182 ALFALFA FARMING IN AMERICA. 
phosphate, acid phosphate or acidulated rock. For its manufac- 
ture the rock phosphates are generally employed, both because 
they are cheaper and because the organic matter in the bones 
interferes with the use of sufficient acid to make all the phos- 
phate soluble. A good sample of super-phosphate or acidulated 
rock contains about 16 per cent of phosphoric acid in a form that 
is soluble in water. Sometimes when insufficient acid has been 
used a part of the soluble phosphate will change into a form in- 
termediate in solubility between the natural phosphate and the 
acid phosphate, and the acid phosphate is said to have undergone 
reversion, and the new compound is called reverted phosphate. 
The latter product is supposed to be more available to the plant 
than the insoluble or natural phosphate, hence the soluble and 
reverted phosphoric acid taken together are known as the avail- 
able phosphoric acid. 
In some instances bone meal is treated with a limited amount 
of sulphuric acid and the product is called acidulated bone. 
This substance contains a much smaller proportion of its phos- 
phoric acid in the soluble form than does the rock superphos- 
phate. When soluble phosphates are added to the soil they soon 
combine with the mineral matter, and are converted first into 
the reverted phosphate, and finally into the insoluble form such 
as is found naturally in the soil. In this way the phosphoric 
acid is fixed and there is no danger of its being lost by leaching. 
Relative Value of Phosphate Fertilizers—The soluble phos- 
phate present in the acidulated goods is generally considered the 
most valuable form of phosphoric acid for use as a fertilizer. At 
first sight it seems useless to go to the expense of making the 
phosphate soluble when it is again rendered insoluble by the 
soil before the plant can make use of it. The real object in mak- 
ing it soluble is to aid in its distribution in the soil. When an 
insoluble phosphate is applied it remains where it falls except 
for the slight distribution it receives by cultivation. In the case 
of the soluble phosphate, on the other hana, the phosphate dis- 
solves in the soil water and is widely distributed before it be- 
comes fixed by the soil. In the former case the roots must go 
to the phosphate while in the latter the phosphate is carried to 
the roots. It follows from what has been said that after the 
soluble phosphate is distributed throughout the soil the indi- 
vidual particles must be very much smaller than is the case with 
the insoluble phosphate. 
There are some soils upon which the superphosphates cannot 
be used without injury, usually those that are deficient in lime, 
the superphosphate in such cases having a tendency to make 
