82 Dlhite Acids and Phosjfhorus Compounds in the Soil 



With the exception of the Woburn soil these all fall into a series, 

 K varying from 19-7 in the heavy Hoos field soil, down to 2-7 in the 

 light Bramford soil, while y shows much less change and at equal times 

 has actually the same value for the heavy as for the light soils. 



The question naturally arises: what do these constants mean? 

 and are they connected with any of the other properties of the soil? 



At the outset it must be observed that these "constants" have no 

 absolute value : they are not constant for the soil but only for the special 

 conditions under which the experiment was carried out. Thus, for 

 Hoos 26*: K varies from 4'1 to 19-7, and no doubt wider variations 

 could be obtained. The figures, therefore, cannot be regarded as 

 absolute measures of any soil property. 



Nor can they be used indiscriminately even for purposes of 

 comparison when different soils are being studied under the same 

 experimental conditions. For however nearly alike the conditions may 

 be at the beginning of the experiment they soon begin to diverge as the 

 experiment proceeds. It is shown on p. 109 that the substances dis- 

 solved out from the soil considerably affect the values of the constant. 



But by exercising due caution it is possible to learn something from 

 them. 



The meaning of K may be arrived at by inspection of the equation. 

 It is the amount of P2O5 that will remain in the soil when (7 == 1, i.e. 

 when unit quantity of P2O5 is present in the solution; it therefore 

 represents the lenaciiy ivifh which the soil kee])s its I*'f>^, or in other words, 

 the reluctance with which the soil parts with its P2O5, under* the condi- 

 tions of the experiment. 



It is high for heavy soils, low for light ones (except Woburn) : it 

 falls off under the continued action of the acid, so that it is less after 

 24 hours than after one hour; it is greater in presence of nitric and 

 hydrochloric acids than of citric acid. 



The meaning of jt is less obvious. The equation shows that it is 

 more complex than K, being the ratio of the logarithms of c and y. 

 Inspection of the curves, and especially of Fig. 4 a, shows that where 

 f is large the curve bends over to the axis of X, i.e. the soil soon ceases 

 to adsorb P2O5, so that large amounts remain in the solution. Where 

 J) is small the curve opens out, and more P2O5 is taken up by the soil, 

 while less remains in the solution, f, therefore, is connected with 

 the manner in which the adsorplive capacity of the soil is satisfied, or in 

 which the soil takes P^O^ out of the solution under the conditions of the 

 experiment. 



