The amount of organic carbon (C) present is important for soil structure, microbial activity and nutrient cycling. It influences the productivity of crops and pastures. Minimising the decline in soil organic C can be of assistance in maintaining the productive capacity of the soil. A survey of alkaline soils in north-western Victoria and the upper Eyre Peninsula of South Australia in 2013 and 2014 found that Victorian soils had significantly lower concentrations of organic carbon (C) and stored about 20 per cent less organic C in the top 30cm than the Eyre Peninsula.
Carbon exists in soils in a number of different forms. A fraction of the soil organic C is dissolved organic C which is very mobile in soil and is actively cycled by microbes. Lowering the component of dissolved organic C in soils and increasing its presence in more stable forms is desirable if organic carbon storage is to be improved.
Soil pH has an important influence on the amount of soil C and dissolved organic C. High pH, especially greater than 8.5, can limit organic C accumulation in soil. The chemical and physical characteristics of highly alkaline soils can limit the growth of crops and pastures and the amount of C that can be returned to the soil. High pH also increases the amount of dissolved organic C in soil, making it more prone to loss and microbial degradation.
Reduction in the pH of alkaline soils may create benefits both from an enhanced ability to accumulate soil organic C and from increased productivity. The benefits of using gypsum and legumes to improve alkaline sodic soils is well documented overseas: in many cases a commensurate decrease in pH has been shown to occur. The question must be asked: can these principles be applied to ameliorate highly alkaline soils in the Birchip region?
To investigate this, three short-term rotation trials were conducted to examine the effects of legume productivity and gypsum application on soil properties. The experiments were designed to test the theory that applying gypsum, together with increasing legume production, would reduce soil pH and thereby improve soil organic C stock.
TAKE HOME MESSAGES
- Gypsum application can reduce pH to depths of 20-30cm within a year in highly alkaline soils
- There is some evidence that gypsum can contribute to increasing carbon retention in alkaline soils
- Improvements in biomass and yield of crops can occur over a number of years with gypsum applications
AIM
To determine whether gypsum and legumes can be used to reduce soil pH and positively influence crop production and retention of soil carbon.
TRIAL DETAILS
Seeding equipment (both sites): knife points, press wheels, 30cm row spacing
Weeds, pests and diseases were controlled to best management practice.
METHOD
Three rotation by gypsum experiments were conducted, two at Birchip commencing in 2012 and one at Watchem commencing in 2013. The soil at the Watchem site was less alkaline than at Birchip.
In each experiment the following cropping sequence was used:
Year 1: Peas (Morgan), vetch (Rasina) and annual medic, sown using conventional sowing and P fertiliser rates and again at double the conventional rates. Prior to sowing, gypsum was broadcast at 0, 2.5 and 5t/ha and incorporated during sowing.
Year 2: Wheat (Elmore CL) was sown to all plots. A post sowing application of 20kg N/ha (Birchip) or 40kg N/ha (Watchem) applied at GS30.
Year 3: Barley (Scope CL) was sown and a post sowing application of 40kg N/ha applied at GS30.
After each crop, the residues were left in the paddock and the subsequent cereal crops were directly sown into the previous year’s stubble. Biomass of the legumes was measured in October and grain yields of peas and vetch were measured at maturity. Estimates of biomass production and grain yield of wheat and barley were made. The wheat at Watchem was not harvested in 2014 because of the very dry spring which caused the crop to fail. Treatment effects at Watchem were assessed on biomass at flowering.
RESULTS AND INTERPRETATION
SOIL PROPERTIES
Only the results from 2014 are shown as they reflect the changes that occurred in 2013. Gypsum was the only treatment to affect both pH and soil organic C. At Birchip applying 2.5t/ha of gypsum reduced pH by up to 0.4 units to at least 20cm within the first year and this effect persisted into the second year (Figure 1). The effect was greater at the 5t/ha gypsum rate, although not significantly more so than the 2.5t/ha rate.
No significant change in soil organic C was measured, but there was a decline in the concentration of dissolved organic C down to 30cm in both years.
At Watchem, gypsum increased pH to a depth of 10cm, but had no significant effect on sub-soil pH. Organic C tended to increase and dissolved organic C decreased with gypsum.
These trends were evident in trials in SA, but the magnitude of the effect varied with soil type. When the trials were repeated on lighter textured, calcareous soils on the Eyre Peninsula, gypsum significantly reduced pH and dissolved organic C down to 30cm within the first year, but had no significant effect on soil organic C. This was consistent with results in the top 20cm at the Birchip site. The sensitivity of dissolved organic C to pH is shown in Figure 2. Once pH increases above 8.5, the dissolution of soil organic C increases significantly, but there is little change between pH 6.5 and 8. This sensitivity to pH helps explain the greater changes in dissolved organic C at Birchip compared with Watchem (Figure 1).
aluminium IN ALKALINE SOILS – SOMETHING FOR FURTHER INVESTIGATION
Another aspect that came from the work in this trial is that there is increasing evidence that high concentrations of aluminium (Al) can occur in alkaline soils, adversely affecting root growth. In this trial, an increase in water soluble Al was observed below 10cm as the pH increased; applying gypsum reduced Al concentrations as pH declined (Figure 3). This may have the potential to improve root growth into the sub-soils of highly alkaline soils. The risk of damage from high Al increases at pH greater than 9. Work on the importance of Al toxicity in alkaline soils is continuing.
GRAIN YIELD
The two treatments that had the largest effect on yield of the cereal crops were the legume crop and the rate of gypsum.
Gypsum application did not affect the biomass or yield of the legumes in the first year. However, gypsum increased the yields of the following cereal crops in two of the three trials (Table 3). In Birchip trial 1, wheat yields in 2013 were increased by about 10 per cent where 2.5t/ha gypsum was ap plied. Despite the very low yields in 2014, barley yields increased in response to the highest rate of gypsum application. There was no response to gypsum in the second Birchip trial in 2013 or 2014.
Although the trial at Watchem was not harvested, biomass production at flowering was significantly increased by the application of gypsum.
At Birchip, yields were highest after peas and lowest after medic; the effect persisted for two years despite the very low yields in 2014 (Table 4).
COMMERCIAL PRACTICE
In terms of changes to pH, soil C and ameliorating the potential adverse effect of high Al on plant growth, the greatest benefits from applying gypsum occur on soils in which the pH is above 8.5. At lower pH values, the buffering capacity of the soil greatly reduces any beneficial effects of gypsum on pH specifically.
This work cannot answer questions relating to the long term benefits of gypsum treatments. It was encouraging that the effects of gypsum, in terms of lowering pH and reducing the proportion of dissolved C in the Victorian (and SA) trials persisted into the second year. However, the longterm benefits of gypsum on soil properties and crop performance in relation to carbon storage need to be verified.
ON-FARM PROFITABILITY
Applying gypsum is expensive. Applying Wimmera gypsum costs around $58/t ($43/t sale price and freight plus $15/t spreading), so applying at 2.5t/ha would equate to $145/ha. The long-term changes cannot be verified from this research, but the benefits of gypsum appear to last for at least two years. The initial cost of the investment in gypsum may be repaid over a longer period of time.
It is also worth noting that the purity of gypsum varies: it is a good practice to check before use. For example, in the SA trials similar changes in pH and dissolved organic C to a depth of 30cm were obtained when 2.5t/ha of gypsum was applied. The purity of this gypsum was 60 per cent; 2.5t/ha gypsum was equivalent to 1.5 t/ha of calcium sulphate.
ACKNOWLEDGEMENTS
The project was funded through the Department of Agriculture National Soil C program: Filling the Research Gap (FR_project_01203.014).
