CROPS AND CROP TILLAGE. 33 



THE IMPORTANCE OF OKCiAXlC MATTER IN THE SOIL. 



A continued supply of organic matter in the soil appears to be abso- 

 lutely essential to successful crop production. It is one of the most 

 important sources of the various forms of nitrogen available to crop 

 plants, and nitrogen in some combined and available form is of course 

 absolutely necessary to plant growth. 



Undecayed organic matter is not only useless to plants, but it is 

 also likely to have an injurious mechanical effect upon the soil, at 

 least in a dry region, by keeping it loose and open, thus making 

 moisture conservation difficult. The processes of decay that take 

 place in the organic matter in the soil are varied and complicated. 

 Some of them result in additions to the supply of soluble salts used 

 by plants, Avhile others lead to the complete loss of the constituent 

 substances through their reduction to gaseous forms and diffusion into 

 the air, as in oxidation and denitrification. It therefore becomes 

 important to consider something more in a tillage method than merely 

 adding raw organic matter to the soil. This must be followed by 

 providing conditions in the soil favorable to the activities of those 

 organisms that reduce the organic matter to forms available for plant 

 nutrition with as little Avaste or loss as possible. 



CONDITIONS FAVORING NITRIFICATION. 



The processes that result in the decomposition of organic matter in 

 the soil and in the formation of nitrates are favored by a moderate 

 supply of oxygen, the presence of capillary moisture, fairly high soil 

 temperatures, and the presence of free bases to combine with the 

 nitric acid formed. Such conditions are very well provided by the 

 method of tillage used in growing wheat in the Great Basin, as pre- 

 viously described. "With that tillage system a large amount of wheat 

 straw with its partially decayed leaves is turned into the soil in the 

 early %autumn. The winter rains penetrate the loosened soil, and the 

 continued surface cultivation during the folio-wing summer largely 

 prevents the loss of this moisture by evaporation. This surface 

 cultivation also promotes the aeration of the soil, which is essential 

 to the most rapid nitrification, while the absence of plant growth to 

 shade the soil gives the high temperature needed. 



The beneficial effects on nitrification processes of the conditions 

 produced by summer fallowing are discussed at some length by Hall <" 

 in describing the work at Rothamsted, England, where the rainfall is 

 sufficient to leach the soil of the field under experiment: 



It will be seen that the produce of Avheat after fallow is considerably higher 

 than when it is grown continuously — 17.1 against 12.7 bushels per acre; but if 



o Hall, A. D. The Book of Rothamsted Experiments, pp. 62-66 ; also pp. 221- 

 223, 1905. 



