Verifying estimates of field capacity for some New Zealand soils

Abstract

Core samples were taken from 7 soils in North Auckland in late winter-spring, a few days after heavy rain. The weights of the cores at sampling were compared with the weights attained subsequently after saturation and drainage to a series of water tensions in a pressure plate apparatus. For both the free-draining brown loams and the slowly permeable yellow-brown earths and brown granular clays the water contents in the field corresponded to tensions of 0.05–0.25 bar (5–25 kPa). Four free-draining soils from the North Island, on which determinations of this type had been made, either as above or in a previous investigation, were re-sampled. The hydraulic conductivities of the samples were measured. At tensions corresponding to the water contents of the earlier determinations conductivities ranged from 1.1 × 10^-9-1.5 × 10^-8 m/s. These conductivities would produce drainage rates of 1.1 × 10^-9-1.5 × 10^-8m/s (0.1–1.3 mm/day) under gravity, representing negligible further drainage.

Keywords:

• soil water
• hydraulic conductivity
• field capacity
• drainage
• water tension