Emil Truog 47 



time, the dropping of needles and leaves provides a continual return 

 of nutrients to the soil. Even with this return, podzols in general are so 

 low in category A and B potassium that soon after being brought un- 

 der cultivation they require heavy applications of potash fertilizer. 



Going now to the slightly podzolized soils of southern Wisconsin, it 

 is found that the supplies of potassium in categories both A and B are 

 usually five to ten times as high as in podzols to the north, although 

 the supplies in category C may be quite similar. This further em- 

 phasizes the fact that transformation from category C to A is very slow. 

 Because of the much greater supplies of category A and B potassium 

 in the less podzolized soils, it is found that many of them may be 

 cropped for 50 years or more without need of potash fertilization. 



The greatest supplies of category A and B potassium are found in 

 the chernozems and other alkaline soils formed under limited rainfall. 

 Here the build-up may be high enough to last for 100 years or more 

 of cropping without the need of adding commercial potash fertilizer. 

 (A more complete discussion of potassium in this connection may be 

 found in references /, 2, and 7.) 



Thus far, nothing has been said about phosphorus, nitrogen, calcium, 

 and other nutrient elements in relation to availability categories. In 

 general, they follow much the same pattern as potassium, but, of course, 

 the types of compounds involved may be quite different. 



Forms of calcium and magnesium in relation 

 to availability categories 



Calcium and magnesium as regards category A are very similar 

 to potassium, also being held there as exchangeable cations. In fact, 

 unless the soil is quite acid or the pH is higher than 9, calcium and 

 magnesium practically always make up the great bulk of the total 

 exchangeable cation content of a soil; and, if the soil is quite acid, 

 then calcium and magnesium are usually supplied in the form of lime 

 to bring up the supply of these elements in category A to the desired 

 level so as to attain a pH of at least 6.5. 



The calcium and magnesium of category A do not become fixed 

 like potassium and revert to category B. In fact, relatively little category 

 B calcium ordinarily exists, unless the exchangeable calcium still re- 



