Furrow sowing with knife points on water repellent sandplain soils is often ineffective with poor crop establishment which can reduce yield potential and crop competition. The problem is exacerbated in severely repellent soils, when dry sowing and in seasons with a dry start. In general, growers have increased seeding rates with the expectation of increased establishment in these situations in an effort to ensure plant numbers don’t limit productivity. Another solution may be to use banded wetting agents which can help water infiltrate more consistently along the crop row.
In order to assess these options in a systematic approach four trials assessing sowing time x seeding rate x soil wetter were conducted with Mace wheat in the Western Australian wheatbelt in 2014. Two seeding rates of 60 and 120kg/ha and two times of sowing were tested at each site. The intention of these experiments was to have one early time of sowing into dry soil and one later time of sowing into wet soil. Soil wetter treatments included an untreated control and seven soil wetters comprising six commercially available products and one new product under development. All soil wetters were banded behind the press wheels at a product application rate of 2L/ha and a water rate of 100L/ha.
Top soil water repellence was measured at all sites using the Molarity of Ethanol Droplet (MED) test. The sites ranged from moderate to very severe water repellence. The nearest towns of the sites were Binnu, Eneabba, Wickipin and Cranbrook.
Soil wetters can have several modes of action, firstly they all contain penetrant compounds that help water penetrate the soil, these are typically surfactants and secondly, some contain retention compounds that hold and retain water in the top soil and in some cases they may be able to hold and exchange nutrients although given the small volumes applied these effects are likely very small (Table 1). Nutrient access may also be improved by better wetting of the repellent topsoil along the crop row. However there is also a risk of leaching of fertiliser micronutrients.
|Wetter||Wetting agent characeristics||Penetrant||Retention|
|Wetter 1||Short term hydrocarbon wetter||Yes||No|
|Wetter 2||Long term hydrocarbon wetter||Yes||No|
|Wetter 3||Organic based wetter||Yes||Possibly?|
|Wetter 4||Carbohydrate based wetter||Yes||Possibly?|
|Wetter 5||Retainer with hydrocarbon wetter||Yes||Yes (dominant)|
|Wetter 6||Hydrocarbon wetter with retainer||Yes (dominant)||Yes|
|Wetter 7||Hydrocarbon wetter with retainer||Yes||Yes|
On the water repellent sands, growers increase seeding rates to reduce the risk of poor establishment. Wetters were not effective at consistently increasing establishment. However there was visual evidence of increased early biomass following application of wetters which growers have also reported. This did not translate into grain yield increases in the studies undertaken in 2014, apart from one product at one time of sowing in Binnu.
Increasing seeding rates was the most effective method for improving establishment, but in drier environments and soils with poor water holding higher plant numbers does not always translate to higher yield.
High seeding rates were not a disadvantage in high rainfall environments but were observed to reduce yields at the later sowing times in low rainfall locations and on poorer soils with very low water holding capacity. High seeding rates on soils with low yield potential were sometimes a disadvantage. Growers should consider optimal seeding rates to account for yield potential as well as the degree of soil water repellence.
|Site||Late April/early May sowing (60kg/ha)||Late April/early May sowing (120kg/ha)||Mid-May sowing (60kg/ha)||Mid-May sowing (120kg/ha)||Least significant difference (LSD)|
|Binnu||98||170||123||155||sowing time x seed rate = 22|
|Eneabba||128||200||98||173||sowing time or seed rate = 10|
|Wickepin||115||204||not mesured||not measured||seed rate = 7|
Weeds are a risk with early sowing in any environment but a particularly big risk if establishment is variable and staggered on water repellent sands. Delaying sowing provided an opportunity to get a knockdown of weeds which is important on water repellent soils where staggered weed germination and reduced herbicide activity reduces efficacy of in-crop and pre-emergent applications.
|Site||Late April/early May sowing (60kg/ha)||Late April/early May sowing (120kg/ha)||Mid-May sowing (60kg/ha)||Mid-May sowing (120kg/ha)||Least significant difference (LSD)||Site mean yield (t/ha)|
|Binnu||0.41||0.44||0.71||0.58||LSD TOS(0.05) = 0.10||0.6|
|Eneabba||1.27||1.26||0.81||0.69||LSD TOS(0.05) = 0.48||1.0|
|Wickepin||n/a||n/a||3.44||3.33||Not significant||3.4 (mid-May sowing 2 only)|
|Cranbrook||2.48||2.76||3.71||3.68||LSD TOS (0.05) = 0.21||3.2|
This research was funded by DAFWA and GRDC through a collaboration of DAW00204 (Delivering agronomic strategies for water repellent soils in WA) and DAW00218 (Wheat agronomy systems profitability). Thanks to LivingFarm for management of sites at Wickepin and Cranbrook. The provision of sites and support of growers Ray Falconer, Kyle Carson, David Stacey and Kim Oliver is greatly appreciated. Thanks to Trevor Bell, Anne Smith, Stephanie Boyce, Paul Bartlett, Virginia Smee and Greg Brown for technical support. Thanks to West Midlands Group, Northern Agri Group, Facey Group, Gillami Centre and the soil wetter product manufacturers for ongoing support.