SOILS AND FERTILIZERS 493 



beneficial. Deep-rooted plants are decidedly preferable to shallow- 

 rooted ones because they penetrate into the subsoil. In this way 

 air and water find entrance, especially after the roots decay. Thus 

 in a way every deep-rooted plant is a subsoiler. It is also supposed 

 that such plants, especially when plowed under, tend to enrich the 

 surface soil with potash and phosphorus from the subsoil, thus 

 bringing these substances within the reach of shallow-rooted plants. 



Leguminous plants are more valuable for green manuring than 

 others because they not only provide humus but also have the ability 

 to use the nitrogen of the air, which upon decaying they add to the 

 soil. For this reason green-manure crops should always be legumes 

 if such are adapted to the particular locality where needed and can 

 be sown at the time desired. 



Use of Green Manures. Green manuring as a definite farm 

 practice can be recommended only under certain conditions. It is 

 profitable in upbuilding poor soils and in improving the physical 

 condition of sandy, clayey, and adobe soils. In orchards green 

 manures may, as a rule, be used very advantageously, as they do 

 not interfere with the fruit crops. Green manuring can not be 

 recommended on good soils, except at long intervals when there is 

 reason to believe that there is need either of more humus or more 

 nitrogen. Where red clover or alfalfa can be used in rotation 

 the need of a special green-manure crop is seldom felt. The reason 

 lies in the fact that about one-third of the weight of the clover 

 plant and nearly one-half of that of alfalfa is in the root, so that 

 these plants virtually produce a green-manure crop under the 

 ground in addition to the regular crop of hay. Sweet clover is an- 

 other plant of this class and of wide adaptation as to soil and climate, 

 but unfortunately the hay is not readily eaten by cattle, so that it is 

 used in limited sections only. 



A serious objection to a green-manure crop lies in the fact that 

 it must ordinarily take the place of a regular crop, so that the in- 

 come from the land is lost for the season. The value of the practice 

 in any particular case must be measured by the results secured in 

 the subsequent crop. Frequently it will happen that as good or 

 nearly as good results can be obtained by the use of commercial 

 fertilizers; in such cases it is often best to use them, thus retaining 

 the use of the land for the season. Sooner or later, however, the 

 humus of the soil becomes depleted and must be replaced. Where 

 clover or some similar large-rooted crop can not be used in rotation, 

 recourse must be had either to green manuring or to the application 

 of barnyard manure. 



The use of green manures in semiarid regions is as a rule im- 

 practicable, as with insufficient moisture vegetable matter decays 

 very slowly. The soil is thus filled with air spaces and loses much 

 more water by evaporation. 



The Principal Leguminous Crops. There are in the United 

 States 15 leguminous field crops that are more or less extensively 

 grown. In the approximate order of their importance they are as 

 follows: Red clover, alfalfa, cowpeas, alsike clover, crimson clover, 



