The indirect determination of the “wilting coefficient” by the freezing-point method, and the influence of the salts upon the pF at that critical moisture content

Abstract

The results obtained in the preliminary investigation were entirely confirmed, the pF at the “wilting coefficient”, as measured by the modified freezing-point method, varied from 4·0 to 4·4 (round figures), with an average of 4·2.The variation observed bears no relation to the soil texture, neither can it be explained by uncertainties in the freezing-point determinations which have proved to be accurately reproducible. Freezing-point measurements after leaching, conductivity measurements and freezing-point determinations in saturated soil and at the moisture equivalent proved that part of the variation is due to the presence of soluble salts, the more saline soils having a higher pF at the “wilting coefficient”. When the salt content does not exceed about 500 p.p.m. the influence of the salts is hardly detectable, and the pF at the “wilting coefficient” lies between 4·0 and 4·3. Besides unavoidable errors in the wilting experiments several other factors may account for this variation. They are all the factors that have any role in the “history” of the soil. In view of these uncontrollable sources of error a variation of 0·3 pF units can be considered very small.It can therefore be confidently concluded that in ordinary agricultural soils with a salt content of less than about 500 p.p.m. permanent wilting occurs when a critical pF value (lying between 4·0 and 4·3) is reached.