SOIL FERTILITY 



393 



Plant Food in Soil Areas of Kentucky. 3 Pounds in Surface, to 7 Inches, 

 Two Million Pounds 



Ky taking the figures of crop requirements given in the tables, 

 pages 390 and 398, it will be easy to calculate the length of 

 time necessary to completely exhaust the soils by growing maximum 

 crops. This will give a fair idea of the deficiency of certain plant 

 foods in the soil. 



Nitrogen. Nitrogen is one of the most limited of the plant 

 food elements in soils, it occurs in organic matter in combination 

 with hydrogen, oxygen, carbon, sulphur, and other elements, and 

 all non-leguminous plants are indirectly dependent upon this form, 

 legumes also use the nitrogen of the organic matter. Free nitrogen 

 occurs in the soil air in large quantities, hut this can be used only 

 by legumes. One hundred corn crops of fifty bushels each would 

 use all the nitrogen in the surface soil of the average brown silt 

 loam, provided the stalks were turned back, while if the stalks and 

 "rain were both removed the nitrogen would all be used in a little 



~ cj 



more than sixty-five years. 



Through the activity of soil organisms the soil nitrogen of the 

 organic matter is slowly made available. The process takes place 

 principally in the plowed soil and the amount of nitrogen made 

 available each season is approximately two per cent of the total 

 nitrogen in this stratum. If there are four thousand pounds in 

 the plowed soil, about eighty pounds will become available, or an 

 amount sufficient to produce a fifty-bushel crop of corn. 



Nitrogen is the limiting element over large areas of soil, and 

 its increase and maintenance becomes one of the most important as 



