BIOLOGY OF ACIJKTT.TrKAL l'l;( »I )l ( TK )N 99 



Relative "Supply and Demand" «>f Seven Elements 



Essen t i a i. Plan r-Ft »< > i » 

 Elkmknts 



Phosphorus 



rotassiuni 



Magnesium 



Calcium 



Iron 



Sulfur 



Nitrogen (vir2:in N.W. soil)i 

 Nitrogen (in air over acre)i 



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 jilants and from aninuil wastes. The 

 table does show what tends to ha[»pen 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 absokitely essential eleineiits which are likely to 

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

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

 combination as a mineral in the soil, but nnist be addc(l fruni 

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

 terial action. Of the other two, phosphorus is likely to ])c the 

 limiting element, but potassium compounds, as well as those 

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

 coniplelely 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 

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

 in the ^Maryland Exi)eriments with Lime.^ 



^ Added to table from p. 550. 



^ Cf. Hopkins, Soil Fertility and rermanent Agriculiiuv, p. 107. 



