No. 6. DEPARTMENT OF AGRICULTURE. 311 



sidered fertile, are relatively high in bacteria as compared with those 

 in the opposite condition. 



In this we must distinguish between active and potential fertility. 



A soil is actively fertile when plant food is being elaborated 

 therein at a greater rate than required by the maximum demand of 

 growing crojjs. 



Such soils not merel}' contain an abundance of crude plant food, 

 but the latter is being actively digested. 



Such soils are, furthermore, always high in bacteria, showing that 

 the latter are functionating vigorously under conditions most favor- 

 able to them, 



A soil is potentially fertile when it is rich in plant food, but owing 

 to unfavorable conditions or environment the soil bacteria are dor- 

 mant, and thus either cease to digest plant food or do it so inactively 

 as to fail to keep up with the demand of growing crops. 



Thus forest and woodland soil are rich in huraus and other crude 

 plant food, but owing to their usually acid condition, as well as to 

 their compacted state, the bacteria therein are able to develop but 

 slowly, and but little available plant food is elaborated. Such 

 soils are low in bacteria; but let this virgin forest soil be brought 

 under active cultivation, especially if its acidity be corrected at the 

 same time by means of a liberal dressing of lime, conditions favorable 

 to bacterial life are at once created, the number of bacteria rises, and 

 an actively fertile soil is the consequence. 



Old pasture lands and permanent meadows possess potential 

 rather than active fertility. In such soils the number of bacteria is 

 relatively low, and plant food is but slowly digested. But such lands 

 are at once converted into an actively fertile condition when brought 

 under cultivation or when other means to stimulate bacterial life in 

 the soil are utilized. 



It is the function of the agriculturist to understand how poten- 

 tial can be converted into active fertility; in other words how land 

 rich in crude plant food can be made large producer of crops. 



An average of the results of 49 analyses of typical soils of the 

 United States showed per acre for the first eight inches of surface 

 soil 2,600 pounds of nitrogen, 4,800 pounds of phosphoric acid and 

 13,400 pounds of potash. The average yield of wheat in the United 

 States is 14 bushels per acre. Such a crop will remove 29.7 pounds 

 of nitrogen, 9.5 pounds of phosphoric acid and 13.7 pounds of potash. 

 Now if all of the potential nitrogen, phosphoric acid and potash could 

 be rendered available there is present in such an average soil, in the 

 first 8 inches, enough nitrogen to last 90, enough phosphoric acid for 

 500 and enough potash for 1,000 j^ears. 



This is what is meant by potential fertility, and yet such a soil 



