These are called respectively tri-calcic phosphate, di-calcic 

 phosphate and mono-calcic phosphate. 



It is in the form of these calcium or lime combinations that 

 plants obtain their phosphoric acid. These three substances 

 vary in the way in which they dissolve in water, and hence are 

 not equally available as plant-food. 



The mono-calcic phosphate dissolves in water as does sugar or 

 salt and consequently its phosphoric acid is directly available. 

 The di-calcic phosphate does not dissolve in water, but becomes 

 soluble in the soil water, which contains carbonic acid gas in so- 

 lution. The di-calcic phosphate is therefore available phosphoric 

 acid. The mono-calcic phosphate tends to pass into the di-calcic 

 condition in the soil; when so changed it is said to be "reverted." 



The total available phosphoric acid in a fertilizer is that which 

 it contains in the forms of mono- and di-calcic phosphates. The 

 tri-calcic phosphate is soluble neither in water nor in soil water, 

 and is known as the insoluble phosphoric acid. 



10. These phosphates are made mostly from bone {either recent or 

 fossil.) — Bones are the chief source of phosphoric acid. The phos- 

 phate rock deposits found in the South are from ancient bone de- 

 posits. In bones the phosphoric acid is insoluble: that is, it is there 

 as tri-calcic phosphate. To make it available, the bones are treated 

 with sulfuric acid (oil of vitrol) and water. In the action which 

 takes piace, the sulfuric acid takes from the phosphoric acid one 

 or two parts of its lime and puts water (H^O) in their places. 

 The lime which is taken away by the sulfuric acid, unites with 

 the sulfuric acid and forms sulfate of lime (gypsum or land plas- 

 ter) , Gypsum is always a constituent of a fertilizer containing 

 available (or treated) phosphoric acid. 



11. Potash {KjO) is also an essential fei'tilizer mate?ial. — Most 

 soils contain more potash than phosphoric acid. The potash in 

 soils is mostly locked up in compounds known as silicates. Pure 

 sand is silica. These silicates are insoluble in water, and hence 

 the potash is not readily available. Perhaps it is well that nature 

 locked up the potash in such a strong combination as a silicate, or 

 it might have run out, as the nitrogen has from many soils. 



1 2 . Potash is made available by tillage, and the decay of humus. — 



