutrient salts are carried beyond the reach of plant roots. 



Here we have an analogous situation to that in the deep parts of 

 the aquatic environment, where decomposition is often completed 

 at depths below those at which green plants can grow. In water 



