Gypsum application is the most common way to tackle the soil structural problems caused by too much sodium. When wet, sodic soils disperse, destroying soil aggregates and structure. When dry, the soils set hard, inhibiting crop emergence and root growth and increasing run-off.

Signs that you have a dispersive soil include cloudy/milky puddles in the paddock, water running off or infiltrating very slowly, and the paddock getting boggy quickly. Always check for dispersion by placing small air-dry aggregates in rain or de-ionised water and checking after 10 minutes, two hours and 24 hours. If the water turns cloudy, the soil is dispersive and most likely gypsum responsive (Pluske 2020).

Gypsum tackles soil sodicity in two ways. It has a short-term effect at the soil surface and a longer-term effect throughout the soil profile.

Short-term: the salt effect

Gypsum starts working as soon as it rains. As gypsum dissolves, the salinity (ionic strength) of the soil solution increases. As paradoxical as it may sound, increased salinity suppresses dispersion, improving soil structure at the surface, rain infiltration and plant establishment.

Growers tend to see improvements in crop establishment and growth in the year of application, with the effect petering out by about the third season, though this depends on rainfall and the amount of gypsum applied. Lower application rates and less rainfall will have less of an effect. Once all the gypsum has dissolved and leached, the ionic strength of the soil reduces, and the soil is at risk of sealing and hard-setting again. Regular gypsum applications are necessary to keep the salt effect going.

All salts can have this effect on soil dispersion, which is why saline soils need a higher level of sodium to disperse than non-saline soils. The salt effect works mostly in the topsoil as ionic strength is more buffered at depth and has less of an impact.

Longer-term: sodium displacement

In the long term, the calcium in gypsum replaces sodium on soil particles, helping form aggregates and improve soil structure. Sodium ions are then ideally leached deeper and out of the root zone.

The amount of gypsum required to displace sodium with calcium throughout the root zone is much higher (more than 20 tonnes per hectare) than that applied for the short-term effect. Such high rates are usually uneconomical in the short term and, in some cases, the amount of calcium needed to displace all sodium is so high the crop will suffer salt damage if all the gypsum was applied in a single application. Repeated applications that displace sodium with calcium within the root zone may well be economical when applied over decades.

How long gypsum takes to displace sodium in the profile depends on how deep and severe the sodicity is and how much gypsum is applied. Surface-applied gypsum will gradually move through the profile with rainfall, but because the soil already has low permeability this can take many years.

One calculation in the eastern states suggests it could take more than 50 years for the required amount of gypsum (more than 30t/ha) to amend sodicity to 90 centimetres depth in a grey clay. On a sandy loam in Western Australia (described below), very high rates of gypsum took about eight years to reduce soil sodium to 70cm depth. In low-rainfall areas, gypsum could take a decade or more to move and in very dense subsoil any displaced sodium may be unable to leach.

Figure 2: Exchangeable sodium percentage comparison between zero gypsum and gypsum dump area. Source: VRT Solutions.

Where subsoil sodicity is the issue, getting gypsum to depth can be tricky, as disturbing sodic soil can make the problem worse. Remediation with gypsum is unlikely to be profitable because, unlike when there is sodicity at the surface, there are no short-term, surface benefits.

Lime also supplies calcium but the soil needs to be acidic for it to affect soil sodicity. In neutral to alkaline soils (which most sodic soils are) lime is not very soluble, so little calcium is released to be of use in displacing sodium.

To manage soil sodicity, repeated gypsum applications should have a cumulative effect. If it is profitable in the short term, applying 2 to 3t/ha of gypsum every two to three years can manage soil sodicity through the salt effect, with the potential added benefit of displacing sodium with calcium in the subsoil in the longer term.

Gypsum dump results

Gypsum left over from a gypsum dump has provided an insight into efficacy and timeframe for one grower in Salmon Gums, WA. After spreading gypsum across the paddock, the remaining gypsum in the dump area was pushed around with a front-end loader, resulting in an area having an estimated rate of 80 of 100t/ha of gypsum. The soil was sandy loam at the surface, grading to clay at about 50cm depth.

The grower noticed improved crop growth each year on the old gypsum dump area. Data collected eight years later, comparing the dump area to a zero-gypsum strip, showed that in the soil under the gypsum dump, exchangeable sodium percentage (ESP) decreased down the profile, even at 70cm depth (Figure 2). The gypsum dump soil was no longer sodic (ESP more than six per cent) to 40cm depth.

The high gypsum rate also:

• increased the calcium:magnesium ratio to 40cm depth, with the biggest difference in the zero to 10cm sample;
• increased sulfur to 70cm depth;
• increased electrical conductivity (salinity) in the top 30cm, as expected because of the large amount of gypsum salts;
• decreased boron noticeably from zero to 20cm; and
• decreased chloride at all depths.

This article was produced as part of the GRDC ‘Maintain the longevity of soils constraints investments and increase grower adoption through extension – western region’ investment (PLT1909-001SAX). This project is extending practical findings to grain growers from the five-year Soil Constraints – West suite of projects, conducted by the Department of Primary Industries and Regional Development, with GRDC investment.