4 ILLINOIS NATURAL HISTORY SURVEY CIRCULAR 52 



in the soil near the tree roots. Surface-applied phosphorus re- 

 mains near the soil surface and is available only to the plants 

 with roots in this region. Almost no phosphorus is carried away 

 by water. 



Potassium 



Most of the rocks from which soils were formed contained 

 potassium. Soils usually contain more potassium than nitrogen 

 or phosphorus. The salts of potassium are readily soluble in 

 water and may be carried away in areas of heavy rainfall, 

 especially in sandy soils. Soil containing clay or organic matter 

 has a large amount of potassium in a form unavailable to plants. 

 However, it is slowly released in a form which plants can use. 



Potassium is present in woody plants in quantities larger 

 than those of all mineral nutrients but calcium. It is not found 

 in any permanent structure but is involved in changing a plant's 

 food into forms it can use for growth and other functions. It acts 

 as a balancing agent between root growth and top growth and 

 between nitrogen and phosphorus utilization. A visual symptom 

 indicating potassium deficiency is not readily apparent on most 

 trees and shrubs. 



The most important commercial fertilizer containing potas- 

 sium is potassium chloride, commonly called muriate of potash. 

 Potassium is distributed from the point of application in the 

 soil somewhat faster than phosphorus but not nearly as rapidly 

 as nitrogen. Potassium-containing fertilizers should therefore be 

 placed in the soil and not applied to the soil surface. Only those 

 roots quite near the point of application can absorb ample quan- 

 tities of potassium. 



Formulations, 



Most granular or crystalline commercial fertilizers contain 

 nitrogen, phosphorus, and potassium in guaranteed amounts. 

 Also some calcium, magnesium, sulfur, and micronutrients are in- 

 cluded either as impurities or in combination with nitrogen, 

 phosphorus, or potassium. The guaranteed analysis of most 

 fertilizers is shown on the bag as three numbers, for example, 

 12-12-12. The first number gives the percentage of nitrogen (N) ; 

 the second number gives the percentage of phosphorus as phos- 

 phoric acid (P 2 5 ) ; the third number gives the percentage of 

 potassium as potash (K 2 0). 



