Key points

• Combined application of lime and gypsum results in an additive plant productivity effect at Kalannie in Western Australia
• Early root growth of cereals appears to be promoted with combined lime and gypsum in acidic WA soil
• Soil profile 're-engineering' where acidity is ameliorated and compaction removed can significantly improve plant productivity

Combining lime and gypsum appears to create an additive effect that can reduce the time it takes to neutralise acidity and aluminium toxicity in Western Australian wheatbelt soils.

Soil acidity remains one of the main production constraints in WA and while surface liming is a well-known ameliorant, experience has shown it can take many years before sustained yield increases are achieved.

In response to growers' need for more rapid methods with which to correct subsurface soil constraints, GRDC has invested in a series of Department of Primary Industries and Regional Development (DPIRD) projects, including the soil acidity project, to address this specific challenge.

Over the past two years the project has included on-farm paddock trials at Kalannie, along with detailed glasshouse experiments at the University of Western Australia and Northam. The glasshouse experiments considered the effect of soil pore water chemistry on root growth and crop emergence and early root establishment in acidic sandy subsoil ameliorated with lime and gypsum.

The trials were also complemented by a 'soil re-engineering' field experiment, which tackled the combined constraints of acidity and compaction.

The background to the project was the knowledge that while incorporating lime in subsurface soil using tillage increases the rate of pH change, this additional operation can make the liming process too expensive for many growers. And while there is some evidence that applying gypsum to the surface can decrease the effects of aluminium toxicity in the subsurface soil, the results are not consistent and the mechanism is unknown.

Consequently many growers want more information before applying gypsum as part of their management strategy for acidic soil.

For these reasons, it was decided to evaluate the combined, interactive effect of lime and gypsum on subsoil acidity, aluminium toxicity and grain yield in low-rainfall regions.

On-farm trial

Established in March 2017 on the Nixon family farm at Kalannie on a soil with a loamy-sand texture throughout the profile (for further characteristics see Table 1), the trial comprised three factors - tillage, lime rate and gypsum rate. Lime and gypsum were applied on the surface before tillage treatments, which were (i) no cultivation and (ii) incorporation to 20 centimetres. The trial was sown to Mace wheat in 2017 and 2018.

The two seasons had contrasting rainfall - in 2017 an average season following a dry start and in 2018 an above-average winter with a dry spring.

It was in the drier 2017 that the lime-gypsum treatment had the biggest impact. The application of six tonnes per hectare of lime with 3t/ha of gypsum without incorporation produced 30 per cent more grain (1.04t/ha) than the ripping-only control (0.79t/ha). By comparison, 6t/ha of lime alone increased yield to 0.99t/ha and 3t/ha gypsum alone increased yield to 0.85t/ha.

Similarly in 2018, though under a different rainfall pattern, the combined application of lime and gypsum increased yield by considerably more than the application of lime or gypsum individually. Incorporation of 6t/ha lime plus 3t/ha gypsum produced 23 per cent more grain (2.05 t/ha) than the ripping-only control (1.66t/ha). By comparison, 6t/ha of lime alone increased yield to 1.86t/ha and 3t/ha of gypsum increased yield to 1.82t/ha.

Root growth

One of the knowledge gaps the research sought to fill was the effect of lime and gypsum amendments on crop establishment and early root growth.

A glasshouse trial examined the effects of these amendments on soil solution chemistry and its impact on early root growth in wheat, barley and canola. Experimental soils were collected from Kalannie on-farm trial site as described in Table 1.

The results showed that the interaction of lime, gypsum and soil type to improve root length was significant for barley and wheat but not for canola (Figure 1).

Figure 1: Glasshouse response of barley, wheat and canola root length to incorporation of lime at different rates (0, 0.32, 0.63 and 1.25t/ha) with (0.63t/ha) and without gypsum in a subsoil and 50:50 mixture of topsoil and subsoil from Kalannie, WA. Source: DPIRD

For barley, the influence on root length of increasing lime rates was more prominent in subsoil (Figure 1a, up to 144 per cent increase in 1.25t/ha lime treatment due to an increase in soil pH from 4.04 in the control to 6.29) than in mixed soil (Figure 1d, up to 17 per cent increase in 0.32t/ha lime treatment due to an increase in soil pH from 4.23 in the control to 4.75).

The mixed soil was a 50:50 mix of surface and subsoil. Barley seedlings produced longer root length in mixed soil (2.18 metres per plant) than in subsoil (1.48m per plant). The significant interaction of lime level, gypsum level and soil type occurred when gypsum was applied with lime in mixed soil (Figure 1d) and with 0.63t/ha lime in subsoil (Figure 1a).

