SOIL FERTILITY 



397 



well as one of the most diflicult problems for the farmer (Figs. 190 

 and 1!)1). The methods that have been recommended for main- 

 taining the organic matter will usually maintain the nitrogen, li 

 is the most expensive plant food element, and more of it is required 

 hy crops than of the other elements. When the market price is 

 eighteen cents per pound the cost of the nitrogen for a bushel of 

 corn is twenty-seven cents, for Avheat thirty-six cents, and for oats 



Fro. 190. Whorit ({rowing on a soil very deficient in nitrogen. Note the effect of the 

 addition of nitrogen (N). Average yield for nitrogen, 32 grams per pot, without nitrogen 

 3 grains. (Illinois Soil Report.) 



Fio. 191. Legumes turned under have the samp effect ns the addition of nitrogen. 

 Yields for a four-year average were u.s follows: No nitrogen, 4 grams per pot, legumes 18 

 grams, and for nitrogen 1!() grams. (Illinois Soil Report.) 



about eighteen cents. This price makes its purchase almost, if not 

 entirely, prohibitive for ordinary grain crops. 



Nitrogen can be readily incorporated with the soil by turning 

 under a crop of inoculated le(/urncs. These mav be grown in con- 

 nection with some of the money crops, such a< corn, col ton. wheat. 

 rye, oats, and others, and turned under for soil enrichment. The 

 cotton belt and the southern part of the corn and wheat belt arc 



