E. J. Russell and J. A. Pkescott 83 



It is not much afEected by the soil, but it falls off with the time of 

 action. 



It has been pointed out that the heavy soils show a high value for 

 K, i.e. a high reluctance to part with their P2O5, while the light Bramford 

 soil only shows a small reluctance and parts with it fairly easily. We 

 are unable to explain why the Wobum soil behaves otherwise. The 

 immediate reason is that the diffusion process does not come to an end 

 as speedily as in other cases, — a phenomenon which would be readily 

 explained if small nodules of coprolite were present, — as indeed they 

 are known to be elsewhere in the district. 



Whatever the explanation of the behaviour of the Woburn soil 

 there is evidence that this distinction holds generally : it is difficult 

 to get clean cut illustrations, but as a rule sandy soils respond less to 

 phosphates than clay soils, not only because there is often more there, 

 but also because plants can make more use of what is actually present, 

 in other words, the sandy soils part with their phosphates more readily 

 than heavier soils would do. This is clearly a promising field for enquiry, 

 and we hope that fresh data will be obtained so that a fuller discussion 

 may become possible. 



It is less easy to say much about p because it varies so little with 

 changes in the soil, and so much with changes in the solution. It is 

 apparently less directly connected with the properties of the soil than 

 K, and is therefore of less interest to us. And as physicists themselves 

 have not come to any agreement as to its meaning in spite of a vast 

 amount of work, we may safely leave it alone for the present. 



The hearing of the results on soil analysis. 



Since the action between dilute acids and soil consists of two parts : 

 a direct and a reverse reaction, it follows that the amounts of P2O5 

 brought out by the ordinary treatment with dilute acids do not represent 

 any definite extractable material in the soil, but only the difference 

 between the quantity dissolved by the acid and that adsorbed by the 

 soil, which varies with the acid and the conditions of the experiment. 



Thus it is incorrect to call the net amount brought out the " available 

 P2O5" as if it were something real in the soil : it is simply an analytical 

 result, which will vary when the conditions of treatment are varied. 

 If two soils have widely different adsorptive capacities they may give 

 different analytical results even when they contain similar amounts 

 of phosphate. Of all the acids examined citric suffers least complication 

 from adsorption, a fact which shows the wisdom of Dyer's choice in 



6—2 



