BIOLOGY OF AGRICULTURAL PRODUCTION 99 

 RELATIVE ** SUPPLY AND DEMAND " OF SEVEN ELEMENTS 



Of course the problem is not as simple as this table would indicate, 

 because these elements are being returned to the soil in various ways 

 from the air and from the decay of plants and from animal wastes. The 

 table does show what tends to happen in the ordinary process of deple- 

 tion from continuous cropping, if care is not taken to thus return the 

 needed elements to the soil. 



The three absolutely essential elements which are likely to 

 limit productivity of a soil are nitrogen, phosphorus, and po- 

 tassium. Nitrogen, the most vital of all, does not exist in 

 combination as a mineral in the soil, but must be added from 

 the decay and waste matters of animals and plants or by bac- 

 terial action. Of the other two, phosphorus is likely to be the 

 limiting element, but potassium compounds, as well as those 

 of calcium, are so easily soluble that they are likly to be 

 completely leached away, as was the case in the peaty loam 

 soil (Fig. 48). No matter what the abundance of the others, 

 lack of any essential element limits plant growth ; it is like a 

 storehouse full of food, with the key lost. This is well shown 

 in the Maryland Experiments with Lime. 2 



1 Added to table from p. 559. 



2 Cf . Hopkins, Soil Fertility and Permanent Agriculture, p. 167. 



