Soil strength and hard-setting behaviour of some structurally unstable British soils
A study was made of the physical properties of a number of structurally sensitive soils some of which exhibited behaviour characteristic of hard-setting soils (soils which when wet slump and set hard, on drying presenting problems in terms of ease of cultivations and root growth). Work concentrated on an examination of soils of the Wick series at two sites at the Institute of Horticultural Research, Wellesbourne, where there is a documented history of consistent differences in crop yields between sites. The worse site (Big Ground) had been intensively managed for considerably longer than the better one (Plum Orchard). Dry bulk density measurements over the growing season suggest that slumping occurred on both sites. Big Ground had the greatest bulk density (typically over 1.65 g/cm3). Field and laboratory penetrometer measurements have shown that under relatively dry (an 8% moisture content) conditions roots would experience severe mechanical impedence on both sites. Root counts at final harvest showed that conditions for rooting were considerably worse in Big Ground where all roots were confined to the top 30 cm. Root growth was better in Plum Orchard and was concentrated in between peds, which did not exist at Big Ground. Laboratory strength (unconfined compressive and indirect tensile) and friability measurements on equilibrated samples also showed up differences between the two sites; the greates differences existing between 1 and 10 bar tension with Big Ground samples exhibiting the greatest strengths and least friabilities. On both sites strengths were observed to increase sharply for a comparatively small decrease in moisture content. Implications of these results are discussed with reference to ease of cultivation and rootability. Another light texured soil from Elgin, known for its tendency to erode, was chosen as a contrast to the Wellesbourne sites. Soil at this site was shown to have much less of a tendency to slump and to create problems for root growth, compared to the Wellesbourne sites. The Elgin soil was also considerably weaker, and the sharp increase in strength observed at Wellesbourne was not observed in Elgin. A new test for water suspendable solids, performed on the Wellesbourne and Elgin soils as well as on 5 other soils known for their structural instability showed that, with the exception of the Elgin soil, a large amount of silt sized material could be brought into suspension with little soild disturbance. An explanation for hard-setting behaviour which is based on those results is suggested.