PLANT FOOD IN THE SOIL 75 



80. How much plant food is available in a season? The pro- 

 duction of 50 bushels of corn, which is double the average acre- 

 yield in the United States, requires approximately 74 pounds of 

 nitrogen, 1 1 pounds of phosphorus, and 35 pounds of potassium. 

 It is estimated that about 2 per cent of the nitrogen and i per 

 cent of the phosphorus and one fourth of I per cent of the 

 potassium l in the soil may be available in a season for the use 

 of crops. Therefore to produce 50 bushels of corn at least 

 3700 pounds of nitrogen, noo pounds of phosphorus, and 

 14,000 pounds of potassium must be within the reach of the 

 plant roots. 



Our best soils originally contained from 6000 to 8000 pounds 

 of nitrogen, from 2000 to 3000 pounds of phosphorus, and from 

 30,000 to 45,000 pounds of potassium to the acre in the first 

 twelve inches, but continuous cultivation, heavy cropping, sur- 

 face washing, and leaching have reduced the store of plant food 

 in most soils much below these amounts. Reasonably produc- 

 tive soils after having been in cultivation a generation or more 

 contain as little as 3500 pounds of nitrogen, 1200 pounds of 

 phosphorus, and 30,000 pounds of potassium. It is evident that 

 if most soils are to produce heavy yields, either the food they 

 contain must be made available more rapidly than it is in the 

 average soil or the crops must be helped with a manure. 



81. How phosphorus and potassium are made available to 

 plants. Phosphorus and potassium are mineral elements derived 

 from the mineral part of the soil. They are made available to 

 plants partly through the action of the weather upon the soil 

 and partly also through the effects of decomposing organic matter. 

 Among the products set free in decomposing organic matter are 

 nitric acid and carbonic acid, as well as other and more complex 

 organic acids. These acids, particularly the carbonic acid, are 

 taken up by the soil water and exert a much greater solvent action 

 upon the undissolved soil compounds than does pure water, thus 

 greatly increasing the solution of the soil minerals. Moreover, 

 the phosphorus and potassium stored in the organic matter are 



1 Hopkins, Soil Fertility and Permanent Agriculture, p. 107. 



