AVAILABILITY OF PHOSPHATIC FERTILIZERS 121 



to the soil is one soluble in water. Even in localities where 

 heavy rains may abound, there is not much danger of loss of 

 soluble acid by percolation. As has before been indicated, the 

 soluble acid tends to become fixed in all normal soils and to re- 

 main in a state accessible to the rootlets of plants and yet free 

 from the danger of exhaustive leaching. For this reason the 

 water-soluble acid is regarded by most agronomists as more 

 available than that portion insoluble in water, yet soluble in 

 ammonium citrate. 



In many of the States the statutes, or custom, prescribe that 

 only the water and citrate-soluble acid shall be reckoned as avail- 

 able, the insoluble residue being allowed no place in the esti- 

 mates of value. In many instances such a custom may lead to 

 considerable error, as in the case of finely ground bones and some 

 forms of soft and easily decomposable tricalcium phosphates. 

 There are also, on the markets, phosphates composed largely of 

 iron and aluminum salts, and these appear to have an available 

 value, often in excess of the quantities thereof soluble in ammo- 

 nium citrate. 



As a rule the apatites, when reduced to a fine powder and ap- 

 plied to the soil are the least available of the natural phosphates. 

 Finely ground bones also tend to give up their phosphoric acid 

 with a considerable degree of readiness in most soils. The soft, 

 finely ground phosphates, especially in soils rich in humus, have 

 an agricultural value, almost if not quite, equal to a similar 

 amount of acid in the acid phosphates. Natural iron and alumi- 

 num phosphates, have also, as a rule, a high degree of availability. 

 Next in order come the land rock and pebble phosphates which, 

 in most soils, have only a limited availability. In each case the 

 analyst must consider all the factors of the case before rendering 

 a decision. Not only the relative solubility of the different 

 components of the offered fertilizer in different menstrua must be 

 taken into consideration, but also the character of the soil to which 

 it is to be applied, the time of application and the crop to be grown. 

 By a diligent study of these conditions the analyst may, in the end, 

 reach an accurate judgment of the merits of the sample. 



