Food from the Soil 141 



The same thing is also true of the farmer's crops, 

 and it is for this reason that he varies them, not 

 growing the same crop year after year, or even two 

 years running, on the same soil, lest it should be 

 exhausted and unable to feed them. 



Corn-crops, for instance, take up much flint, which 

 goes chiefly to give the hard, glossy coating to their 

 stems ; and they want from a fifth to a tenth part as 

 much potash. Turnips and beet, on the other hand, 

 take in little flint, but more lime and potash; and 

 turnips and carrots will use up the sulphuric acid; 

 while clovers want little sulphuric acid, but much 

 potash, lime and soda. 



All plants need more or less of several mineral sub- 

 stances, and even when it is Mess,' they cannot do 

 without this lesser quantity, be it never so small. 

 When, therefore, we say that corn-crops take up much 

 silica or flint, it is not at all meant that they do not 

 take some proportion of lime, potash, soda, sulphur, 

 iron and phosphoric acid as well; for they use them 

 all, in larger or smaller quantities. 



The amount taken up of each varies in different 

 kinds of corn ; wheat, oats, barley, etc., have all their 

 special needs, and so, too, have different varieties of 

 the same kind of corn. More than this, different plants 

 of the same variety differ slightly in this respect, as 

 if they had their own individual preferences ; but the 

 difference is very slight, and in plants of the same 

 species, the proportion always remains nearly the 

 same. 



And this is true, no matter where the plant may 

 grow. If it grows at all, its ash that is, the mineral 

 substances which it has taken from the soil will always 



