486 FIELD CROPS 



ibr it is usually cheaper to grow their feed than to purchase 

 it. It is generally profitable to keep some cattle, hogs, and 

 sheep, or at least one of these classes of animals, to con- 

 vert much that is grown on the farm into more readily 

 marketable or more valuable products, and at the same time 

 to return to the land in the manure a large part of the fer- 

 tility which is removed by the crops. Live-stock farming 

 will postpone soil exhaustion much longer than grain farm- 

 ing if no fertility is brought to the farm from outside sources. 

 Among the crops which may be grown for live-stock feeding 

 are corn, grass, clover, alfalfa, oats, and barley. 



666. A Crop to Supply Vegetable Matter and Nitrogen. 

 It is necessary to conserve the supply of vegetable matter in 

 the soil, in order to maintain profitable crop yields. The 

 exhaustion of the vegetable matter makes the soil ''hard to 

 work"; it becomes stiff and lifeless, bakes and clods badly, 

 and dries out very quickly. Vegetable matter improves 

 the physical condition of the land and increases its moisture- 

 holding capacity. The acids formed through the deca>' 

 of this organic matter also help to unlock the unavailable 

 supply of some of the elements of plant food by changing 

 the nature of the compounds and by acting as a stronger 

 solvent than water. Nitrogen, the most expensive of the 

 three elements of plant food usually purchased in the form of 

 commercial fertilizers, can be added to the soil very cheaply 

 through the medium of leguminous crops. The grasses 

 increase the supply of vegetable matter; the legumes increase 

 the supply of both vegetable matter and nitrogen. Vege- 

 table matter is also added to the soil in corn and cotton 

 stalks, straw, stubble, and manure. The more important 

 crops to supply vegetable matter are clover, alfalfa, the 

 perennial grasses, cowpeas, soy beans, field peas, and green 

 manure crops such as rye, vetch, and rape. 



667. What Crops to Grow. The crops which are included 

 in the rotation depend entirely on xne kmd of i arming wnicn is 



