EXPERIMENT STATION BULLETINS. 235 



of what is commonly called reverted phosphoric acid. By reverted phos- 

 phoric acid is meant chemically di-calcic phosphate. This form is sup- 

 posed to have been once soluble in water but aftter standing some time to 

 have reverted or go7iG lack to the di-calcic phosphate, hence its name, re- 

 verted phosphoric acid. This reverted or di-calcic phosphate is completely 

 soluble in a solution of citrate of ammonia, hence its name, also, citrate- 

 soluble phosphoric acid. In this form it is supposed to be readily avail- 

 able to plant growth and, therefore, in the Bulletin the water soluble and 

 the citrate soluble are taken together and called ''available phosphoric 

 acid." 



3. Insoluble phosphoric acid. In this form the phosphoric acid is as 

 tri-calcic and tetra-calcic phosphate of lime and is completely insoluble 

 in water and citrate of ammonia. It is not, therefore, immediately avail- 

 able to plants. Fertilizers containing their phosphoric acid in this con- 

 dition should not be used where immediate results are expected, but they 

 may be used to reclaim exhausted soils. 



Generally speaking, the fertilizer of most value from a phosphoric acid 

 standpoint is the one containing a relatively large proportion of available 

 and a relatively small proportion of insoluble phosphoric acid. 



SUPER-PHOSPHATES. 



In true super-phosphates the phosphoric acid is all available. Super- 

 phosphates are formed by treating insoluble bone and phosphate rock 

 with sulphuric acid. The sulphuric acid combines with the lime forming 

 sulphate of lime or gypsum, while the phosphoric acid being freed from its 

 combination with the lime is rendered available. Super-phosphates are 

 known in fertilizers also as acid phosphates. 



POTASH. 



Potash (K.O) is a combination of the metal potassium (K) two parts 

 with one part of the gas oxygen (O). Potassium in the free state is not 

 known in nature for it has such an aflSnity for oxygen that it will destroy 

 other substances to obtain their supply of oxygen. In contact with water 

 it takes fire, thus decomposing the water and depriving it of its oxygen ; 

 it, like phosphorus, is not estimated in its pure state, but as potash. 



Potash is a constituent of many rocks and in organic forms in vege- 

 tables it is quite abundant. Roots contain a large amount of potash. 

 It is available to commerce, mainly in the forms of sulphate and muriate 

 of potash. Wood ashes contain considerable amounts of potash (about 

 5^) in the form of carbonate of potash. Only one form of potash is 

 recognized in fertilizers, and that is what is soluble in water. 



USES OP NITROGEN. 



1. Nitrogen gives a healthy growth to plants by promoting the forma- 

 tion of chlorophyll, or the green coloring matter of the stems and leaves. 



2. It is useful in promoting leaf growth but if present in too large 

 available quantities it retards maturing of the plant. 



