Root penetration rate - a benchmark to identify soil and plant limitations to rooting depth in wheat

Data on wheat rooting depth was compiled from 36 agronomic experiments conducted in southern NSW from 1990 to 2004. Rooting depth was measured by direct soil coring and observation of roots using core-break or root washing techniques. Maximum rooting depth varied from 80 to 180 cm and was influenced by the depth of soil wetting, soil type and the duration of the vegetative phase (sowing to anthesis) as determined by interactions of sowing date, variety and seasonal conditions. The root penetration rate (RPR cm/day), defined as (maximum root depth measured at or after anthesis)/( days from sowing to anthesis), emerged as a simple but unifying parameter which could be used to estimate the potential rooting depth of wheat on different soils. RPR, expressed on a thermal time basis, was highly correlated with that expressed on a simpler time basis (r = 0.92). Incomplete wetting of the soil profile reduced maximum rooting depth and RPR in 12 of the 36 crops studied, and root penetration in the subsoil was clearly restricted in soil layers with less than 45 to 50% plant available water. Soil type influenced the RPR. The average RPR for wheat was 1.13 +/- 0.04 cm/day on Red Kandosols (n = 11), 1.01 +/- 0.07 cm/day on a Red Sodosol (n = 3) and 0.79 +/- 0.03 cm/day on Red Chromosols (n = 10). The RPR was relatively constant across cultivars and sowing dates within these soil types, although there was some evidence for a reduction in RPR with later sowing independent of time or thermal time. We suggest that the RPR (cm/day) established for wheat on various soil types provides a useful tool for wheat growers to estimate the rooting depth and available water and nutrients in-season. It also provides a benchmark to indicate potential subsoil limitations to crop growth, and for researchers investigating opportunities to increase the maximum rooting depth of wheat through management or breeding.