Crop responses to compacted soil: capture and efficiency in the use of water and radiation

Abstract

We combined field and modelling experiments to investigate crop-level responses to soil compaction. Our working hypotheses are that the effect of soil compaction on crop growth is (i) primarily mediated by reduction in capture of water and photosynthetically active radiation (PAR), and (ii) secondarily affected by reduced transpiration efficiency (biomass per unit transpiration) and radiation-use efficiency (biomass per unit intercepted PAR). Three field experiments were carried out in the Mediterranean-type Mallee region of south Australia where the landscape alternates sand dunes (hills) and swales (flats) of sandy loam soil. All three experiments compared wheat crops grown in compacted (control) soils, and soils in which compaction was alleviated with deep tillage (ripped); additional sources of variation include season and soil type as related to topography. All soil and crop responses to ripping were more marked in sand hills than in sandy loam flats. Penetration resistance of undisturbed soil had a peak ∼2 MPa at 0.1–0.2 m depth in sandy loam flats and ∼3 MPa at 0.2–0.3 m depth in sand hills. Ripping dramatically reduced soil penetration resistance between 0.10 and 0.3–0.4 m. Control crops yielded between 1.2 and 2.9 t ha −1 and yield improvement attributable to alleviation of soil compaction ranged from nil to 43%; yield response to ripping remained for at least two cropping seasons. Increased transpiration and PAR interception fully accounted for the increase in crop growth associated with alleviation of soil compaction; ripping did not affect transpiration efficiency or radiation-use efficiency. The proportion of evapotranspiration accounted for by soil evaporation (E:ET) declined from 0.58 in controls to 0.36–0.45 in ripped sand hills. A limited modelling study showed that water availability, as characterised with the lower limit of plant available water, could partially account for the effect of soil compaction and deep tillage on crop growth and evapotranspiration. Long-term simulations indicated important changes in the fate of water in response to ripping in sandy soils, including a moderate increase in evapotranspiration, a substantial reduction in E:ET, and important reductions in the frequency and rate of drainage beyond the crop root zone.