Clay soils have the potential to support crops and for those crops to do well as long as the soil is managed well.

An unstable clay soil suited to gypsum application will display specific characteristics that a grower can easily detect. A farmer will notice that the soil has a hard-set surface when dry. On further investigation, it will be hard to push a pencil into a dry surface. When dry, the soil surface may form widely spaced deep cracks. Other indications include poor rainfall infiltration and water ponding on the soil surface after rain, and the ponding remains for days or weeks. The soil becomes boggy and slippery after a bit of rain, making machinery operations difficult. Seed beds are difficult to produce, and the seed bed breaks down rapidly after rain, resulting in poor seedling emergence. Crop growth is patchy.

Clay soils consist of many small aggregates that are tightly packed together, so they lack soil pore space and aeration to allow rainfall to infiltrate the soil. Plant roots struggle to grow in clay soils due to the lack of air spaces and typically end up having shallow root systems, limiting their access to water and nutrients. As a result, plants often suffer from drought stress very quickly during extended dry periods.

Soil particle aggregation in clay soils is promoted by high calcium content, high organic matter content, and active earthworm populations, which provide the biological glue to join soil particles together (1) and a diversity of soil micro biology.

Encouraging earthworm populations to establish will create macropores in the soil profile from their burrowing activity, which aerates the soil and promotes water infiltration. In addition, earthworms prefer soil surface residue to feed on, adequate calcium levels, and a neutral soil pH. 

Gypsum’s chemical composition is calcium sulphate dihydrate. The calcium in Gypsum helps micro particles in clay soils to aggregate, resulting in favourable soil structure for root growth and air and water movement. (2))  When roots are more abundant and can grow deeper into the soil profile, they take up more water and nutrients, even during the drier periods of a growing season. (2)

Other ways to manage clay soils include keeping the soil covered to reduce soil moisture loss and protect the soil from exposure to high soil temperatures. (1)

Covering the soil can be as simple as mulching crop residues and leaving this on the soil surface or growing a multi-species cover crop for up to 8 weeks and cultivating it into the topsoil. 

After heavy rain it is best to delay activities such as ploughing, using heavy machinery or placing stock in the area for grazing as this will create compaction issues. 

Gypsum is best applied to clay soils annually for at least three years and then applied biennially from then on. Gypsum is BioGro certified.   Gypsum is a multitasker.

For more information on Gypsum application rates, talk to your fertiliser advisor. 

References

1. The Biological Farmer, G Zimmer with Leilani Zimmer- Durand, 2017, Acres USA
2. Gypsum as an Agricultural Amendment, Bulletin 945, Ohio State University, USA, W A Dick, L Chen.