UTILIZING OUR SOIL RESOURCES — SALTER 327 



effect of accumulative crops. Manure also contributes directly to the 

 humus content of the soil, since it supplies both organic matter and ni- 

 trogen. For a given amount of plant nutrient supplied, manure adds 

 about twice as much to the humus content of the soil as do chemical 

 fertilizers. About half of the conservation effect of manure arises 

 from increased residues from the larger crops grown, whereas the other 

 half represents the direct contribution from the organic matter sup- 

 plied in the manure itself. Eight tons of manure per acre has half as 

 much effect on soil productivity as a crop of clover. 



I can think of no single improvement in farming practice that would 

 yield as big dividends in soil conservation and improved soil produc- 

 tivity as the general adoption of practical and effective measures for 

 the preservation and use of animal manures. 



Chemical fertilizers. — The use of chemical fertilizers in farming is 

 already making a substantial contribution to our productive capacity. 

 During the last 10 years fertilizer use has increased two and a half 

 times. Consumption in 1949 totaled 18,542,000 tons compared with 

 7,912,000 tons in 1939. Fertilizer now accounts for about 25 percent 

 of the total United States production. 



Recent experimental findings suggest hundreds of new opportunities 

 for increasing crop yields through greater use of fertilizers. Usually 

 the opportimities are greatest when fertilizer is used in adequate quan- 

 tities in combination with several other improved practices. The 

 greatest returns from the use of green-manure legumes, for instance, 

 come when higher rates of phosphate and potash fertilizers are applied 

 to the green-manure crop. 



Evidence from east of the Great Plains and irrigated areas clearly 

 indicates that the use of balanced fertilizers on grasslands, in conjunc- 

 tion with other improved practices, could easily double forage pro- 

 duction on 250 million acres of grasslands. Other experiments show 

 that on intensively cultivated crops such as cotton, tobacco, and pota- 

 toes, where heavy applications of phosphate and potash have been used 

 over a long period of years, these elements have accumulated in some 

 soils to a level where continued heavy usage is uneconomical. On much 

 other land, however, use or heavier use of these elements would increase 

 yields. 



More extensive use of nitrogen fertilizer on crops holds the greatest 

 of all potentialities among the various fertilizer elements, provided 

 that nitrogen is not used as a substitute for legumes, manure, and other 

 sources of organic matter, or to overcultivate sloping erosive soils. 

 We are just beginning to appreciate the values that can be gained 

 from heavy applications of nitrogen fertilizers. Agronomists in gen- 

 eral are raising their sights as to the quantities that can be used 

 efficiently. 



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