THE REACTION BETWEEN DILUTE ACIDS AND 

 THE PHOSPHORUS COMPOUNDS OF THE SOIL. 



By EDWAED JOHN RUSSELL and JAMES ARTHUR PRESCOTT. 



{Rothamsted Experimental Station, Harpenden.) 



Few reactions are more important to the soil chemist than that 

 involved in the action of dilute acids on the phosphorus compounds of 

 the soil, but, owing to its complex nature, little has been definitely 

 ascertained about it. The importance of the reaction lies in the fact 

 that it afiords a distinction between those phosphorus compounds which 

 are fairly easily soluble, and may therefore be expected to enter the 

 plant root without much difficulty, and the less soluble compounds 

 which are of less value in the nutrition of plants. 



The necessity for such a distinction was emphasised in a classical 

 memoir published in 1845 by Daubeny^, who used the terms "active" 

 and "dormant" to express the more and less soluble constituents 

 respectively. He suggested that a solution of carbonic acid might be 

 used to discriminate between them, but the manipulative difficulties 

 proved considerable, and the suggestion was gradually forgotten, and 

 along with it the distinction it was intended to emphasise. It was not 

 till- 1894 that general attention was once more directed to the need for 

 the distinction by the publication of Dyer's important paper^ in which 

 he uses the terms "available" and "unavailable" for these groups,^ — 

 terms which have since been generally adopted in this country. From 

 the circumstailce that the earlier analyses were expressed not only in 

 percentages but also in pounds per acre, the idea gradually arose that the 

 "available" and "unavailable" compounds were sharply distinct. 



^ Daubeny, C. G. B., "On the rotation of crops and on the quantity of Inorganic 

 Matters abstracted from the soil by various plants under different circumstances." Phil. 

 Trans. 1845, 179-253. 



^ Dyer, B., "On the analytical determination of probable available mineral plant 

 food in soils." Trans. Chem. Soc. 1894, 65, 115-67; aisoPhil. Trans. 1901, 194B, 235-90. 

 Joum. of Agric. Sci. vni 5 



