ROTATION OF CROPS A NECESSITY. 279 



COMPOSITION OF THE ASH OF 



This striking difference prevails through the whole list of 

 hard and soft woods. 



Evergreen trees require a large quantity of silica, and in 

 gathering this from the soil, separate it from its combina- 

 tions with potash, lime, and magnesia; leaving these in the 

 soil in an available condition to accumulate in excess of the 

 requirements of the then growing trees. When in course of 

 time, the lumberer, or the farmer, or the accidental confla- 

 gration, removes the pines from the land, a forest of hard 

 woods soon takes their place; and vice versa; when the hard 

 woods have taken up the potash and lime, and have left a 

 large accumulation of silica, and they perish, or they are 

 cut off in their prime, the evergreens succeed them. 



Precisely a similar occurrence takes place in the growth of 

 farm crops. If the table given in a preceding chapter (Chap. 

 XVI, page 100) is referred to, it will be seen how a crop of 

 wheat, in the straw, takes much silica and little potash from 

 the soil; while red clover takes more than three times as 

 much potash; 8* times as much lime; and only a twentieth 

 as much silica as the wheat. But more than this, that as a 

 large quantity of the red clover consists of roots and stub- 

 bles, and these are left in the soil, a considerable quantity 

 of nitrogen, potash, and lime, are thus accumulated after a 

 crop of clover has been removed ; and the stubble has been 

 plowed under; and these furnish precisely the kind of food 

 which will contribute to the needs of a crop of wheat. 

 Moreover, clover is a deep rooted plant, and finds its food 

 far below the reach of the shallow rooted wheat; so that the 

 clover brings up to the surface a large quantity of plant 

 food, and leaves it there, just where the wheat can find it. 

 Hence it is that an abundant wheat crop follows clover; 

 and a crop of clover plowed under is the very best prepara- 



