82 1-ERTlLITV AND FERTILIZER HINTS 



This soluble phosphate is called super or acid phosphate and is 

 a saturated compound. It is also known as monobasic, mono- 

 calcic, and one lime phosphate. 



This compound may be graphically represented as : 



Lime ( 



Water - Phosphoric acid. 



Water ( 



This form is entirely soluble in water and readily available as 

 plant food. It is the highest valued form of phosphate of lime 

 and is called soluble phosphoric acid. 



3. Reverted Phosphoric Acid. — Between the soluble and in- 

 soluble phosphate of lime there is another form known as re- 

 verted, citrate soluble, dicalcic, and two lime phosphate, in which 

 there are two parts of lime and one part of water as represented : 



Lime ( 



Lime - Phosphoric acid. 



Water ( 



The name reverted is applied to this form of phosphate of 

 lime because it is formed by reversion or retrograding of some 

 of the soluble towards the insoluble. This form is not as soluble 

 as the soluble phosphoric acid and is more soluble than the in- 

 soluble form. It is insoluble in water, but the weak acids of the 

 soil render it favorable for plant food. The sum of the soluble 

 and the reverted is called available, because both forms may be 

 used by plants. 



4. Basic Slag Phosphate. — It used to be accepted that the three 

 forms just described were the only forms of phosphoric acid. 

 However, the phosphate of lime in basic slag is in another form. 

 It was supposed that one part of phosphoric acid was combined 

 v^^ith four parts of lime, and in this form it was known as tetra- 

 calcic, tetrabasic, and four lime phosphate. 



Lime ( 



]^^l \ Phosphoric acid 

 Lime ( 

 Recently, however, there seems to be some uncertainty as to 

 whether or not the phosphoric acid in basic slag exists as tetra- 



