stop erosion, but if harvested they do not build soil. That is possible 

 only when they are turned under. Even when they are turned 

 under, little benefit may be derived from the nitrogen unless another 

 crop is ready to use it. Turning under a summer legume in the fall 

 and leaving the ground bare is a wasteful practice. Most of the 

 nitrogen derived from the legume is lost during winter by erosion or 

 leaching. This is not the fault of the crop but of the practice. 



All annual legumes have relatively poor root systems, usually not 

 exceeding 10 to 12 percent of the total weight of the plant. In red 

 clover, on the other hand, about one-third of the total plant weight 

 is underground. Even when a red clover crop is removed, consid- 

 erable quantities of organic matter and nitrogen remain to maintain 

 soil productivity. 



When soil-building crops are discussed, therefore, it is not enough 

 to suggest the use of legumes. It is necessary to specify not only the 

 kind of legume, but also how it must be used to help 

 Soil Building in soil building. Soil building is an extremely slow 

 a Slow Proc- process. The reference here is not to the geological 

 ess concept of soil building, which is a process going on 



from below. The term is used in its agricultural 

 meaning of permanently raising the productive capacity of a soil. 

 In this sense, soil building by cropping can take place in only one 

 direction — an increase in the organic-matter content of the soil. If 

 this can be accomplished, other effects, physical and chemical, will 

 follow. The mineral content of a soil, to be sure, will not be increased 

 by cropping, though it may be conserved by preventing wastage. It 

 has been shown that in a sandy soil in a hot climate a permanent in- 

 crease in organic matter simply does not occur. A heavy crop of 

 legumes plowed under on such a soil is burned up by cultivation in 

 one season. On heavier soils and in a cooler climate there is some 

 increase in organic matter when plants are turned under, but it is very 

 slow. If a crop of vetch that would cut a ton of hay per acre is turned 

 imder, one-half or more is quickly lost by decay, mostly as carbon 

 dioxide. Of the remainder, some is later lost, and only a small part 

 is added to the organic matter of the soil. 



Allowing for loss of organic matter in the cultivation of the summer 

 crop, it would probably be more than 100 years before the organic 

 matter in a soil containing 2 percent of organic matter could be 

 doubled by the practice of turning under a good green-manure crop 

 every year. It is more correct, therefore, to speak of green manuring 

 as a soil-conserving rather than as a soil-building practice. The best 

 that can be hoped for is to maintain the organic matter at a level satis- 

 factory for crop production. Green manuring combined with erosion- 

 control practices should maintain the productivity of the soil. 



The soil building commonly referred to in agricultural practice is a 

 temporary process. It is quite true that when a green-manure crop 

 is turned under, the soil is temporarily more productive than it would 

 have been if that crop had not been turned under. This is due in 

 part to the additional nitrogen supplied when a legume is turned 

 under and in part to an increase in organic matter. If this organic 

 matter were not later removed by cultivation the soil might be built 

 up, but it is removed, at least in great part. The organic matter must 

 be constantly renewed if the productivity of the soil is to be main- 

 tained. This is soil conservation rather than soil building. 



