Iron-Man-Gypsum and Clay for Phosphorus – Using soil improvers to keep nutrients in sandy soils

Iron Man Gypsum (IMG) is a gypsum iron-rich by-product of mineral sands processing which contains no contaminants that have been found that might be a risk to the environment leaching from the blended soils.

In this project, both IMG and different clay types have been blended with topsoil, they have been found to be very effective at preventing dissolved Phosphorus (P) from leaching to groundwater.

Above: Piles of IMG at trial site in the Scott River

Many grey sands on the Scott River contain a lot of free Phosphorus that makes up part of what pastures need to grow – but this is highly prone to being leached or washed off each winter.

Three single application trials were set up in 2022 and 2023 on two dairy farms in the Eastern Scott River. These trials show that a single top-dressing of IMG reduced the free-P in soils which limited the phosphate-P that could be leached or washed off the paddocks. This retained P is now showing up as improved plant available P in the soils.

Iron Man Gypsum and Clay were added to improve nutrient retention on two properties in three different trial and monitored for two years:

Clay was spread and rotary hoed into the top soil at a rate of 50 to 250t/ha at Boley’s Dairy
Clay was top-dressed at a rate of 20 to 40t/ha with a disc seeder at Boley’s Dairy
IMG was top-dressed at a rate of 10t/ha and 20t/ha at LAZTANZ 2 Dairy

The findings so far show that there are mixed pasture growth responses:

IMG has increased winter pasture growth by at least 10 % on Lactanz-2 possibly because of improved plant nutrition
Clay is shown to increase early and late season growth, depending on the season.

The correct clays and/or IMG can reduce P leaching loss by reducing free-P, meaning less P needs to be added to offset this each year. Although, 10 times more clay than IMG in necessary to have the same effect, unless the Phosphorus Buffering Index (PBI) of your clay is very high. Free-P is the part of plant available P that is easily washed through or off soil.

IMG and Clay also has other lasting effects on soils:

IMG is a long-lasting source of Sulphur and increases pH slightly
IMG also adds Calcium to soils – reducing microbial stress in acid sands
Clay breaks the water-repellence in sands, which helps improve the early season ‘wetting-up’ of the soil

Above: Clay top-dressing at Boley’s Dairy (left) & Trial of clay mixed in or left on soil surface (right)

The next steps for the project:

Continuing to follow soil and pasture responses
Looking to try IMG on other properties
Looking to support first time trials of clays with interested farmers

For further information contact:

Jourdyn Wells: [email protected]

Brad Degens: [email protected] about IMG

Brooke Anderson: [email protected] about Clay

This project is a part of Healthy Estuaries WA – a State Government program that aims to improve the health of our South West estuaries