Clay amendments for WA coastal sands
I get asked a lot about what sort of clay and how much to use in sands. This is not an easy question to answer. Clays as we know them are seldom pure minerals and therefore all clays tend to be different and so will react differently. Bentonite (which is partly composed of montmorillonite) from one source will not be the same as that from another. The bentonite mined in WA is calcium bentonite. Most of the other bentonite sold in WA for sealing dams is sodium bentonite from interstate or overseas.
Clays are also dynamic and will respond differently depending on the environment into which they are put.
Kaolins are only a 1:1 layer lattice and so do not hold much water. Montmorillonites are a 2:1 lattice and can absorb large quantities of water. Depending on how these clay minerals were formed they may have calcium or sodium in the internal lattice. Sodium montmorillonite can hold 15-20 times its weight in water whereas calcium montmorillonite holds only 1-5 times its weight. This is why sodium bentonite is used for sealing dams.
But, if you put a sodium bentonite clay into an environment where there is lots of calcium and magnesium, over time the sodium in the lattice will be replaced by the other cations. That means the swelling ability of the bentonite will decrease. Calcium bentonite is a more stable clay than sodium bentonite.
In one paper I came across, a comparison of water retention curves obtained for two natural bentonites, one predominantly calcium and magnesium and the other mainly sodium, showed the water retention capacity of the first bentonite to be greater, although the difference between them became smaller towards low suctions. In another paper, retention and release of water were greatest in bentonite followed by illite and then kaolinite. As suction increased, the difference in water retention became less. At 10 centibars, bentonite retained 244 per cent more water than kaolinite, whereas at 15 bars, it was merely 9 per cent.
Trials in Western Australia showed no difference between the sodium or the calcium version of clays in their long term impact on water repellency but kaolinite seemed to improve water repellency better than smectites at the rates used (up to 1.6% by weight).
Cation exchange capacity
If your main purpose in using clay is to increase the retention of fertiliser then the CEC of the clay is relevant. And in that case almost anything is better than kaolin or spongelite. But a good cation exchange capacity is only relevant to cations (positively charged ions). So if you apply all your nitrogen as nitrate then it won’t reduce leaching. But don’t think you can apply all your nitrogen as ammonium either because that could be toxic to the plant. Nitrate is the preferred form of nitrogen by most plants but is not held well by soils and moves freely in soil water.
So what does this mean for amending my soil?
That is the $64M question! Firstly, clays aren’t the only means of amending soil – there are organic materials as well and they tend to have much higher CEC's than clays. And they also usually contain nutrients. BUT in our coarse sands, trying to amend soils with only organic matter is difficult. Keeping the organic matter content above 2% is nigh on impossible because it turns over and burns off so quickly. There is a sort of cap on the amount of organic matter that can be held in soils that relates to the amount of clay in a soil. So to increase organic matter you need to also increase clay content. Organic matter can be quite water repellent so adding clay also sorts that problem out.
Water holding capacity is the area I think where there is the greatest lack of information. But the soil with the greatest water holding capacity is not necessarily what we are after. If we are growing plants, we need that water to be readily transferrable to the plant and that is where the real lack of information is. Clays hold lots of water but don’t necessarily give it up easily. And in our coarse sands, for water to be plant available, the soil needs to be much closer to field capacity than is traditionally accepted – because they ARE coarse and hydraulic conductivity is extremely low. The bottom line is that we lack meaningful soil moisture retention curves for our situation. The other thing to think about is that a soil that holds more water will also hold more nutrients in the root zone even if it’s not held by the organic matter or by the clay. Sure, if you get a downpour it will be washed away but there is capacity there for nutrients to be held just in the soil solution and not bound by other mechanisms.
Working in this area is difficult. Nothing is one dimensional. We are dealing with complex systems. Create a better soil environment for your plant and it may respond by growing better. And by doing that it may NEED more water and nutrients. Add clay to your soil and not only may it hold onto nutrients better but in the case of phosphorus it may not give it all back – at least in the first year or so anyway. So adding clay may change the way you need to fertilize as well as the way you need to water.
Confused? I can assure you, you are not alone!
Aileen Reid is a professional horticulturist with a NZ Honours degree in horticultural sciences. She has worked in the WA Ag Department for many years. This extract is from her website, 'Gardening WA Style' and is reposted here with her permission.