The moisture-holding properties of Waikato soils and methods of their determination

Abstract

Comparisons have been made of the amounts of plant-available water stored in profiles of the following soils derived from volcanic ash in and adjacent to the middle Waikato Valley: Hamilton clay loam and Ohaupo silt loam, which occur on low hills on the valley floor, and Horotiu sandy loam and Te Kowhai silt loam of the flood plain. Undisturbed samples from pits were taken at intervals from the surface down to depths of 30 in., and available water was determined as being the difference between moisture held in these samples at tensions of 200 cm of water and of 15 atmospheres. The methods by which these determinations were made have been brought into line with ideas published in other countries in recent years.

The A horizons of all four soils had similar water-holding capacities under pasture. The heavy-textured, compact B horizon of the Hamilton clay loam, a brown granular loam, stored less water to a depth of 30 in. in the profile than did the open, friable B horizons of the Ohaupo silt loam and Horotiu sandy loam, classed as yellow-brown loams. However, some profiles of Horotiu sandy loam had lower than average totals of available water because of shallowness of the soil and consequently of the effective root zone. The highest contents of available water were found in the fine pumiceous B horizons of the Te Kowhai silt loam, a yellow-brown pumice soil.

These results agree with field observations on the relative susceptibilities to drought of pastures on these soils and point to the Band C horizons of the soils as the source of the differences observed.

The verification in the Waikato area that, at equal rainfalls, a lower content of available water is associated with more severe drought damage to pastures indicates that measurements of available water can be applied elswhere with confidence to determine whether observed drought damage in pastures is attributable to soil properties or merely to low rainfall.