234 SOILS 



Georgia, and in Florida. As taken from these mines, this 

 rock contain" from twenty-six to thirty-five per cent, of 

 phosphoric acid, the remaining portion of the rock being 

 such impurities as sand, clay, limestone, and water. In 

 its raw or natural state, phosphate has three parts of lime 

 united with the phosphoric acid. The chemists call this 

 tri-calcium phosphate. It is very insoluble in water, and 

 plants cannot use it. To make it soluble in water and fit 

 it for plant food, the rock is finely ground and treated with 

 sulphuric acid, which acts upon it in such a way as to take 

 from the three-lime phosphate two parts of its lime, thus 

 leaving only one part of lime united to the phosphoric 

 acid. This one-lime phosphate is what is known as water- 

 soluble phosphoric acid. 



On long standing, this water-soluble phosphoric acid 

 has a tendency to take lime from every substance in con- 

 tact with it, and in so doing becomes less soluble. This 

 gives rise to the term "reverted" or "gone-back" phos- 

 phoric acid. 



In this product there is supposed to be two parts of 

 lime in combination with the phosphoric acid and is thus 

 an intermediate product between soluble and the original 

 rock. Of course, in treating with sulphuric acid, some 

 of the ground rock is not acted on by sulphuric acid, and, 

 hence, is left in its original insoluble condition. In this 

 we get insoluble phosphoric acid, as our fertilizer bags 

 often indicate. Available phosphoric acid is made of the 

 water soluble and reverted ; it is the sum of these two ; 

 and the available and insoluble make the total phosphoric 

 acid it includes all the phosphoric acid present. 



When you buy fertilizers again', just bear these facts 

 in mind. 



I believe you will be more interested hereafter in get- 

 ting available phosphoric acid than total phosphoric acid, 



