Plant Food in the Soil 85 



The R6le of Potassium is a metallic element, and like 



Potassium 



the Plant 



Potassium in ^j^^ ^^^io: elements cannot be used by plants 



in its elemental form, but must be had in 

 some combination. We get potassium in the form of 

 the oxide of potassium, or as it is commonly called pot- 

 ash. No plant can thrive in the absence of available 

 potash in the soil. In our clay soils, which are the 

 result of the decomposition of granitic or feldspathic 

 rocks, there is usually a large supply of potash in the form 

 of an anhydrous siHcate, which becomes slowly soluble 

 through the action of the lime in the soil and the carbon 

 dioxide in the rain water, and it can be rendered more 

 rapidly available by fresh applications of water-slaked 

 lime, which, as one chemist has said, "pushes out" the 

 potash from its insoluble sihcate. While nitrogen rapidly 

 leaches from the soil in the drainage water, there is but a 

 slight loss of potash in this way, for the soil holds the 

 phosphorus and potash much more strongly than it does 

 nitrates, and but the merest trace of these is usually found 

 in the drainage water. Potash is much more slowly used 

 up from the soil by crops than phosphorus is, since in the 

 ripening of grain the potash largely returns to the roots. 

 Still there are crops Hke potatoes and tobacco which 

 select far more potash than other plants and hence these 

 exhaust the soil supply most rapidly. Potash also exists 

 in nature in the combinations with chlorine, sulphur, 

 and carbon, making the muriate, sulphate, and car- 

 bonate of potash. These are all readily soluble forms, 

 and are the forms in which potash is most commonly used 

 in fertihzers. It is also combined with nitrogen in the 

 form of the nitrate of potash, or as it is commonly called, 



