Q23. Why does adding gypsum help improve soil structure on clay soil?

Gypsum helps improve soil structure by suppressing dispersion. It will only do this on dispersive soils; gypsum won’t help improve soil structure on soils that are dense or compacted but not dispersive.

Dispersive soil often has high levels of sodium (Na+), which is why it is often called sodic soil. Sodium ions have a positive charge. When the soil gets wet, the Na ions repel each other (imagine two positive ends of a magnet), pushing the soil particles apart.

Gypsum stops this from happening in two ways:

1. A short-term electrolyte effect.
2. By replacing sodium with calcium in the long term.

When gypsum dissolves, it increases the salinity (ionic strength) of the soil solution. This increase in salinity temporarily suppresses dispersion (stops the aggregates from breaking up). The effect lasts until the gypsum has dissolved and leached, and usually takes a few seasons, though it depends on how much gypsum was applied and how much rainfall was received. Lower application rates and less rainfall will have less of an effect. Ongoing gypsum applications are necessary to maintain this effect.

If enough gypsum is applied, over time, the calcium in gypsum replaces the sodium on the soil particles, so it no longer disperses when wet.

In the Mallee, gypsum rates are usually 2–5 t/ha (one-off rates), or about 0.5 t/ha as a cheap source of sulphur for canola. Gypsum follows water and in low rainfall areas, it can be hard to get gypsum into a dispersive soil with limited rainfall.

If enough gypsum is applied, over time, the calcium in gypsum replaces the sodium on the soil particles, so it no longer disperses when wet.

In the Mallee, gypsum rates are usually 2–5 t/ha (one-off rates), or about 0.5 t/ha as a cheap source of sulphur for canola. Gypsum follows water and in low rainfall areas, it can be hard to get gypsum into a dispersive soil with limited rainfall.