430 FOOD SUPPLY PROBLEM 



"Let us remember," said Sir William, "that the plant creates 

 nothing; that there is nothing in bread which is not absorbed 

 from the soil, and unless the abstracted nitrogen is returned to 

 the soil its fertility must ultimately be exhausted." 



What is Soil Fertility? From 65 to 95 per cent of the soil, 

 by weight, is made up of finely disintegrated rock. The black 

 color is given to the soil by the humus, that is, the decayed vege- 

 table and animal matter the so-called organic matter of all 

 kinds. This matter usually ranges from 2 to 5 per cent. The soil 

 also contains a certain amount of water and of air. These ele- 

 ments, taken altogether, constitute the plant food. Much of 

 this plant food is not fit for use by the plants until it has been 

 acted on by an additional element in the soil, namely, the soil 

 bacteria. Presence of plant food and soil bacteria constitute 

 fertility. The human body, for instance, contains iron; but 

 the unhappy person who lacks iron in his tissues cannot supply 

 the need by swallowing " raw " iron. He will probably eat 

 foods like celery, lettuce, carrots, etc., containing iron in a 

 digestible form. Thus also, with wheat for instance, which 

 cannot feed on raw nitrogen. Assume then, that a soil has 

 enough water, enough sunshine, enough warm temperature to 

 insure plant growth, enough lime to prevent acidity, what are 

 the chief plant foods which are subtracted from the soil by con- 

 tinuous cropping? The three chief plant foods mined from the 

 soil by cropping are nitrogen, phosphorus and potash. According 

 to forty-nine analyses of soils in different parts of America, the 

 average acre of soil contained the following plant food : 



Nitrogen 3,000 pounds 



Phosphoric acid 4,000 pounds 



Potash 16,000 pounds 



Taking the average wheat crop of the United States as 13.8 bushels 

 per acre (as it was for the period 1899 to 1908), this wheat and this 

 straw removed each year from the soil about 14.5 pounds of nitro- 

 gen, 10.6 pounds of phosphoric acid, and 14 pounds of potash 

 per acre. 



Similar calculations may be made for other crops, some taking 

 more, some less plant food from the soil. Continuous cropping, 

 without rotation and without putting anything back into the soil, 

 gives warning of soil depletion in the form of diminished yields. 

 And yet how little attention the average farmer in the newer parts 

 of the country gives to this warning. And herein lies one of the 

 chief evils of the American short-term tenancy system, for the 



