Clay on sand: How much helps and how much costs you

Jul 21,2026 Broadening Article, Sustainable Solutions for Sandy Soils Project, 2026, Projects, RAD2413,

By Simon Kruger, WMG

Clay is drawing renewed interest as a way to lift the productivity of sandy soils. On deep sands that hold water and nutrients poorly, and that often repel water at the surface, working clay into the profile offers something wetting agents and other seasonal treatments cannot, a change that lasts. The appeal comes with a catch. How much the practice pays depends heavily on how much clay ends up in the soil, and a rate that lifts yield on one paddock can hold it back on another.

Guidance on where that balance sits comes from Soil CRC Project 3.3.003, Organic and clay amendments to improve the productivity of sandy soils, led by Emeritus Professor Richard Bell at Murdoch University. As part of that project, researchers brought together 270 records from clay and organic amendment trials across southern Australia and analysed them for the effect of amendment on yield and soil organic carbon. Western Australia contributed 66 of those records, second only to South Australia, so the findings reflect local conditions rather than a single site or season. Pooling results this way gives up the detail of any one trial in return for the patterns that hold across many soils and seasons.

The productive band

The clearest finding to come out of the analysis is that there is a band of surface clay concentration where sands perform best. Treatments that lifted clay in the amended sand to between 6 and 10 per cent were optimal for crop productivity. That sits comfortably alongside the rules of thumb already in use across the southern cropping regions. Around 3 per cent clay is commonly considered enough to overcome water repellence, 6 per cent is often the target growers work to in Western Australia, and 9 per cent is more typical of South Australian practice. The analysis suggests the WA target sits at the lower edge of the productive band rather than in the middle of it, which leaves some room to move for growers whose goal is production rather than repellence alone.

Figure 1. Surface clay concentration bands in amended sand, showing the band optimal for productivity, the range the evidence does not resolve, and the point at which a yield penalty appears.

Where more clay starts to cost

Where the findings become more useful is at the upper end. Once clay concentration in the amended sand rose above 15 per cent, a yield penalty appeared. More clay did not keep delivering more production, and past a point it started to take yield away. For a practice that is expensive to apply and effectively permanent once the clay is in the profile, that is the more consequential half of the result. Overshooting is not a matter of spending more than you needed to, it is a matter of spending more to end up worse off, with no practical way to reverse the decision.

Yield and carbon do not move together

The picture changes if the goal shifts from yield to soil carbon. The greatest increases in organic carbon occurred when clay concentration went above 15 per cent, the same range where the yield penalty appeared. The analysis also found a relationship between organic carbon and yield, but only within a window: where carbon in the top 10 centimetres sat between 0.75 and 1.5 per cent, more carbon went with higher yields. Below 0.75 per cent there was no relationship at all, and above 1.5 per cent the yield response flattened out. The researchers suggest that where carbon is very low, other factors are probably limiting production or limiting the conversion of organic inputs into stable carbon in the first place. A grower weighing a clay rate against both production and carbon goals is not choosing between two settings on the same dial, and the evidence does not say which way that call should go on any given paddock.

Concentration is not a spread rate

One point of clarification sits underneath all of these figures. They describe the concentration of clay in the amended soil, not the rate of clay spread on the surface. The tonnage required to reach a given concentration varies enormously with the clay source and with the depth it is mixed through. Work in the same project makes the scale of that variation plain: reaching a 3 per cent clay concentration took 18 tonnes per hectare of bentonite but 56 tonnes per hectare of subsoil kaolinite, and reaching 6 per cent took 70 tonnes of bentonite against 224 tonnes of kaolinite. Those figures come from a pot experiment rather than a paddock, so they are not spread rates to work from directly, but the ratio between them is the useful part. A target concentration only becomes a spread rate once the clay content of the source material and the intended incorporation depth are both known, which is a soil test and a calculation rather than a rule of thumb.

No single number decides it

Running through all of this is a caution the researchers state directly. Amendments for sands cannot be chosen on a single soil characteristic. The analysis found that responses differed according to water repellence, water holding capacity, low pH, the rate applied, the incorporation depth chosen, and the depth of sand sitting over clay. Clay concentration is one variable among several, and a rate that suits one paddock’s combination of constraints will not necessarily suit the next. The same work found that seasonal rainfall distribution influenced both organic carbon and biomass production, which puts a limit on how precisely any target can be set in advance.

The other caution concerns time. Organic carbon accumulates well after any productivity gain shows up, so a season or two of yield data will not tell a grower whether the carbon side of the investment has worked. The recommendation from the project is to sample to at least 30 centimetres and to resample over periods of five to 20 years, which is a longer commitment to measurement than most amelioration decisions come with. For growers weighing clay against other options on sand, the figures here are worth treating as a starting point for a conversation with an agronomist about a particular paddock, rather than as a rate to order against.

Tools and guidance for a WA context

The figures from this research describe what happens once clay is in the soil. Turning that into a decision on a particular paddock takes local information about the clay source, the cartage, and the soil being amended.

DPIRD’s EasyClay Calculator, on the WA Soil Knowledge Base, converts between the two figures this article has kept separate. It will calculate the resulting clay percentage from a given application rate, or work backwards to the rate needed to reach a target percentage, using the clay content of the subsoil being spread and the intended mixing depth. A more detailed setting takes on-farm data including bulk density, gravel content and subsoil nutrient levels. The calculator assumes clay is mixed evenly through the profile, which the developers note is often not achieved in the paddock with current machinery, so the result is a starting approximation rather than a prescription. It is described as an initial prototype and is being developed further.

DPIRD’s factsheet Claying to ameliorate soil water repellence covers the question this article does not, which is whether a given clay source is worth using. It advises testing the subsoil before committing, looking for 30 to 50 per cent clay content, aggregates that slake or disperse in water, and acceptable pH, salinity and boron levels. It also addresses cartage distance, which often decides whether the exercise is viable, and notes that claying is best suited to the medium and higher rainfall parts of the WA cropping zone.

For the operational side, Spread, Delve, Spade, Invert (Davenport and Hughes 2011) remains the best practice guide to adding clay to sandy soils and is among the references drawn on by the Soil CRC project. GRDC’s GroundCover has also reported on WA work combining clay spreading with strategic tillage, including the finding that clay applied without adequate shallow incorporation can hard-set the topsoil and reduce early establishment.

Acknowledgement

This article was produced as part of Soil CRC Project 1.4.007, Accelerated Adoption Program, which supports farming systems groups to extend completed Soil CRC research to growers. The underpinning research is from Soil CRC Project 3.3.003, Organic and clay amendments to improve the productivity of sandy soils. This research was funded by the CRC for High Performance Soils (Soil CRC), an Australian Government Cooperative Research Centre bringing together researchers, industry and farmers to develop practical solutions for Australia’s soils. This article further supports the West Midlands Group’s work in the Sustainable Solutions for Sandy Soils Project, jointly funded through the Australian Government’s Future Drought Fund Resilient Landscapes Program and the Government of Western Australia, led by the Department of Primary Industries and Regional Development, supported by the Mingenew Irwin Group and Edith Cowan University.

Source: Bell, R.W., Yeap, S., Schapel, A. and Robinson, N. (2025). Sandy soils: Organic and clay amendments to improve the productivity of sandy soils. Final report for research project 3.3.003. CRC for High Performance Soils, Callaghan, NSW.