THE SOIL AND ITS IMPROVEMENT. 107 



tains will not be without value, especially on sandy or clayey 

 soils. 



Green Manures. As the practical instructions concerning 

 green manures are given in the appropriate chapter, we here 

 need to consider only the scientific principles connected with 

 their use. 



When a farmer grows a heavy crop of clover or rye, and 

 plows it under, he adds to the soil a large amount of vegetable 

 matter perhaps eight tons to the acre. The composition of 

 this would be about as follows : 



Water, 12,600 Ibs. 



Carbonaceous matter, ...... 3,100 



Lime, sand, etc., ....... 120 



Nitrogen, ........ 80 



Potash, 78 



Phosphoric acid, ....... 22 



Total, 16,000 " 



Of this, the one hundred and eighty pounds of nitrogen, 

 phosphoric acid, and potash are valuable plant food, and were all 

 obtained from the soil. The carbonaceous matter was obtained 

 from the air, and is not plant food. The question, then, nat- 

 urally arises : " Seeing that all the plant food which this crop 

 adds to the soil was taken from it by the crop, how can the 

 soil be enriched by the process? Does a man get rich by 

 taking a dollar out of his pocket and then putting it back 

 again ?" 



It must be remembered that there are two ways of improv- 

 ing the soil : one by adding plant food to it, the other by ren- 

 dering available that which it already contains. The green crop 

 enriches the soil by the latter method. The clover plant gathers 

 from the soil large quantities of plant food which other plants 

 can not use, and when it decays in the soil it leaves this plant 

 food in available forms. It also sends its roots deep into the 

 soil, and gathers food which is out of reach of other crops ; when 

 it decays it leaves this in the surface soil, within reach of other 

 crops. But it has yet another effect : By keeping the soil moist 

 and shaded, and full of decaying vegetable matter, it furnishes 



