No. 6. DEPARTMENT OF AGRICULTURE. 473 



serviceable in improving the physical character and absorptive prop- 

 erties of soils. The members of this group include the cereal grains, 

 the grasses, buckwheat, turnips, rape, etc., yet, because of their 

 time of growth and period of rapid dt^'elopment, it is often desira- 

 ble to grow crops of this class, particularly where the primary pur- 

 pose is to prevent losses of plant food rather than to rapidly build 

 up the soil in nitrogenous organic substances. 



The second group of crops includes those plants which belong 

 to the legume or clover family, and which do not depend solely 

 upon soil sources for their nitrogen, but can obtain it from the air, 

 these in their growth and removal from the soil need not materially 

 reduce the content of soil nitrogen, but rather particularly when 

 plowed down directly add to the crop-producing capacity of soils 

 by improving their physical character and by increasing their store 

 of nitrogen. In order that a plant of this group may obtain its 

 nitrogen from the air, however, the soil must contain originally, or 

 must be inoculated with a specific organism, the presence of which 

 in the soil is manifested by the growth of nodules upon their roots 

 and through which it is believed the plants obtain their nitrogen, 

 though the exact process is not yet fully understood. 



Advantage of Legumes. 



The use of this class of plants, therefore, possesses a- three-fold 

 value in the improvement of soils, first, in absorbing and retaining 

 the soluble food in the soil; second, in providing vegetable matter, 

 or humus-forming material, which contributes to the physical im- 

 provement of soils; and third, by adding to the store of nitrogen in 

 the soil, the element most likely to be deficient, and which can be 

 used by plants whose sole source of nitrogen is the soil. Fortu- 

 nately, because of the number of plants belonging to this group, 

 and because of their wide range of adaptation to the various con- 

 ditions, it is possible to introduce one or more of them into the 

 regular system of farm practice, without interfering with useful 

 and profitable rotations. Many of them, for example, the various 

 clovers, red, crimson and alsike, are already grown extensively, and 

 their value in the rotation well understood by practical men. There 

 are many others, however, whose characteristics have not been 

 carefully studied until recent years, and whose usefulness are just 

 beginning to be appreciated. Among these are the Canada field 

 pea, the soy bean, the cow pea and the various varieties of vetch, 

 all possessing that valuable power of appropriating for their use 

 the free nitrogen of the air, and thus contributing directly to the 

 potential fertility of the soil. The plants of this group are often 

 called, and properly, "nitrogen gathering" crops, and their renovat- 



