322 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



productivity for economic sustained production on such soils is some- 

 what lower than in the virgin state, high levels of productivity can 

 be maintained. Already, productivity is on the "come-back" on many 

 farms where yields were falling off because of declining soil fertility. 

 The job we face is one of bringing the greater portion of the 

 land now in use — both naturally fertile and infertile soils — to a 

 high level of economic production on a sustained basis. We need 

 to offset soil deterioration, and on many soils go even further — build 

 them up to higher levels of productivity. 



HOW SOILS DETERIORATE 



Soils deteriorate through numerous changes, many of which are 

 not readily visible. Deterioration can be classified into two basic 

 types, both important but varying as to conditions. The first is actual 

 removal of soil by erosion — by wind or water. The second includes 

 changes within the soil itself under cropping — changes hard to detect 

 on sight. Both processes often go on simultaneously and affect each 

 other. 



Erosion. — The physical removal of soil by erosion or wind blowing 

 may in extreme cases render the soil totally unsuited to cultivation, as 

 where severe gullying occurs. The more usual damage, however, is 

 that resulting from loss of surface soil. In removing the surface soil, 

 erosion usually exposes soil layers that are more compact and less 

 favorable for root growth than the original topsoil. Erosion — even 

 sheet erosion — causes serious fertility losses far greater than the weight 

 of soil removed might indicate. In most agricultural soils the plant 

 nutrients — nitrogen and phosphorus particularly — are concentrated 

 in the surface layers. Losses due to erosion are especially serious for 

 those two nutrients. 



Damage to cultivated land by erosion is a serious problem in much 

 of the country. Not all soils, however, are subject to erosion, which 

 naturally is related to the kind of soil and the slope of the land. For 

 those that are, it is often necessary to check cropping losses and in- 

 crease fertility before erosion losses can be checked. In any given 

 instance the relative importance of the factors contributing to erosion 

 depends upon the type of soil, especially the character of the several 

 layers or horizons and slopes, the crops grown and their culture, and 

 the climate. 



Cropping. — Soil deterioration through cropping is at least as serious 

 as losses resulting from erosion. The majority of people, however, 

 are more fully aware of the damage from erosion. They do not realize 

 that on an acre of fertile soil growing a cultivated crop such as corn, 

 the productivity decline from cropping may equal the loss of 20 tons 

 of topsoil through erosion. 



