48 Mineral Nutrition of Plants 



maining in distinctly acid soils be thus designated because of its rather 

 low availability compared with much of that existing at pH 6.5. Mag- 

 nesium like potassium tends to form secondary silicates to a much 

 greater extent than calcium, and, hence, its greater tendency to go into 

 forms which fall in category B. 



Because calcium feldspars and other calcium silicates and most of 

 the primary magnesium silicates weather relatively rapidly, the supply 

 of calcium and magnesium in category C is generally very much lower 

 than potassium, especially in soils of the humid regions. That is one 

 reason why in these regions it is generally necessary, from time to time, 

 to add large amounts of calcium and considerable magnesium in the 

 form of lime. 



Availability categories of phosphorus, nitrogen, and sulfur 



Since phosphorus exists in soils largely as the anion P0 4 ~ _ , it 

 cannot exist in category A in the same form as most of the calcium, 

 magnesium, and potassium. It can, nevertheless, probably exist attached 

 to aluminum and iron hydroxides and silicates as an exchangeable 

 anion, and a small portion of this may possibly be considered as be- 

 longing to category A. However, the main bulk of category A phos- 

 phorus in soils well supplied in this respect exists as calcium phosphate. 

 Experience teaches that for good yields in general farming, a soil should 

 contain at least 50 pounds per acre-plow-layer of category A phosphorus. 

 For truck and garden crops, double and treble this amount is required 

 for best results. 



The forms of phosphorus which make up categories B and C have 

 not as yet become well defined. Possibly, much of the phosphorus 

 attached to aluminum hydroxide and the silicates can be considered as 

 belonging to category B, and most of that attached to iron hydroxide 

 and all existing as apatite as belonging to category C. Details regarding 

 the manner in which phosphates and other nutrient anions are held 

 and become available in soils are given in another paper of this sym- 

 posium. 



Because nearly all of the nitrogen of soils is generally associated with 

 organic matter rather than mineral matter, it might be expected that 

 the pattern of availability categories for nitrogen would be radically 



