THE SOIL AND ITS IMPROVEMEMT. 109 



easily reached as is commonly supposed. No soil that was orig- 

 inally fertile has ever been entirely exhausted of plant food by 

 cropping alone. A soil of ordinary fertility contains in the upper 

 twelve inches plant food enough could it all be utilized to 

 produce a crop of thirty bushels of wheat per acre every year 

 for two hundred years, supposing the entire crop to be removed 

 and nothing returned. Of course, it is not possible to draw from 

 the soil in this manner, but these facts show that it is a matter 

 of fully as much importance to know how to render available 

 the plant food the soil already contains, as to apply more. Of 

 course this wonderful abundance of material provided by nature 

 should not cause the farmer to be prodigal or wasteful of this 

 supply, but the farmer who uses clover or other green manures 

 intelligently and in connection with barn-yard manures, and a 

 proper rotation of crops, need have no fear of reaching the ul- 

 timate exhaustion of his soil. 



Fallow. In olden times the system of summer fallowing 

 was largely relied on for the improvement of soils. It was 

 called " resting the land," an erroneous term, as the soil being 

 simply passive in the matter of producing a crop, can need no 

 rest. Experiments have shown that a field cropped with wheat 

 each alternate year, and cultivated as a fallow the intervening 

 years, produced more wheat in a given time than another field 

 cropped continuously. 



By fallowing, nothing is added to the land except a little 

 ammonia, which is absorbed from the air by the exposed sur- 

 face. The gain of fertility is due to the fact that by the ex- 

 posure to the air, and constant stirring, chemical changes are 

 induced by which plant food in the soil is rendered available. 

 Fallowing is, therefore, similar in principle to green manuring, 

 but is inferior to it, the disadvantages being : 



A crop is lost the fallow year. 



The humus in the soil instead of being increased is de- 

 creased. 



It costs more to keep the soil constantly stirred than to grow 

 a crop of clover. 



The chance of a gain of nitrogen under the influence of shade, 



