— 23 IREC Farmers' Newsletter No. 201 — Autumn 2019 — 23 IREC Farmers’ Newsletter No. 201 — Autumn 2019 Identifying ‘hard’ soil problems in the Riverina Soils of the irrigated areas of the Murray and Murrumbidgee valleys are known to have dense sub-soils with low final infiltration rates. While these are well suited to growing rice, restricted infiltration and low available water holding capacity present difficulties for other crops. These ‘hard’ soils have the potential to adversely affect plant growth through reduced root growth and plant available water. TO INVESTIGATE the nature and extent of hard soil problems in the region, both soil water potential and soil penetration resistance (i.e. soil strength) were measured at 23 irrigated wheat sites across the Murray and Murrumbidgee irrigation areas. This allowed the non-limiting water range (NLWR) concept to be used to assess the effect of soil structure on crop growth. Non-limiting water range is the range of soil water content in which plant growth is not restricted by waterlogging, drought stress or hard soil. Plants are most productive when the soil is drier than field capacity (i.e. not saturated and pores contain greater than 5% air), but not so dry and hard that roots are unable to penetrate the soil. Soil water potential was measured instead of water content as it provides a measure of both aeration status and plant water stress, while not requiring calibration for different soil types. A cone penetrometer was used to measure penetration resistance. The relationship between penetration resistance and water potential defines the soil strength characteristic. Different locations, similar characteristics Most soils showed a similar soil strength–water potential relationship at the 3 cm and 30 cm depths. This group comprised 20 of the 23 sites and included all soil types commonly found in the Murray and Murrumbidgee irrigation areas: red-brown earths, transitional red-brown earths, non self-mulching clay and self-mulching clays. At three sites however, all self-mulching clays, the soil was markedly different, having a lower penetration resistance across the range of water potential measured at all three depths, as well as a lower rate of increase in penetration resistance with increasing water potential at the 15 cm and 30 cm depths. There was a clear difference between the duplex soils (red-brown earths and transitional red-brown earths) and the uniform clays (self-mulching clays) at the 15 cm depth. The duplex profile had a zone of very high soil strength at 10–15 cm, whereas the self-mulching clays did not. Soil strength in the duplex soils increased more rapidly in the 10–15 cm zone than in the rest of the profile when drying, whereas in the self-mulching clays it increased more uniformly through the profile (Figure 1). Figure 1. Soil penetration resistance in self-mulching clays (top) and transitional red-brown earths (bottom) showing changes in soil strength with wetting and drying. Soil penetration resistance (MPa) Soil penetration resistance (MPa) 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0 1 2 3 4 5 Soil depth (m) Soil penetration resistance (MPa) 29-Aug 28-Sep 12-Oct 20-Oct 27-Oct 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0 1 2 3 4 5 8-Sep 28-Sep 29-Sep 9-Oct 12-Oct Soil depth (m) Soil depth (m)