272 IxMPROVEMENT OF THE SOIL. 



were surrounded by substances, which will keep up with it, 

 the vigor of the galvanic action. 



In addition to the mechanical effect upon the soil, we 

 would suggest whether similar roots may not form with the 

 soil similar galvanic circles of similar power and mode of ac- 

 tion, and that the interchange of crops changes this action or 

 restores its activity. We know that different metals require 

 different substances to excite the voltaic currents, and that 

 rest or a change of materials will restore the action of a bat- 

 tery, when its power is exhausted. The reason why some 

 plants exhaust the soil more than others, is partly due to their 

 structure. In this respect plants are divided into three classes. 



1. The culmiferous plants, so called from culm, the stalk, 

 which is usually hollow and jointed in order to afford support 

 both to the leaves and seeds. Wheat, barley, oats, rye, In- 

 dian corn, tobacco, cotton, flax, hemp and the grasses, are 

 of this class. All of them, save some of the grasses, are 

 termed exhausters of the soil, and in all cases exhaust it 

 more during the ripening of the seeds than during any other 

 period of their growth. Flax and hemp are the most ex- 

 hausting crops, because their leaves are small, and hence but 

 a small quantity of their substance can be obtained from the 

 atmosphere. They also return but a small quantity of mat- 

 ter to the soil, in the form of stubble and roots. 



The smaller grains rank next in their power of exhausting 

 the soil, because their leaves are narrow and roots small. 

 They, however, return more to the soil in the form of stubble. 



Indian corn, tobacco and rice, have larger leaves, and de- 

 rive more of their substance from the atmosphere. The roots 

 of culmiferous plants, being fibrous, do not penetrate and di- 

 vide the soil so perfectly as those of the next family ; and, on 

 this account, do not leave the soil in so good a condition for 

 succeeding crops. 



Von Thaer has attempted to determine experimentally 

 the different degrees in which different kinds of grain exhaust 



