ACTIVE PLANT FOOD, ETC. 187 



Significance of the Dissolved Plant Food. 1 In considering the 

 significance of the dissolved plant food, it is necessary to regard 

 the active phosphoric acid and potash, the "acid consumed," and 

 the fixing power of the soil. The fixing power is of importance 

 chiefly in connection with soils which fix more than 80 per cent, 

 phosphoric acid. With such soils, the extracted phosphoric 

 acid may, or may not, represent the soluble phosphates. 



The "acid consumed" is a measure of the bases dissolved by 

 the solvent, and is estimated by titrating 10 cc. of the solution 

 after the extraction is complete. 



When 10 parts per million of phosphoric acid, or less, is 

 extracted, associated with a fixing power of less than 50 per cent., 

 and with acid consumed less than 90 per cent., it indicates that 

 practically none of the phosphoric acid of the soil is present as 

 apatite, calcium phosphate, or similar compounds, but must be 

 present as basic iron or aluminium phosphates or in organic com- 

 bination. When 10 parts phosphoric acid, or less, are present and 

 the soil has a high fixing power for phosphoric acid (75 per cent, 

 or more), calcium phosphates may or may not be present. That 

 is to say, the method can not in this case distinguish between 

 phosphoric acid which goes into solution from calcium phosphate 

 and is then removed by fixation, and that which comes from the 

 basic phosphates of the soil. The origin of the soil will throw 

 some light upon the matter. If the soil is geologically old, the 

 phosphoric acid has probably all been converted into basic phos- 

 phates. If the soil has been recently formed from rocks contain- 

 ing apatite and other phosphatic minerals, it is possible that cal- 

 cium phosphate may still be present and the same is true if the 

 soil has been fertilized. In the majority of soils having a high 

 fixing power and a low content of phosphoric acid, provided that 

 .they have not been fertilized, the phosphoric acid is probably 

 present as basic iron and aluminium phosphates. 



A soil of high fixing power such as above mentioned would 

 yield up the same quantity of phosphoric acid to the solvent, 

 whether fertilized or not fertilized, unless a very heavy applica- 

 1 Texas Station Bulletins 126 and 145. 



