Soil moisture changes under grassland as measured by neutron probe in midland England

The moisture regimes of three soils under grassland in midland England were investigated using a neutron probe moisture meter from May 1976 until August 1978. The main aim of the experiments was to compare the differential build-up of moisture deficits in the three soils of different texture by monitoring moisture extraction down to 1.50 m depth. The moisture deficits measured by neutron probe were also compared with those predicted from meteorological data using the Grindley (1970) model. The sandy (Newport) soil dried out more quickly and more completely than the clayey (Worcester) or fine loamy over clayey stagnogley (Salop) soil, particularly in 1976. This reflected the fact that the former had the smallest available water, most of which is held at low suctions, whereas the Salop soil, with much of its water bound at high suctions, continued to supply small amounts to plant roots when the available water in both the other soils was exhausted. The soil moisture deficits determined from neutron probe measurements were much larger at all three sites in 1976 than in the following 2 years. The deficit increased more slowly in the Salop soil than in the other two in the early part of 1976 and more slowly than predicted by the Grindley model. However, during July and August of the same year, deficits in the Salop and Worcester soils increased more quickly than predicted and maximum deficits were in proportion to the available water capacities of the three soils. Deficits in 1978 were very small (less than 70 mm) in all three soils because of the wetter weather. The results of this neutron probe study clearly show that the patterns of moisture extraction and the maximum deficits which developed in the three soils, particularly in 1976, are closely related to their physical properties, as determined from undisturbed soil cores. The authors would like to acknowledge the cooperation of Mr D. S. Prince (Forestside Farm), Mr F. Thompstone (Byrkley Park) and Mr R. Hall (Darley Oaks), on whose land in Needwood Forest (Staffs) the experiments were sited. Thanks are also given to members of Department Met 08C at the Meteorological Office, Bracknell, for providing the rainfall data and estimates of soil moisture deficits and to J. M. Ragg, D. Mackney, B. W. Avery and members of the Physics Department, Rothamsted, for helpful criticism of the text. © 1983, Cambridge University Press. All rights reserved.