324 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



practices to achieve both conservation and high production — and get 

 them into use. It is important to understand that most of the steps 

 required to increase crop yields tend to lessen the effect of soil dete- 

 rioration from cropping and reduce the erosion hazard. 



Seldom can our goal be achieved by a single practice. Instead, in 

 nearly all areas, a sound program of land use will include a combina- 

 tion of such practices as adequate liming to control soil reaction, ad- 

 equate acreage of legumes and grasses in the cropping system, return 

 of adequate quantities of plant residues, including the proper conser- 

 vation and use of animal manures, use of adequate amounts of mineral 

 fertilizer, improved treatment and management of pastures, and spe- 

 cial erosion-control and water-conservation measures. Cropping sys- 

 tems and management practices must be adjusted to the capabilities 

 and needs of local soil types. 



Lime. — On most soils east of the Great Plains lime is a basic factor 

 in building soil productivity. By far the most effective method for 

 overcoming soil deterioration, brought about by erosion or due to chem- 

 ical and biological forces, consists in expanding the acreage devoted 

 to the soil-building sod legumes. The successful establishment of 

 these legume crops requires a fair abundance of lime in the soil. Only 

 by liming acid soils can they be made to grow the more efficient 

 soil-improving legumes. 



Despite large increases in the use of liming materials, especially 

 since the introduction of the Agricultural Conservation Program, the 

 annual return of lime to soils in the humid area is still far short of that 

 needed to offset removals by crops and drainage. Farmers are now 

 applying lime at an annual rate ranging from 25 to 30 million tons, 

 which is eight to ten times as much as they were applying 15 years ago. 

 Yet, it still falls far short of needs. Most acid soils need a substantial 

 application of lime every four or five years. Profitable use can be 

 made of two to three times the amount now being applied. 



Limestone resources of the United States are ample to supply liming 

 materials at such high levels for hundreds of years. 



Legumes and grasses. — The values from including grasses and le- 

 gumes, especially deep-rooted legumes, in crop rotations have long 

 been recognized. They are our best antidote for the destructive effects 

 of row-crop cultivation. Yet too few farmers use enough of them. 



Long-time field experiments have revealed that each time a full- 

 season cultivated crop is grown without return of residues, the soil 

 loses through biological oxidation about 2 percent of its organic matter 

 and nitrogen. (Erosion frequently causes additional loss.) Under 

 small grain, the annual loss is about half as great — or 1 percent. 



These experiments also show that growing legume sod crops in the 

 rotation will offset these losses. Gains from a crop of red clover, even 



