MANURING AND WORN-OUT SOILS 333 



actual gain of plant food, except nitrogen if legu- 

 minous crops are grown. Ho green manures 

 return to the soil any more potash and phosphoric 

 acid than they took from the soil. No matter how 

 long and how skilfully green-manuring is- con- 

 ducted, it will not enrich the soil with one pound 

 of the mineral plant foods, although it may make 

 the mineral foods already in the soil more available, 

 which may amount to the same thing, so far as 

 crop production is concerned. A sharp distinction 

 should be made here : green-manuring may actually 

 enrich the soil in nitrogen, but it cannot enrich the 

 soil in potash and phosphoric acid ; it may, however, 

 so improve the texture of the soil that plants can use 

 more of the potash and phosphoric acicl already there. 

 When we remember that most farm soils, 'even 

 the poorest, contain tons of plant food, we' can 

 believe that in practical effect, though not in 

 reality, green-manuring may enrich the soil in all 

 the plant foods. The mere amount of plant food 

 in the soil is nothing to us: it is the ability of the 

 soil to transform this material into plants that inter- 

 ests us. Green-manuring helps the soil to do this 

 as no other farm practice does, except the use of 

 barn manures. We cannot expect green-manuring 

 to relieve us of the necessity for buying and using 

 the mineral plant foods; but we do expect that, in 

 certain systems of farming, it will make unnecessary 

 the purchase of nitrogen, and that it will greatly 

 reduce the amount of the mineral plant foods that 

 need be applied. 



IN06WLATING THE SOIL 



Under some conditions a leguminous crop that is 

 plowed under may make the soil richer by a 



