Emil Truog 49 



different from what it is for, say, potassium. This, however, is not the 

 case. In order for the soil to be a frugal custodian, it appears necessary 

 or desirable that the same general pattern be followed for all of the 

 nutrient elements. 



The nitrogen of category A usually exists largely as fresh crop resi- 

 dues and recently applied manure and nitrogen fertilizers. Very small 

 amounts may be held in exchangeable form as NH 4 ~. 



When organic matter accumulates for long periods as it has in 

 chernozems, and mucks and peats, true humus is formed, and much 

 of the nitrogen contained therein, possibly two-thirds or more, is of 

 low availability and belongs to category C; the remainder falls in 

 category B and largely provides the nitrogen for crops when these soils 

 are brought under cultivation. Under continuous cropping, very little 

 of the nitrogen added in the form of crop residues, manure, and ferti- 

 lizer becomes transformed into category B or category C. However, 

 when sod crops occupy the land continuously for several years, such 

 transformation does take place to an appreciable extent. Thus, it is ap- 

 parent that categories B and C with respect to supplies of nitrogen 

 can only be built up in practical farming by keeping the soil in sod 

 crops a good share of the time. 



Sulfur, as regards availability categories, follows a pattern quite 

 similar to nitrogen when it is present as a constituent of organic matter. 

 It also exists in mineral forms which fall into all three categories, but 

 often to the greatest extent in category C. 



Availability categories of the minor nutrient elements 



The four minor nutrient elements — iron, manganese, copper, and 

 zinc — which exist in soils primarily as cations follow much the same 

 pattern as calcium and potassium. Contrary to this pattern, iron and 

 manganese particularly when the pH is above 6.5, readily form the in- 

 soluble higher oxides which belong to categories B and C. Boron fol- 

 lows a pattern quite similar to nitrogen except that it changes rapidly 

 to category C at moderate alkalinity, and again rapidly to A and B at 

 high alkalinity. 



To be sure, many if not most of the details of the matter just dis- 

 cussed are still to be worked out, especially with respect to the minor 



