33 2 SOILS. 



soils for comparison ; although they have served as a basis of comparison 

 lor years in the Washington Bureau of Soils. 



Solubility of Soil Phosphates in Water. The solubility of 

 the phosphate contents of soils has been elaborately investi- 

 gated by Th. Schloesing fils. 1 He found in the case of a num- 

 ber of soils investigated by him that the amount of phosphoric 

 acid P2O5 in the soil-solution ranged from less than one mil- 

 lionth (or one milligram per liter of water) in a poor soil, to 

 over three milligrams in a rich one. He also found that for 

 one and the same soil the amount so found was constant, if 

 about a week's time were allowed for saturation. He calcu- 

 lates that while in general the amount of phosphoric acid capa- 

 ble of being supplied to the crop during a growing season of 

 twenty-eight to thirty weeks would suffice for but few crops, 

 the supply so afforded is in no case a negligible quantity, fre- 

 quently amounting to more than half of the crop-requirements. 

 Experiments with various crops prove that these dilute solu- 

 tions are utilized by all of them, sometimes to the extent of 

 completely consuming the content of the solution. The much 

 smaller content of phosphoric acid in drain waters is accounted 

 for by the lack of time for full saturation during the time that 

 the flow lasts. Whitney, (Bureau of Soils, Bulletin 22) has 

 extracted the soil-solution by means of the centrifuge from 

 several soils ; the contents of phosphoric acid thus found are in 

 general of the same order as those shown in the preceding table 

 by King, but much in excess of Schloesing's figures ; notwith- 

 standing the fact that Whitney's soils had been in contact with 

 water for only twenty-four hours. The cause of this wide dis- 

 crepancy is not clear. 



Practical Conclusions from Water Extraction. As regards 

 the practically useful conclusions to be drawn from the ex- 

 traction of soils with pure water, the data given above, and 

 especially the results obtained by King, seem to prove that 

 there is a more or less definite correlation between the immedi- 

 ate productiveness of soils and the amount and kinds of in- 

 gredients dissolved ; especially in the case of phosphoric acid, 

 the adequacy of the supply of which for immediate production 



1 Ann. de la Sci. Agron., 2de serie tome i, pp. 416-349; 1899. 



