UTILIZING OUR SOIL RESOURCES — SALTER 323 



Any agricultural soil, under any cropping system, is undergoing 

 both improving and depleting processes at the same time. Even under 

 natural conditions both downgrade and upgrade processes go on, but 

 they tend to reach a unique equilibrium for each level of natural 

 en^'ironment. 



When placed under cultivation, a new environment is created. 

 Usually the downgrade changes are accelerated, and the soil deteri- 

 orates. Mineral elements are removed by both crops and drainage, 

 often faster than new supplies are made available. The annual plant 

 nutrient removal from United States soils by crops is about three 

 million tons of potash, nearly two million tons of phosphate, and 

 over three million tons of nitrogen. Except for phosphate, not nearly 

 that much is returned. We need to put back much more to build up 

 responsive soils to a high level of productivity. 



Mineral nutrient removals represent only a part of the soil deteri- 

 oration resulting from cropping. Loss of organic matter is even more 

 serious. Organic matter is highly important to inherent soil produc- 

 tivity. The superiority of most productive soils is largely the result 

 of organic matter. Stable soil humus is a reservoir for about 98 per- 

 cent of all soil nitrogen, about 50 percent of the phosphorus in surface 

 soil, and smaller amounts of other nutrients. It makes for good soil 

 structure essential to proper movement of air and water in the soil. 



The importance of organic matter — and the seriousness of its loss — 

 is better understood when we realize that in the fertile soils of the 

 Corn Belt each 1 percent of soil organic matter adds about 10 bushels 

 to the acre-yield of corn. Thus, in soil with 3 percent organic matter, 

 inherent productivity will account for a yield of about 30 bushels of 

 corn per acre; while in soil with 6 percent organic matter, inherent 

 productivity will account for a yield of about GO bushels per acre. 



Organic matter is destroyed through the activity of bacteria and 

 other microorganisms in the soil. The process is essentially one of 

 oxidation, and requires air. Each time the soil is stirred, air is intro- 

 duced and the rate of organic decomposition is speeded up. Nitrogen 

 and other nutrients are then released to the crop, or made subject to 

 leaching. This explains why destruction goes on so fast under inter- 

 tilled crops such as corn. 



Organic matter also functions as a binding agent to promote good 

 aggregation and porosity. As organic matter is depleted, soils be- 

 come more compact; porosity is less, which means that the soils 

 become less permeable to air and water; and the soil becomes more 

 and more susceptible to erosion. 



BUILDING SOIL PRODUCTIVITY 



If we are to reverse the declining trend on the majority of our soils, 

 we need to determine for each farm the optimum combinations of 



