ARTICLES 583 



point by the absorbing action of a plant root. This capillary 

 moisture was regarded as available for the plant roots, whereas 

 the hygroscopic moisture was not, because of the great attrac- 

 tion exerted on it by the soil particles. In the early days of 

 soil physics Davy suggested that the amount of hygroscopic 

 moisture taken up by a dry soil when exposed to an atmosphere 

 of saturated water vapour was a measure of the fertility of 

 that soil. Schiibler showed, however, that this was not neces- 

 sarily so, since an infertile clay would absorb more water 

 vapour than some soils of great fertility. Actually it is a fair 

 measure of the relative fineness of texture of different soils, 

 because the deposition of vapour is essentially a surface pheno- 

 menon, and, since a clay soil presents a greater total particle 

 surface than an equal weight of sandy soil, we shall normally 

 get more moisture deposited on the former. A standardised 

 method for measuring this quantity was worked out in America 

 by Hilgard, and is still used extensively in the physical specifi- 

 cation of soils, under the name of the Hygroscopic Coefficient. 



German investigators have paid considerable attention to 

 hygroscopicity measurements, and we may notice here the use 

 made of it by Mitscherhch in studying compound particles. 

 The water vapour is supposed — with good reason — to condense 

 on the total particle surface, which includes the interstices 

 within the soil aggregates, whereas an organic liquid of high 

 molecular weight condenses only on the outer surfaces of the 

 soil crumbs. It will be seen that comparison of these two 

 condensations for different soils allows an estimate of the rela- 

 tive degree of aggregation to be made. 



After these physical divisions of soil moisture were defined, 

 attention was directed towards their relations with the moisture 

 absorption by plant roots ; it was found that the original 

 divisions were too broad, and further subdivisions and equili- 

 brium points were defined. This work was done almost 

 exclusively by American investigators — Briggs and his fellow- 

 workers. We have only space to notice two of the equilibrium 

 values — the Wilting Coefficient and the Moisture Equivalent. 

 The former is defined as the amount of water left in the soil 

 when permanent wilting occurs of a plant growing therein. 

 The earlier work of Briggs showed that under his experimental 

 conditions the coefficient was the same for all plants in a given 

 soil, but varied for different soils, which indicated that the 

 soil factors were the dominant ones. Later work — of Caldwell, 

 Alway, and others — shows that the plant factors also come in. 

 The Moisture Equivalent is more of a physical equilibrium 

 point. Saturated soil is subjected to a centrifugal force 1,000 to 

 3,000 times the force of gravity, which removes the water in the 

 coarser interspaces of the soil. The percentage of water not 



