E. J. Russell and J. A. Prescott 69 



of plotting they might be made to fit a curve of the Schloesing and de 

 Sigmund type, or, for that matter, almost any other. Arrangements 

 were therefore made for shaking at constant temperature. Under 

 these conditions uniform results are obtained, but the figures show no 

 constancy over any period (Fig. 2, Table III). Thus the evidence of 

 constant action disappears, and with it the evidence for the view that 

 the action of dilute acids on soil is a simple solvent action, an easily 

 soluble phosphate being first attacked, and then more difl&cultly soluble 

 phosphates. We have just seen also that the action is not of the 

 ordinary mono-, di-, or tri-molecular type. 



The significance of these conclusions is very considerable. The 

 soil was for a long time regarded as a collection of insoluble inert mineral 

 fragments admixed with small quantities of more soluble substances 

 some of which arose by weathering or other decomposition processes. 

 This view had the advantage of simplicity, and it allowed of the applica- 

 tion of simple and chemical and physical laws to soil processes. But 

 unfortunately it leads to inaccurate conclusions. If soil is simply a 

 mixture of insoluble inert minerals with phosphates, etc., there is no 

 reason why it should behave abnormally towards dilute acids. In like 

 manner the view leads to wrong conclusions as to the phenomena of 

 evaporation of water from the soil^. 



It thus appears that this simple conception of the constitution of 

 the soil is inaccurate, and must be discarded. We have been able to 

 show that another conception is more in accordance with the facts. 



The most convenient method of studying the reaction between dilute 

 acids and the phosphorus compounds of the soil is to shake a definite 

 weight of the soil, — we used 50 or 100 gms., — with a uniform volume, 

 — 1 litre in our experiments, — of the acid at constant temperature 

 (23° C.) for a definite timfe, and then to estimate the amount of phos- 

 phorus compounds in the solution. Thus all the factors are under 

 control and can be varied one at a time, all the others remaining 

 constant. The results obtained are briefly summarised below. 



1. When a soil is shaken for a definite period at constant tempera- 

 ture with a dilute acid the amount of action is found to increase 

 continuously with the concentration of the acid. The increase is nearly 

 proportional to the concentration of the acid, but not quite, and on 

 plotting the results they are seen to fall on perfectly smooth curves 



1 This Journal, 1914, 6, 456-475; also Annual Report oj the Rothamsted Experimental^ 

 Station, 1914, p. 6. 



