THE SOIL 13 



nished a habitation and of being provided with a 

 certain quantity of the sugary material which they 

 take from the phmt sap is amply repaid by their 

 nitrogenous excretions which are in a form to be 

 quickly available to the host plant. Plants of this 

 family are thus enabled to grow and thrive in soils 

 that are very deficient in nitrogen, since they are 

 able to obtain a sufficient supply of this most neces- 

 sary material from the atmosphere through the agency 

 of these minute inhabitants of their root tubercles. 

 The tissues of leguminous plants are rich in nitrogen, 

 and they are more useful than any others to plow 

 under for green manuring, since they really add to 

 the soil large quantities of this ever desirable ele- 

 ment. Other green crops and weeds when plowed 

 under only return to the soil what they have pre- 

 viously taken from it. The fact that clover and 

 other similar legumes were useful for green manur- 

 ing has been known since the earliest times, but the 

 reason why they are better than other kinds of plants 

 for this purpose has only been known for a compara- 

 tively few years. Even when crops of clover, cow- 

 peas, or other similar legumes are cut and removed 

 from the land, the roots and stubble contain enough 

 nitrogen to be of considerable use to succeeding crops. 

 This is a matter of tlie utmost practical importance, 

 since by establishing a proper rotation and by keep- 

 ing the ground continuously covered by some legu- 

 minous catch crop when not occupied by the principal 

 crop it is possible, at slight expense and without losing 

 its use for a single season, to constantly maintain the 

 nitrogen content of the soil and that without pur- 



