92 SOILS AND FERTILIZERS 



The fact that plants feed on humate compounds 

 and that decaying animal and vegetable matter pro- 

 duce humates from the inert potash and phosphoric 

 acid of the soil, has an important bearing upon crop 

 production, because it indicates a way by which inert 

 plant food may be converted into more active and 

 available forms. It also explains that stable manure 

 is valuable because it makes the inert plant food of the 

 soil more available. 



107. Amount of Plant Food in Humate Forms. 

 In a prairie soil containing 3 y 2 per cent, of humus 

 there are present 100,000 pounds of humus per acre. 

 Combined with this humus there are from 1,000 to 

 1,500 pounds each of phosphoric acid and potash. 

 Similar soils which have been under long cultivation 

 without the restoration of any humus contain from 

 300 to 500 pounds each of humic potash and phos- 

 phoric acid. 18 A decline in crop-producing power has 

 in many cases been brought about by the loss of 

 humus. 



108. Loss of Humus. Loss of humus from soils 

 is caused by oxidation and by fires. Any method of 

 cultivation which accelerates oxidation reduces the 

 humus content. In many of the western prairie soils 

 which have been under continuous grain cultivation 

 for thirty years or more, the amount of humus has 

 been reduced one-half. Summer fallowing also causes 

 a loss of humus. When land is continually under the 

 plow, and no manures are used, the humus is rapidly 

 oxidized, and there is left, in the soil, organic matter 

 which is slow to decay. 



Forest and prairie fires have been very destructive 



