46 Mineral Nutrition of Plants 



which is two to three times as high. Fortunately, the level of potassium 

 in category A is easily determined by means of a chemical test. 



The amount of potassium in category B may be several times that in 

 category A, but ordinarily during a growing season crops probably get 

 no more than 10 to 20 per cent of their potassium needs from the 

 former. The very important consideration in this connection is that 

 during the time when crops are not growing, there occurs a flow of 

 potassium from category B to A to make up some of what the previous 

 crop has removed from A. Transformation of potassium from category 

 B to C and the reverse of C to B is extremely slow. In fact, there is 

 some question whether or not these transformations in the case of 

 potassium take place at all, especially from B to C. 



Now, what happens when soluble fertilizer potassium is added to a 

 soil in which categories A and B are at equilibrium? At first, most of 

 this potassium rapidly goes into the exchange form of category A. This 

 causes a disturbance of the previously established equilibrium between 

 A and B, and as a result there occurs a flow of potassium from A to B 

 until equilibrium is again established. That is why it takes more than 

 the calculated amount of potash fertilizer to raise the level of potassium 

 in A to some prescribed point. However, it is important to recognize 

 that this potassium which is transformed to category B is not lost. It 

 is simply stored more safely against leaching and reckless use by care- 

 less cropping. Yes, the soil is a frugal custodian. 



What is the situation in virgin soils as regards levels of potassium in 

 the three availability categories? Let us consider first the conditions 

 under which the podzols are formed. Here, leaching is so severe that 

 even a frugal soil is unable to stem the tide of relatively rapid loss of 

 potassium from category A. This causes a continual flow from B to A, 

 and, as a result, the level of potassium in categories A and B becomes 

 low. The flow of potassium from C to A is so slow that virgin podzols 

 in northern Wisconsin, containing less than 100 pounds per acre-plow- 

 layer of category A potassium, have on the same basis 30,000 to 40,000 

 of this element in category C. This is further evidence that category C 

 cannot be depended upon to furnish much of the current needs for 

 profitable cropping other than forestry where the annual drain is low 

 and only one crop may be removed in 50 years. Moreover, during this 



