xx INTRODUCTION 



(3) Of the ten different chemical elements absolutely required 

 for the growth of every agricultural plant, three come directly from 

 air and water in practically unlimited amounts, and these three 

 (carbon and oxygen from air and hydrogen from water) constitute 

 about 95 per cent of the common mature crops. Nevertheless, each 

 one of the seven elements obtained from the soil, though aggre- 

 gating only 5 per cent, is absolutely necessary to the life and full 

 development of the plant. Indeed, if any one of these elements 

 be entirely lacking, the soil would be infertile and barren. So 

 important are these plant-food elements, that soils are found so 

 deficient in some essential plant food that the addition of a single 

 element will more than double the crop yield. 



(4) The five elements, potassium, magnesium, calcium, iron, and 

 sulfur, are contained in most normal soils in such large amounts, 

 compared to the requirements of crops, that the supply rarely 

 becomes depleted. Thus, in most cases, the problem is narrowed 

 to the two elements, nitrogen and phosphorus, although, for 

 various reasons, potassium also has come to have a recognized 

 money value in commercial fertilizers. 



(5) Nitrogen is contained in the air in inexhaustible amount, 

 but the legumes (clover, alfalfa, peas, beans, etc.) are the only 

 agricultural plants which have power to utilize the free nitrogen of 

 the air. Nitrogen in limited amount is contained in the soil in the 

 organic matter, the principal material which gives a good soil its 

 dark color. If the supply of organic matter is maintained, by 

 plowing under farm manure, clover, cowpeas, or other green 

 manures, then the supply of nitrogen will also be maintained. 



(6) The plowed soil of an acre (2 million pounds, for a depth of 

 6 inches) of rich, well-balanced normal land in the Corn Belt 

 contains about 8000 pounds of nitrogen, 2000 pounds of phosphorus, 

 35,000 pounds of potassium, and 15 tons of calcium carbonate 

 (limestone) . 



(7) The surface soils of the United States vary in composition: 



(a) in nitrogen content, from 1000 pounds to 35,000 pounds; 



(b) in phosphorus content, from 160 pounds to 15,000 pounds; 



(c) in potassium content from 3000 pounds to 60,000 pounds, 

 per acre; and many soils not only contain no lime, but are markedly 

 acid and thus require heavy applications of lime, while some pro- 



