PERMANENT AGRICULTURE 335 



period of several years, increasing during the first three or 

 four years. 



Calcium add phosphate, called also simply acid phos- 

 phate, soluble phosphate of lime, one lime phosphate, 

 superphosphate, etc. It is soluble in water and hence 

 gives quick results though the effects are not seen so long 

 as in the case of floats. An average sample contains 

 about seven per cent of phosphorus. 



Ground or steamed bone, which is chemically nearly the 

 same as floats, and which contains from ten to eleven per 

 cent of phosphorus in addition to about two and a half 

 per cent of nitrogen. 



Basic slag, a by-product of the manufacture of steel, 

 is considerably used as a source of phosphorus. It con- 

 tains from six to nine per cent of this element. It dis- 

 solves very slowly and hence should be finely ground. 



Potassium. — Most new soils contain enough potassium 

 to supply the needs of plants for a very long time. How- 

 ever, sandy' soils, or newly drained swamp land, may be 

 deficient in this element as well as those soils which 

 have been long under cultivation. It may be restored 

 only in the form of barnyard manure or commercial 

 fertilizers. 



The chief commercial fertilizers containing potassium 

 are : 



Potassium chloride, called also muriate of potash. 

 It gives satisfactory results for ordinary crops ; but pota- 

 toes, tobacco, sugar beets and other plants which use 

 much potassium appear to be injured by the chlorine 

 present. 



Potassium sulfate, which is of special value for those 

 crops which are injured by the chloride. 



Kainite, which contains from ten to twelve per cent of 

 potassium in the form of both chloride and sulfate. 



