PLANT NUTRITION 269 



or on the surface and can be mined by power shovels. Greensand is 

 classified as glauconite (KKeSi^(),j-//l L.O), ihe potassiinn ol which is 

 ^•ery slowly availal:)Ie. Its greatest \ahie seems lo be as a raw material 

 for the production of K2SO4. 



Potassium chloride (KCl) is the chief constituent of muriate of 

 potasii, manure salts, and kainite. The chloride is the most common 

 form of potassium fertilizer and can be used on most crops. However, 

 the quality of some plants is adversely affected by the application of 

 large quantities of chlorides. The sugar content of beets is lowered, as 

 is the quality of potatoes and the burning quality of cigar leaf tobacco, 

 when KCl is used as a fertilizer. 



Sulfate of potash and sulfate of potash-magnesia are marketed as 

 potassium fertilizers. The sulfates are somewhat more expensive than 

 the chlorides, and their use is limited to special-purpose fertilizers in 

 which the chloride is undesirable. Sulfate of potash-magnesia is par- 

 ticularly desirable when magnesiimi is lacking in the soil. This double 

 salt of potassium and magnesiiun contains about equal amounts of 

 potassium sulfate and magnesium sulfate. Its K2O content (about 25 

 per cent) is about one-half that of sulfate of potash. 



Other potassium fertilizers include potassium nitrate, wood ashes, 

 potassium phosphate, and tobacco stems. Sewage sludge and the 

 various seed meals, such as cottonseed meal, also contain small and 

 variable amounts of potassium. 



Potassium salts used as fertilizers are water soluble and therefore 

 shoidd be readily available to plants. However, potassitmi salts added 

 to certain soils with high clay contents appear to be changed to forms 

 in which the potassium is as unavailable as that in soil minerals. This 

 fixation of added potassium is a problem that has occupied the atten- 

 tion of investigators for many years. Experimental workers have se- 

 cured good results from potassium fertilization by making one ex- 

 tremely large application of potassium salts to satisfy the fixing power 

 of the soil. In subsequent years this is followed by small additions of 

 potassium fertilizers for each crop. 



Farm Manure 



Farm manure is the solid and Jicjuid excrement of animals, alone or 

 mixed with litter. Litter is the straw, sawdust, or other absorbent 

 material used in stables and barns. The term nuniurc is sometimes 

 applied to any fertilizer but is more often used, as it will be in this 

 chapter, to mean farm manure. The value of manme for maintaining 

 and improving soil productivity has been recognized from the earliest 

 times. Its beneficial influence on soil fertility is much greater than can 



