26 THE PRINCIPLES OF AGRICULTURE 



yet the figures ''reveal the fact that even the 

 poorer soils have an abundance of plant -food 

 for several crops, while the richer soils in 

 some cases have sufficient for two hundred to 

 three hundred crops of wheat or maize." Yet 

 these calculations are made from only the 

 upper eight inches of soil. 



43. Happily, this food is not all du-ectly 

 available or useful to plants (being locked up 

 in insoluble combinations), else it would have 

 been exhausted by the first generations of 

 farmers. It is gi-adually unlocked by weather, 

 micro-organisms, and the roots of plants ; and 

 the better the tillage, the more rapid is its 

 utilization. Plants differ in the power to unlock 

 or make use of the fertility of the soil. 



44. Nature maintains this store of fertility 

 by returning her crops to the soil. Every 

 tree of the forest finally crumbles into earth. 

 She uses the materials, then gives them back 

 in a refined and improved condition for other 

 plants to use. She repays, and with interest. 



45. Man removes the crops. He sends them 

 to market in one form or another, and the 

 materials are finally lost in sewage and the 

 sea. He sells the productive power of his 

 land ; yet it does not follow that he impoverishes 

 his soil in proportion to the plant -food which he 

 sells. Given the composition of any soU and 



