844 ANNUAL, REPORT OF THE Off. Doc. 



Several investigators have tested the solubility of rock pho«phateg 

 in hiiniic acids and in humates of ammonia, and it has been shown 

 that the humic acids have really a considerable solvent power for 

 phosphates, which would seem to exjjlain the good elfect* produced 

 by certain phosphates on peaty or muck soils. 



INFLUENCE OF THE KIND OF CROP UPON THE AVAILABILITY. 



The root system and habit and periods of growth vary considerable 

 in different classes of crops, and hence their ability to get at and use 

 plant foods, which accounts for some crops using insoluble phos- 

 phates more readily than others. 



For instance, some experiments seem to show that turnips possess 

 to an especial degree the ability to feed upon undissolved phos- 

 phates, while potatoes seem to have but little ability in this direc- 

 tion. 



THE INFLUENCE OF THE SOURCE OF THE PHOSPHATE UPON ITS 



AVAILABILITY. 



As has been stated, the availability of a phosphate depends upon 

 the source and the relation which it bears to the soil, the crop and 

 the conditions under which it is applied. So that phosphate which 

 is available under one condition may be unavailable under another. 



If the availability of the phosphate used depends upon the changes 

 which take place after it is applied to the soil, it will generally be 

 found that organic phosphates will act quicker than those of mineral 

 origin, as all organic matter is subject to decay and thus responds 

 to the action of the natural agencies which exist in most cultivated 

 soils. While the mineral phosphates are fixed and more or less 

 stable compounds, which, if not natural food for crops become so but 

 slowly, as they yield very gradually to chemical changes. 



When the material is being used in a dissolved condition it makes 

 but little difference whether it is derived from the animal or mineral 

 source. 



THE AVAILABILITY AND LASTING EFFECTS OF PHOSPHATES AS DE- 

 TERMINED BY MECHANICAL CONDITION OR DEGREE OF FINE- 

 NESS. 



The finer or more highly sub-di\aded a material is the greater sur- 

 face it presents and, hence, the more easily is it acted upon by either 

 organisms of decay or dissolved by the soil water, or the solution 

 sent out by the roots of plants for the purpose of preparing plant 

 food. 



The finer the material the more easily is it disseminated in the 

 soil and consequently placed in better position to be used by plants. 



The chief ultimate value, as explained elsewhere, in reducing the 

 phosphate to a soluble condition is the fine division and great dis- 

 semination which they get in the soil by being reverted or precipi- 

 tated. 



