ACTIVE PLANT FOOD, ETC. 185 



known fixing power with the solvent, one part with the acid alone, 

 and the other with a known quantity of phosphoric acid or potash. 

 The quantities should be such as might be dissolved from the 

 soil. The following is an example i 1 



Phosphoric acid 

 Parts per million 



Extracted from soil alone 8.5 



Added to solvent 194.0 



Total present 202.5 



Actually recovered 48.0 



Absorbed by soil 154-5 



With 17 soils, the fixation of the added phosphoric acid ranged 

 from 5 to 94 per cent, of the quantity added. Thus the phos- 

 phoric acid extracted by the N/5 nitric acid does not necessarily 

 represent that which has gone into solution, but represents the 

 resultant of the solvent, and the fixing power of the soil. With 

 some soils, the fixing power is so high that it must be considered 

 very seriously in interpreting the results of the analysis. Fixation 

 also takes place from acids stronger than fifth-normal. 



Fixation of potash takes place under the same conditions as the 

 fixation of phosphoric acid, but to a much less extent, and the 

 factor of fixation is much less important with potash. 



Soils containing easily-soluble phosphoric acid or potash com- 

 pounds give decreasing amounts to successive extractions, but 

 soils containing little or no compounds of high solubility give 

 successive extracts of nearly constant composition. 



Solubility of Constituents of the Soil. The solubility of the 

 constituents of the soil must be considered as a factor in the 

 analysis of soils with weak solvents. If any quantity of the soil 

 passes into solution, phosphates will thereby be exposed to the 

 action of the solvent, which were protected from the action of 

 soil moisture and roots, and which are really physically unavail- 

 able. This factor must be given careful consideration. For 

 example, N/5 nitric acid dissolves 320 parts per million of lime 

 (CaO) from one soil, while from another soil it dissolves 53,250 

 parts, which corresponds to nearly 10 per cent, carbonate of 

 1 Texas Station Bulletin 126, p. 16. 

 13 



