— 24 IREC Farmers' Newsletter No. 201 — Autumn 2019 — 24 IREC Farmers’ Newsletter No. 201 — Autumn 2019 Identifying ‘hard’ soil problems in the Riverina When comparing the water use efficiency (yield achieved per millimetre of water applied), transitional red-brown earths achieve a higher water use efficiency on average, ranging from 11.5 to 17.5 kg/ha/mm. Duplex soils had a wider spread ranging from 6.0 to 17.2 kg/ha/mm. Three compacted problem soils confirm a trend of reduced water use efficiency with a smaller non-limiting water range due to ‘harder’ soils (Figure 2). Water availability limited at 85% of sites Soil aeration is considered limiting in soils wetter than field capacity (i.e. water potential between 0 and -10 kPa). Irrigation should be applied to avoid drought stress and obtain maximum yields of cereals when water potential at the bottom of the active root zone (i.e. 30 cm) dries to -60 kPa. Furthermore, very few roots are able to penetrate a soil when penetration resistance is greater than 2.4 MPa. Based on these criteria, 85% of the sites tested in this survey had a non-limiting water range restricted by high soil strength at 15 cm and 30 cm. There was not a strong correlation between non-limiting water range and yield, with good irrigated wheat yields achieved at the majority of sites studied. Crop root growth through these soils is only likely to occur when they are fairly moist (i.e. between -10 and -35 kPa). As winter crops grow under conditions of high soil moisture and low evaporative demand, the high soil strength of these soils did not appear to adversely affect the yield of winter crops. In contrast, summer crops are likely to experience a period of low root growth during every irrigation cycle, particularly in the duplex soils as penetration resistance at 15 cm and 30 cm in these soils becomes limiting well before the -60 kPa irrigation trigger point is reached. The effect of this on yield and water use is still not understood for summer crops. Although there is no strong relationship with wheat yield and soil strength in the paddocks investigated, we do see a trend indicating that high water use efficiency is harder to achieve in soils with a narrow non-limiting water range, as seen in the duplex soils with a ‘hard’ pan at 10–15 cm (Figure 2). This study found that just over 85% of sites investigated had soils in which penetration resistance would inhibit root growth at a water potential of only -35 kPa, well short of the recommended irrigation trigger point of -60 kPa. This highlights the prevalence of hard sub-soils in all soil types commonly found under irrigated systems in southern NSW. To investigate the impact of this on productivity, its cause and possible solutions, the Grains Research & Development Corporation (GRDC) is funding research through active learning groups. It is important to get involved in your local group as this will lead to better ameliorating techniques tailored to your soil. Contact the authors Alex Schultz Research Development Officer, Water and Irrigation Telephone: 03 5881 9905 Mobile: 0429 952854 Email: [email protected] Figure 2. Water use efficiency shown for different soil groups. SMC = self-mulching clays 0 2 4 6 8 10 12 14 16 18 20 Water use efficiency (kg/ha/mm) Three SMC SMC Duplex soils Compact problem soils BACK TO CONTENTS PAGE