METABOLISM 71 



The amount of nitrogen taken from the soil, annu- 

 ally, by an average crop in Alsace was estimated 

 at approximately 46 Ibs. to the acre. Less than 

 half this amount is returned directly to the soil 

 (mostly as volatile ammonia). Moreover, every 

 inch of rain falling on the land and draining through 

 it causes an additional loss of something like 2J Ibs. 

 of nitrogen to the acre. There must be an excess of 

 nitrogen, therefore, in the soil over and above 

 what the plant life demands. Here again, as was 

 the case with the carbon, the demand would seem to 

 greatly exceed the supply. 



Since man first began to develop the art of agricul- 

 ture, he has practiced various methods of replenishing 

 the soil from which his crops have taken their foods. 

 Especially has he used various sorts of manures, 

 which contain nitrates and phosphates. Modern 

 man has added to these fertilizers various mineral 

 substances which he quarries from the earth, such 

 as phosphate of lime and " potash " (potassium 

 phosphates and sulphates). The so-called " basic 

 slag," a residue from metal smelting, which contains 

 12-20 per cent of phosphoric acid, is nowadays 

 finely ground and largely used to replenish the supply 

 of phosphates. The weathering of the rocks also 

 slowly adds to the soil the soluble components 

 needed by organic life, and of course this was their 

 original source. But whatever man can return to 

 the soil is obviously insignificant compared with 

 what Nature's crops remove. The huge stores 

 of nitrogen and carbon constantly built up into 



