A MODIFIED PERMEABILITY TEST FOR MEASURING THE COHESION OF SOIL CRUMBS

Abstract

SummaryThe technique previously described (Emerson, J. Soil Sci., 6, 160, 1955) has been improved. A I‐cm. thickness of soil crumbs of standard size (1–2 mm. diameter) is supported on a 1‐cm, layer of glass spheres (0·5–1 mm. diameter), and the permeability measured by percolation of 0·05 N NaCl‐the so‐called ‘initial’ test. Then 3 litres of the same solution are percolated in 24 hours, after which the permeability measurement is repeated—the ‘final’ test. The ratio, K2/K1, of final and initial permeabilities of the crumbs is regarded as a measure of soil cohesion.The technique has been applied to a wide range of soils, on some of which other structural tests had already been made. The results, in four tables, show: (a) a continuous arable system produces very bad structure; (b) a short‐term ley produces an improvement‐as does wheat‐and some of the improvement is detectable after a subsequent year in arable cultivation; (c) attainment of maximum stability is very slow, for 100 years in grass may not be enough; (d) farmyard‐manure residues increased crumb stability on a clay soil, but not on a sand;(e) a ley, and lucerne too, produces a greater increase than root crops.Limitations of the test are: (a) It should not be used unless the soil acidity is approximately the same for all samples. Very acid clay (pH 4 to 5), known to possess a very poor structure, gives a value of K2/K1 close to unity. (b) It may not be safe to compare soils of differing clay content. A poorly structured sandy loam (12 per cent. clay) gives a much higher value of K2/Kl than a clay loam (31 per cent. clay) that field experience would assess as having the better structure.