Emil Truog 45 



similar type of mineral. Since biotite contains ferrous iron which suffers 

 oxidation on exposure to air and water, this mineral is quite readily 

 subject to breakdown and release of its potassium. 



The potash feldspars and muscovite contain practically all of the 

 slowly or difficultly available potassium. Muscovite, unlike biotite, con- 

 tains no ferrous iron and, as a result, is tremendously resistant to 

 weathering and release of its potassium. Potash feldspars, being dense 

 crystalline silicates, weather slowly. This is particularly true of micro- 

 cline. Although soils frequently contain 30,000 to 40,000 pounds per 

 acre-plow-layer of muscovite and feldspar potassium, usually not more 

 than five to ten pounds of this become available annually for crop use. 

 However, if little or no plant growth is removed from land for several 

 years and leaching is not too severe, there results a sufficient accumu- 

 lation from this source to be a potent factor in supplying potassium to 

 one or more subsequent crops. The so-called "resting of land" owes its 

 virtues to processes of this kind. 



Let us now suppose a crop of corn is growing on the soil in question. 

 What happens? There is a heavy drain on category A and, hence, its 

 pressure for transformation to category B drops. As a consequence, the 

 rate of transformation from B to A exceeds the reverse transformation 

 and there is a gradual replenishment of A from B. After the growth 

 of a heavy crop, it may take several months or more for the replenish- 

 ment to run its course so that A and B are again in equilibrium. That 

 is why the amount of readily available potassium found in a soil by 

 test in the fall after the removal of a heavy crop is sometimes consider- 

 ably lower than the amount found the next spring. 



It will be noted in the diagram that there is a narrow connection 

 between categories A and C, and the arrows indicate some transforma- 

 tion from C to A. However, the rate of this transformation is so slow 

 that what may be transformed during a growing season is sufficient to 

 supply only 5 to 10 per cent of crop needs. Thus it is apparent why 

 the amount of potassium in A must be kept up to a certain level if a 

 crop is to be adequately supplied. In practice it has been found that 

 category A should contain about 200 pounds per acre-plow-layer of 

 potassium to assure good yields of corn, alfalfa, and other general farm 

 crops. Potatoes, sugar beets, and truck and garden crops require a level 



