124 NATURE AND PROPERTIES OF SOILS 



casionally to replace the organic matter lost by oxidation. 

 Whether such catch-crops are pastured or turned under, they 

 tend to increase the soil organic matter. Weeds, which spring 

 up after the crop is harvested, are often valuable as cover- 

 and catch-crops and when turned under aid in maintaining 

 the organic content of the land. 



Crop residues form no inconsiderable portion of the organic 

 matter produced on the land. If such materials as straw, 

 stubble, cornstalks, and the like are incorporated in the soil, 

 much will be accomplished towards the upkeep of the organic 

 matter. The burning of straw and cornstalks, especially in 

 the Middle West, entails an enormous waste of carbon as well 

 as of nitrogen. The value of crop residues has been demon- 

 strated very conclusively by the Illinois Experiment Station x 

 on their outlying experimental farms. At Bloomington, for 

 instance, the turning under of crop residues for five years 

 increased the wheat yields 4.4, 7.9 and 5.9 bushels in 1911, 

 1912 and 1913 respectively. 



Farm manure is one of the most important by-products on 

 the farm and is especially valuable because of its organic mat- 

 ter. Although only about one-fourth of the organic materials 

 of the original food given the animal ever reaches the land, 

 the use of such a by-product is worth while, since the carbon 

 it contains comes from the air and not from the soil. The main 

 losses that the carbon of the crop undergoes when thus util- 

 ized are due to the digestive influences of the animal and to 

 the leaching and fermentation which goes on in the manure. 

 While sufficient manure ordinarily can not be produced from 

 the crops grown on the farm to maintain the organic matter 

 of its soil, the use of farm manure with green-manure and crop 

 residues in a proper rotation is fundamental in good soil man- 

 agement. 



66. Organic matter and soil conditions. — Improper soil 



*Mosier, J. G., and Gustafson, A. F., Soil Physics and Management, 

 p. 171; Philadelphia and London, 1917. 



