PHOSPHORUS AND LIFE — HOPKINS 237 



phorus in animal life. It is not only an essential constituent of all cell 

 nuclei but also a major constituent (as calcium phosphate) of bone 

 substance; it has, therefore, a dynamic "life process" function and a 

 less dynamic structural function. Nearly 25 percent of the total min- 

 eral content of an animal body may be phosphorus, but as much as 

 four-fifths of this amount is held as calcium phosphate in the skeleton. 

 In the final decomposition of the animal body after death, the phos- 

 phorus in the flesh will be fairly quickly returned to the soil ; but the 

 decomposition of solid bones is exceedingly slow. So, in the soil-* 

 plant-»animal->soil sequence, the final closing of the cycle involves, 

 at any rate for as much as 80 percent of the animal-assimilated phos- 

 phorus, a serious time lag. 



Did the expansion of plant and animal life on land with its increas- 

 ing utilization of rock-derived and dissolved phosphate reduce the 

 flow of land phosphate into the sea and the marine cycle? There 

 would seem to have been always a sufficient release of soluble phosphate 

 from the land to the sea, a fact that supports the idea that soil forma- 

 tion and plant life led to an increase in the previous rate of rock 

 denudation. There is, however, little "washing out" of phosphate 

 from soils by rain for the actual existence of soluble phosphate in soils 

 is quite brief. Like other nutrients, phosphorus can be assimilated by 

 plants only when it is available in the soil solution. But if phosphate 

 in the soil solution is not quickly removed by plant uptake, it is precipi- 

 tated by other soil constituents. Insoluble calcium phosphate is readily 

 formed ; however, this need not be a long or permanent withdrawal 

 from the "system" for calcium phosphate can be equally readily re- 

 dissolved by weak soil acids. But iron and aluminum phosphates are 

 also formed (especially in acid soils) and from these compounds 

 phosphate recovery is slight and slow. These processes are known as 

 soil fixation. Soil fixation prevents a considerable proportion of the 

 "biological currency" of phosphorus in the sorL-^PLANT->ANiMAL-»soiL 

 sequence from re-entering the cycle when it returns to the soil. In- 

 stead, it enters the soil's reserve of unavailable or not easily available 

 phosphate. There is some resemblance in this to the steady loss of 

 deep-sea phosphate from the marine system. (It is possible that a 

 tiny fraction of the fixed soil phosphate enters the marine cycle ; for 

 some of this accumulation in topsoils will pass downward by gradual 

 and physical movement. In contacts with ground waters the fixed 

 phosphates will be at least as extractable as the apatite of igneous 

 rock. These ground waters, out of reach of most plant roots, making 

 their way to streams and rivers, will carry some of the phosphate to 

 the sea.) 



The slow and almost timeless balance of the land cycle was greatly 

 disturbed by farming. Agriculture meant the ever-increasing growth 



