V.J RETENTION OF PHOSPHORIC ACID BY SOIL 149 



solubility, not of the recently added, but of the original 

 soil phosphates. About five extractions remove the 

 phosphoric acid down to this point, further extractions 

 remove very little more, and the sum of the phosphoric 

 acid dissolved in these five extractions approximates 

 very closely to the surplus of phosphoric acid supplied 

 as superphosphate over that removed in the crop. 



Table XL.— Phosphoric Acid soluble in Five Extractions with 

 I PER cent. Citric Acid, compared with that in Manure 

 AND Crop (Roihamsted). 



This shows that phosphoric acid supplied as super- 

 phosphate remains in the surface soil, and in a form 

 that is readily soluble in such weak acids as a dilute 

 solution of citric acid or the natural solution of carbon 

 dioxide occurring in the soil. Doubtless the result 

 would be modified if the soil were not well provided 

 with calcium carbonate, in which case more insoluble 

 phosphates of iron and alumina would be formed. It 

 is a fair conclusion to draw from these results that 

 superphosphate and indeed all phosphatic manures, 

 may be applied to the land much earlier than is usually 

 the case ; because there is not the least fear of their 

 washing out, and it is all-important to get them well 