Increasing the lime rate increased total root length by up to 14-fold through the production of secondary and fine root systems which may allow young seedlings to grow their roots into moist subsoil and improve establishment.

For wheat, liming also improved root length in subsoil (Figure 1b, 100 per cent increase in 1.25t/ha lime treatment due to an increase in soil pH from 4.04 in the control to 6.29) than in mixed soil (Figure 1e, 25 per cent increase in 1.25t/ha lime treatment due to an increase in soil pH from 4.23 in the control to 5.89).

The soil type effect on wheat root length was also significant but opposite to that of barley. Wheat grew longer roots (2.48m per plant) in subsoil than in mixed soil (1.81m). Also gypsum produced longer wheat roots (2.24m) than in the untreated control (2.05m).

The significant interaction of lime level, gypsum level and soil type for wheat occurred only in subsoil when 0.63t/ha gypsum was applied with 0.32t/ha or greater level, of lime (Figure 1b).

Canola produced significantly longer roots when it was grown in mixed soil (4.47m) than in subsoil (1.52m). Increasing lime level had a significant effect on root length and with the application of 1.25t/ha lime there was 43 per cent and 97 per cent increase in root length in subsoil (Figure 1c) and mixed soil (Figure 1f) respectively. Application of gypsum also increased canola root length (3.19m) in mixed soil compared to the untreated control (2.8m).

Crop establishment

In the cereal crop emergence and early root establishment experiment, lime on its own had no effect on crop emergence, but high rates of gypsum slowed plant emergence by two days. However, applying gypsum with lime countered the negative emergence impact by reducing the number of days required to achieve 75 per cent plant emergence. This could be an advantage in farming areas that are sown dry and/or receive only a small rainfall event during sowing.

Increasing the lime rate increased total root length by up to 14-fold in 2.5 t/ha lime treatment (due to an increase in soil pH from 3.95 in the control to 6.91) through the production of secondary and fine root systems, which may allow young seedlings to grow their roots into moist subsoil, collected from Kalannie on-farm trial site. This can be an advantage where the crop experiences a dry spell following germination. Other key findings are illustrated in Table 2.

Soil re-engineering

In the push to hasten subsurface acidity amelioration, in 2018 the research team tested whether 're-engineering' (deep tillage and lime incorporation) soil profiles with multiple constraints can significantly improve rooting depth, water-use efficiency (WUE) and grain yield.

The results showed that soil excavation decreased penetration resistance from 3-4 megapascals (MPa) to less than 1MPa to the depth of excavation, and lime incorporation lifted soil pH well above the minimum recommended pHCa of 5.5 in the surface and 4.8 in the subsurface. Liming also decreased aluminium from a very toxic 18 to 27 milligrams per kilogram in the control subsoil to a non-toxic level of less than 5mg/kg.

The trial at Kalannie, WA (adjacent to the Lime x Gypsum OnFarm trial described above) comprised five soil amelioration treatments: an untreated control and four treatments involving either removal of compaction only, or removal of both compaction and acidity using three lime rates 1.5t/ha incorporated to 10cm, 4.5t/ha to 30cm and 6t/ha to 45cm. Photo: DPIRD

The improved soil physical and chemical properties resulted in significant improvement in root growth. Root growth was restricted to within 20 to 25cm depth for the unameliorated control. For all four treatments, wheat roots grew down to 60 to 65cm. Where lime was incorporated at depths (30cm and 45cm) there were more fine roots and roots hairs in the deeper horizons.

The wheat crop growing on ameliorated soil profiles was found to extract more water and nutrients. In the untreated control plots, a large proportion of the soil water remained unused.

Plant biomass production in the ameliorated soil profiles was doubled (Figure 2a). This did not affect grain fill (that is, harvest index was not different; Figure 2c) despite a dry September.

Ultimately, wheat grain was more than double in the deep lime incorporated treatment compared to the untreated control (Figure 2b). Incorporation of 6t/ha of lime to zero to 45cm depth was significantly better than the incorporation of 1.5t/ha of lime to zero to 10cm soil.

Figure 2: Improvement in wheat (a) biomass, (b) grain yield, (c) harvest index and (d) water-use efficiency in 2018 due to deep incorporation of lime. Vertical bars represent standard error of the mean values of the respected parameters. Scales on Y-axes are different due to differences in response of different parameters. Source: DPIRD

Removal of compaction only also increased grain yield by 70 per cent compared to the untreated control. WUE was doubled (24kg of grain per millimetre) compared to the untreated control (11kg/mm).

These improvements in plant growth, grain yield and WUE strongly correlated with depth of amelioration of soil acidity and compaction.

More information: Gaus Azam, DPIRD, gaus.azam@dpird.wa.gov.au, 08 9690 2159.