Gypsum improves yield on responsive soils in South West WA

Clearer guidelines have been established for gypsum use along Western Australia’s southern coast and how to achieve an economic yield response

Research done in conjunction with the national GRDC Water Use Efficiency Initiative has found that gypsum applied to responsive soils along Western Australia’s south coast lifted water use efficiency (WUE) from 11 to 17 kilograms per hectare per millimetre and crop returns increased by almost $200/ha over four years.

The exciting results were generated from a broader research program known as the Agronomy Jigsaw Project, which investigated the main reasons underlying the large range in WUE across cropping paddocks along the southern coast.

The gypsum response is restricted to specific soils – and growers will need to test soils for exchangeable sodium levels to establish if a positive response to gypsum is likely. The response is also more likely after heavy post-sowing rain on responsive soils.

In this research, responsive soils all had dispersed clay evident, where water had pooled on the surface of paddocks.

Surface and sub-surface sodicity is prevalent across the south coast of WA and often results in poor drainage, surface sealing and transient waterlogging, which in turn results in low germination rates. Low plant-available water from sodicity further reduces crop WUE.

Ravensthorpe grain grower Lloyd Burrell and Precision Agronomics Australia agronomist Luke Marquis checking crop growth and health in an on-farm gypsum trial.PHOTO: Nicole Baxter

Gypsum counteracts sodic soils by reducing clay dispersion, which improves soil structure and drainage and reduces waterlogging.

In the process, crop-limiting constraints such as boron and sodium are also often washed beyond the root zone – offering further yield benefits.

At one site, applying gypsum at 10 tonnes/ha reduced salinity by 20 per cent and sodicity by 30 per cent within the crop root zone. The positive benefits of gypsum can last many years.

Yield potential

Project leader and Department of Agriculture and Food, WA (DAFWA) soil scientist David Hall says the WUE improvements saw a Ravensthorpe crop increase its yield potential from 55 per cent to 85 per cent.

Mr Hall says it is likely the improved drainage allowed water to be stored deeper in the soil profile and prevented root pruning from waterlogging. More water and better root systems resulted in higher yields in a dry finish.

Carried out in partnership with Precision Agronomics Australia (PAA) and the South East Premium Wheat Growers Association, the Agronomy Jigsaw Project assessed the most effective and most economic gypsum rates.

In terms of grain production, PAA analysis indicated between 5 to 10t of gypsum/ha generated the highest grain yields (Table 1).

To estimate the return on investment DAFWA researcher Jeremy Lemon totalled the gross income from each treatment between 2008 and 2011 (barley 2008, field peas 2009, and wheat 2010 and 2011) and then subtracted the initial gypsum treatment and cropping costs. This indicated the highest return on gypsum investment was achieved with the 5t/ha treatment.

To achieve a gypsum response, treated soils need to be highly sodic in the surface layer (pre-washed exchangeable sodium more than 10 per cent). While an immediate yield response to gypsum was found at rates of 2.5 to 5t/ha, only the higher rates of 5 to 10t/ha resulted in deep leaching of salts and boron beyond 30 to 50cm.