Gypsum – Benefiting Soil Structure
Gypsum is a naturally mined substance (calcium sulphate, CaSO4.2H2O) that has been used for many years as a soil conditioner. New independent research in New Zealand has confirmed some of the known benefits of gypsum and shown that the product can affect subsoil structure six months after application to the soil surface.
Gypsum dissolves steadily in soil to release calcium and sulphate. The sulphate is useful as fertiliser value in itself but also to aid the leaching of any excess sodium levels and to take the calcium deeper into the soil. The calcium from gypsum can also provide a fertiliser effect in some cases but it is most recognized for its benefits to soil structure through the process of flocculation.
Flocculation is the forming of crumbs or aggregates in the soil. A calcium ion has two positive charges and this allows it to provide some amount of cohesion between two negatively charged soil particles such as clay minerals or soil humus (organic matter). This helps the soil to forming crumbs that assist with better aeration, water holding within the crumbs and water drainage around the crumbs. The crumb structure provides improved conditions for root penetration, nutrient uptake and reduced potential for root diseases.
Gypsum provides most striking results in heavy clay soils or where soil sodium levels have accumulated. Some clays are prone to swelling (which can again be related to the sodium content) and many can suffer from a surface crust through lack of crumb structure. Gypsum is a well-established ready remedy for both these conditions.
High sodium soils can naturally occur in many coastal flats and semiarid conditions as well as through the irrigation of dry areas when the water contains sodium. Under dry conditions some irrigation water can evaporate off but this leaves behind the small amounts of salts that came with the water. High sodium levels tend to reduce flocculation and soil crumb structure since the sodium ion has only a single positive charge. Gypsum is well recognized for positive benefits on moderate to high sodium (sodic) soils due to the sulphate that helps leach out excess sodium and the calcium that corrects soil structure.
What may be less well recognised is the positive benefit that gypsum can bring to a wider variety of soils through its positive effect on soil crumb formation and an ability to affect deeper subsoil structure where clay content is usually higher than in the topsoil and drainage and root restriction can often impact on the crop above.
A further identified benefit of gypsum has been in the organic sector where it can be a challenge to addresssulphur requirements of crops in a timely manner. Gypsum is a permitted product in organics and provides readily available sulphate on top of any soil conditioner benefits for that soil.
Practical and experimental work with gypsum has focused on rates of between 1 tonne/ha and 5 tonne per hectare depending on the industry and soil structure challenge. For heavy clay soils, still higher rates can be worthwhile to address soil structure. Sulphate levels and soil penetrometer readings (a measure of soil compaction) have shown that in a variety of soils, subsoil can benefit from such gypsum applications. Smaller additions such as 100 to 200 kg/ha can address sulphate requirements in an organic farming situation if soil structural issues are not a major target.
The average effect of gypsum on subsoil compaction (at 60cm depth) in three locations, six months after application.
The effect of gypsum on subsoil compaction was statistically significant (p<0.05, general linear model) across all three replicated locations tested in March 2012 six months after application (see graph).A compaction reading of 2 MPa is sufficient to restrict some root growth andcause issues with drainage. All of the soils studied were silt loams rather than heavy clays, highlighting the existence of structural issues in deeper subsoil even under lighter topsoils. Addressing subsoil compaction can improve land drainage (reducing susceptibility to topsoil water logging and pugging type damage as well as allowing deeper root growth for improved water uptake.
The experiments saw smaller reductions in compaction and soil bulk density in the topsoil also (though this was only statistically significant for bulk density at the vineyards). Readings will continue to be taken for a three year experiment period. For more detail on the results see