PLANT FOODS FOE GRAIN CROPS. 131 



but tliese contributions are small. We must also 

 take into consideration that nitrates may leach, 

 through the soil and escape into the drains. The 

 query then is, what fertilizing material does it take 

 to return to the soil the plant food of which we have 

 robbed it by taking off the crop? 



Six tons of average farmyard manure would just 

 about replace the amounts of nitrogen (sixty 

 pounds), and phosphoric acid (twenty-three pounds), 

 but would more than make good the loss of potash 

 — in fact give an excess of about twenty-five pounds 

 of it. For other ordinary grain crops the figures 

 approximate those given for wheat. 



In stable manure we also replace carbon which the 

 crop has removed, and of which we have taken no 

 account, as it has no quotable value. Uniting with 

 the oxygen of the air, it forms carbonic acid, as 

 already stated, and this not only acts directly as 

 plant food, but also helps in decomposing and ren- 

 dering soluble the locked-up plant foods in the soil. 

 This carbonaceous matter, as it undergoes slow com- 

 bustion, keeps the soil open and makes it warmer, 

 exposing it in a greater degree to chemical action, 

 and is especially valuable as an aid in the process 

 of nitrification. The supply of mineral plant foods 

 in the manure is thus supplemented by the slow de- 

 composition of the soil, which often contains im- 

 mense quantities in its natural state; and the supply 

 of the nitrogenous element is supplemented by 

 additions from the atmosphere. 



Thus, if we, in return for an annual crop of thirty 

 bushels of wheat or its equivalent in other cereals, 

 apply six tons of stable manure year after year, we 

 not only return all the phosphoric acid taken off, 

 but also increase the available stores of potash and 



