Gypsum is sold as a clay breaker and is usually applied to gardens throughout winter to be washed through the soil with soaking rains. Gypsum works by improving the structure of clay soils, causing them to become more crumbly or friable. Once clay soils are more friable there is air movement between particles, these spaces allow organic matter and water to wash through creating pathways for plants roots. What you may not be aware is only certain types of clay soil will benefit from an application of gypsum and a simple test could save many hours of anguish for gardeners.
Take a small piece of soil, equivalent to a dessertspoon and drop into a jar of water. If the clay stays in the clump and the water stays clear, it will be a waste of time adding gypsum and other clay breaking solutions are needed. If the clay causes the water to go cloudy and the soil disperses gypsum may be beneficial for your soil. To determine the suitability, apply a little gypsum to the jar of water, wait 5 or 10 minutes and see whether or not the gypsum has caused the suspended clay particles clump together. If so, gypsum will be beneficial and can be applied by sprinkling over the soil surface in existing garden areas or apply to the base of the hole when planting.
If your soil does not disperse in water when carrying out the simple test, an alternative clay breaking treatment is required. Organic matter, compost and humus are dug into the soil to improve structure along with liquid composts such as Powerfeed.