Assessing the suitability of lower Murrumbidgee valley soils for irrigated cotton production

Abstract

Soil is a finite and irreplaceable resource that underpins agricultural systems, regulates water cycles and presents opportunities for climate change mitigation. For ongoing sustainable management an accurate understanding of inherent soil properties and how they vary spatially, is required. Within the lower Murrumbidgee valley of southern NSW, Australia, there is a paucity of available data and land managers require more information on the properties of their soils and the presence of constraints to agricultural production. This thesis utilised data from 153 soil cores, extracted to a depth of one metre, to classify soils and map how they vary spatially. It is demonstrated that the soils of the lower Murrumbidgee valley are highly variable. Results demonstrate that morphologically described heavy clay and deep sandy soils occur directly adjacent to each other. When this occurs at a within-field scale, cotton crops yield substantially less in the sandy areas. Presently available digital soil mapping (DSM) products for subsoil clay content did not account for this variability when assessed at the point support using data collected in this study. A regional DSM approach mapped soil properties across the region with moderate model quality statistics and pedologically appropriate spatial patterns. A digital soil assessment (DSA) was undertaken, identifying the most limiting factor for irrigated cotton production to be soil chemical constraints, of which sodicity was the constraint predicted to occur most frequently. This, however, is a constraint at a broader scale, with it demonstrated that when there is variability between heavy clay and deep sandy soils within-field, high sand content is most limiting. Overall, the majority of the lower Murrumbidgee valley is considered either suited (31%) or marginally suited (34%) to irrigated cotton production. It is predicted that 17% of the region is well suited to irrigated cotton production while 18% is considered unsuited.