330 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



systems, deep tillage, and the incorporation of lime and fertilizer in 

 the deep soil layers. 



We need to make much more extensive use of winter cover crops 

 on intertilled land. Leaving the ground bare over the winter pro- 

 motes loss by erosion. For the South in general, adequate cover 

 crops are available. Still, more than 80 percent of the cultivated 

 land goes through the winter unprotected. For years we have taken 

 for granted the bare fields with brown cotton stalks and broom sedge 

 all the way from Virginia to east Texas. Now, with practices de- 

 veloped during the last 20 years, fields could be green with growing 

 crops the year around. Here the agricultural plant can work on 

 a 12-month basis, contributing to both soil conservation and increased 

 production. 



SOIL PRODUCTIVITY CONDITIONS BY REGIONS 



Again I want to emphasize that no single practice will accomplish 

 the combined goal of increased production and soil conservation. 

 The goal can be met only by balanced application of known soil and 

 crop management techniques in accordance with specific soil charac- 

 teristics and needs. The complexity of the problem is apparent when 

 we examine the types of soil and how they deteriorate in various 

 parts of the United States. 



Far West. — The prevention of saline and alkali soil conditions 

 is one important factor in the conservation of soil productivity in 

 western irrigation lands. Accumulation of salts in the soil is one 

 of the most difficult problems. In many instances, salt-laden water 

 is the only source available for irrigation. 



Also, the structure of heavy soils is deteriorating under certain 

 irrigation and cropping systems, making it difiicult to get irrigation 

 water into the soil. There is increasing evidence, too, that continued 

 soil productivity will require the maintenance of considerably higher 

 levels of nitrogen. The use of phosphorus fertilizers is also increas- 

 ing under irrigation. But for many years to come, the maintenance 

 of proper salt balance and the conservation of good soil physical 

 properties must be the most important criteria of good land use 

 in this area. 



Great Plains. — In the Great Plains, moisture rather than plant 

 nutrients has been the primary factor limiting the productive ca- 

 pacity of the soils. Subsurface tillage, fallowing, and other cul- 

 tural practices designed to leave crop residues on the soil surface 

 are used to conserve both soil and water. Protection against soil 

 blowing is a particularly important step in sound land use here. 



Despite improvements in mulching and subsurface tillage, land 

 use in this area has been primarily exploitive in nature. Although 

 cropping has not yet generally depleted soil fertility reserves to 



