Water retention, porosity, and composition inter-relationships of alluvial soils in mid Hawke's Bay and their relevance in irrigation planning

Abstract

Water retention, porosity, and density measurements are presented for 15 silty, sandy, or gravelly alluvial soils in mid Hawke's Bay. Correlation coefficients were obtained between wilting point, field capacity, available water, pores air-filled at field capacity (coarse porosity) and various particle size fractions, organic carbon content, and bulk density. Multiple regression was used to generate equations for the prediction of wilting point, field capacity, available water, and coarse porosity from particle size, organic carbon, and bulk density. The regressions explain a large amount of the observed variation in wilting point and in subsoil coarse porosity but are less accurate for predicting field capacity moisture contents. No satisfactory equations were obtained for estima(ing available water and coarse porosity in topsoils from the chosen composition parameters. Profile water retention and porosity characteristics of 4 typical soils are interpreted for irrigation. Coarse porosities of less than 5% are always accompanied by morphological evidence of impeded drainage and thus have implications for irrigation planning. Bulk densities of about 1.7 g/cm³ in these soils appear to prevent root penetration and hence reduce access to some of the available water at depth. In studies on similar soils before irrigation, it is suggested that estimates of subsoil water retention and porosity parameters from the regression equations can be combined, if necessary, with direct measurements on topsoils.

Keywords:

• soil physics
• water retention
• soil water
• porosity
• available water
• field capacity
• wilting point
• permeability
• irrigation
• Hawke's Bay