The Available-Water Capacity of Barbados Soils

Abstract

Large soil monoliths, extracted undisturbed in 44-gallon oil drums, have been used to assess the available-water capacity, and the relation between the growth of sugarcane and soil water deficit for agricultural soils in Barbados. Constancy of field capacity was studied and the effect of cultivation on the storage of available soil water. Deep montmorillonite clays and oceanic soils had storage capacities greater than 20 cm. of water in an 80 cm. profile, whereas sandy or stony montmorillonite clays and most soils developed from kaolinite clays had capacities less than 11 cm. Cultivation significantly increased the water holding capacity of soils but this was rarely as great as for fabricated composts and the water was never so freely available. The data have been used in decisions about cultivation and irrigation, and as the basis for an ecological grouping of sugar estates according to their probable water balance.During 1963/4 the soils of Barbados were mapped at a scale of i : 10,000 by the Regional Research Centre (Vernon and Carroll, 1966). We have attempted to interpret this soil map in terms of the availability of soil water during shorter or longer periods of drought, since this is known to be the main factor limiting yields of sugar in Barbados. The kaolinitic and montmorillonitic clay soils covering most of the Island have a rather massive structure and are frequently stony, and nothing was known about the constancy, or otherwise, of their 'field capacity'. We eventually abandoned approaches based on the pressure membrane apparatus, in-field determination of field capacity, and the other methods of assessing soil water attributes recently reviewed by Salter (1967), concentrating instead on large undisturbed soil monoliths removed from the field and stood side by side at the laboratory for comparison.To extract a monolith the two ends of a 44-gallon oil drum were removed and the resulting cylinder was placed upright on the soil at a carefully selected site. A pit was dug around the cylinder, leaving it standing on a central column of soil, which was trimmed as precisely as possible, with a machette, to the diameter of the drum. The drum was then pushed down over the trimmed soil column as the pit was progressively deepened. When the cylinder touched bed rock, or had been sunk to its full depth, one of the ends previously cut from the drum was placed on the soil surface and four chisel-punches were made under the rim of the cylinder to secure it. A slip-knot in the hawser of a front-mounted Land Rover winch was then drawn tight under the cylinder to cut the soil off neatly, using the hawser in the same way as a wire cheese cutter. The drum was then tilted slightly, and a sheet of metal slipped over the hawser hook and under the drum to form a skid on which the monolith could be winched from the pit. n Expl Agric. 5, 3