Illinois Natural History Survey Circular 56 



hut additional quantities supplied as fertilizers may be 

 needed for maximum growth. 



Plants use from one-tenth to one-fifth as much phosphorus 

 as nitrogen. Phosphorus is found in all living plant tissues 

 and is essential for good root growth, proper tissue 

 development, and the production of flower buds. It is 

 abundant in seeds and other storage organs. Phosphorus 

 has a direct roleasacarrierof energy throughout the plant 

 and is also involved in photosynthesis. When the soil is 

 deficient in phosphorus, plants fail to get a good start at the 

 beginning of the growing season, have poor root growth, 

 and experience delayed flower production. 



Phosphorus is available in commercial fertilizers as super- 

 phosphate, double superphosphate, and with nitrogen as 

 ammonium phosphate. The available phosphate in these 

 fertilizers reacts rapidly with the soil and remains in the area 

 of application. For this reason fertilizers containing 

 phosphorus must be placed in the soil near the tree roots. 

 Surface-applied phosphorus remains near the soil surface 

 and is available only to 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, therefore, 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, this potassium is slowly 

 released in a form that plants can use. 



Potassium is present in woody plants in quantities larger 

 than those of all mineral nutrients except calcium. It is not 

 found in any permanent structure but is involved in 

 changing a plant's food into forms that can be used for 

 growth and other functions. It acts as a balancing agent 



