THE IMPROVEMENT OF SOILS. 45 



These plants have small, knotty growths, called " tuber- 

 cles," on their roots, which are believed to be caused 

 directly or indirectly by certain bacteria which are present 

 in soils in which this class of plants are grown. Recent 

 experiments have shown, too, that soils which do not 

 contain these bacteria may be inoculated by applying a 

 light dressing of soil from a field in which the plants have 

 previously grown to perfection, without direct applications 

 of nitrogenous material. It is through these tubercles that 

 the plants are supposed to gain free nitrogen from the 

 atmosphere. Experiments have shown pretty clearly that, 

 where they have been formed, the nitrogen in the crop is 

 far greater than when they are absent. The fact that 

 such is the case is sufficient for us to make use of this 

 free source of the expensive element, nitrogen; and it 

 makes green manuring with these plants a most impor- 

 tant part of farm work, not only as a means of securing 

 nitrogen for the crop itself, but as a source of nitrogen 

 for crops unable to secure it except from soil sources. 



An acre of an average crop of red or crimson clover, 

 or of cow peas, will contain one hundred and fifty pounds 

 of nitrogen, equivalent to that contained in fifteen tons of 

 average stable manure. Rye or buckwheat, or other plants, 

 which do not possess this power of securing nitrogen, are 

 much less valuable for this purpose. 



Green manuring is particularly useful in the improve- 

 ment of light lands usually deficient in humus, and in 

 that method of farm practice where exhaustive crops are 

 grown without the addition of yard manure or other forms 

 of organic matter. By the use of the legumes as green 

 manures, and the addition of materials furnishing the 



