Sandy soil can be a difficult place to garden. It dries out quickly. It lacks nutrients. The amount of microbial life is very low. But it is easy to dig. It is claimed that clay can be added to sandy soil to improve it’s qualities but does this really work?
Adding clay to sand is a popular technique in some places like Australia and China, but it is not used very much in North America. Why is that?
There are many different kinds of clay – which one is best?
What Is Sandy Soil?
The mineral components in soil consist of sand, silt and clay. A soil that has more than 50% sand is called sandy soil, but in this blog we are discussing soil that has a much higher level of sand. Sand becomes a problem when it makes up more than 80% of the mineral components.
How do you know how much sand you have? There are two simply DIY tests you can do at home this soil texture. One is the soil ribbon test which is simple to do but it does take a bit of practice to get a good estimate. The other option is the jar test described in this video. It takes a bit more time but is a better option for a gardener who has never done a ribbon test.
Why Is Sand A Problem
Sand consists of relatively large particles with a lot of space between them. This allows water to seep through quickly resulting in soil that dries too fast.
The second problem with sand is its chemistry. Sand has a very low CEC (Cation Exchange Capacity). This means it does not hold nutrients very well. They just wash away with the water resulting in low fertility.
Organic matter in sandy soil decomposes faster than in clay which is one reason why most sandy soil has low levels of organic matter. Low organic matter also leads to fewer nutrients and lower levels of microbial life.
You can think of silt as being just small pieces of sand. Silt also has a very low CEC, but due to the smaller particle size it does hold water a bit better than sand. Clay is the complete opposite. It consists of extremely small particles which hold water well and it has a high CEC.
Does Sand + Clay Make Concrete?
There is a common myth floating around that if you add sand to clay it will make concrete. It won’t. Concrete requires cement.
But does adding sand to clay cause the soil to get harder? I have discussed that fully in Does Sand And Clay Make Concrete?
Although some people are concerned about adding sand to clay, nobody claims that adding clay to sand produces hard soil.
Does Clay Improve Sandy Soil?
A good loam contains about equal amounts of sand, silt and clay so it seems obvious that adding clay to sand should improve the soil. It seems odd to me that it is almost never mentioned in North America as a way to improve sand and yet it is a very popular method in Australia. If you have an explanation for this let me know in the comments.
There have been numerous studies that measured soil parameters after adding clay to sand. All of the ones I looked at showed some improvements and none fund found a negative effect.
In 1973 some sandy test plots were amended with waste bentonite at the rates of 0, 3, 6 and 12 kg m-2. Various crops were grown each year. During the first 10-years of the study (1973-1983), it was found that the bentonite addition stimulated the accumulation of organic carbon and total nitrogen, as well as increased microbial populations and fertility. Lower bentonite rates had proportionally smaller effects. After thirty years of cropping and seven years of laying fallow, the same soil was tested again. Treated soil still contained a significant amount of clay, held more water, had a higher amount of organic carbon and was a better quality.
A two year study in Thailand compared bentonite to composted leaf liter and found that bentonite improved the water holding capacity and aggregate stability much better than leaf liter.
Another study using bentonite found an increase in grain yield of maize, an improvement in soil properties and an increase in microbial activity, especially fungi.
” Dryland wheat farmers in Australia mine clay and apply it to their sandy soils with specialized machinery. The results (more uniform germination and better herbicide efficiency) can be economically attractive where a local source of clay is available.”
Clay vs Organic Matter
There is another common method for improving sand and that is to add organic matter. There is no doubt that this also improves sandy soil. So which works better, clay or organic matter?
The benefit of clay is that it lasts a long time. Organic matter decomposes very quickly in sand and needs to be added regularly. This is especially true in hot climates where you find a lot of the sandy soil. Organic matter on the other hand is better for supporting microbes and it may release more nutrients, depending on the clay source.
What does the science tell us? It suggests that the best solution is to use a combination of clay and organic matter. Each one brings something different to the table, but when they are combined the effect on building soil quality is magnified. The organic matter helps aggregate the clay particles into larger aggregates which translates into better soil.
So the best solution is to apply clay to the soil as a one time amendment and then to add annual amounts of organic matter.
Which Kind Of Clay Works Best?
Bentonite seems to be the most popular choice. It is found in most parts of the world and is relatively cheap. Kaolin clay is also used.
Another option is to use a high clay soil. This is probably even less expensive and may add both silt and organic matter to the equation. Soil that is highly aggregated with a high level of organic matter would be a good choice.
How Much Clay Should You Add?
Good question, but the answer is not clear. I found recommendations ranging from 2 lb/sq yd (1 kg/sq m) to 22 lb/sq yd (12 kg/sq m). The government of Western Australia suggests increasing the clay content to about 5-10%.
Most of the studies I looked at tested various amounts of clay and usually the higher addition produced the best results. The highest value was 22 lb/sq yd (12 kg/sq m).
If the product you are using is not 100% clay you will need to adjust the amount you apply.
The other factor is mixing depth. How deep should you mix soil after adding the clay? Suggestions range from eight inches to a foot (20 -30 cm).
Applying The Clay
The best way to apply clay is to spread it over bare ground and then dig or rototill it into the soil.
For an existing landscape or a lawn, either spread the clay over the surface and water it in, or make a water/clay slurry and pour that on the surface. This method is not as good since the clay will not reach the root zone as well as when it is dung right in.
How Long Does Clay Remain In the Soil?
Individual clay particles are extremely small. You would expect that they would easily settle between the sand particles and move to lower soil levels fairly quickly. Surprisingly, they are slow to do this provided you select the right kind of clay.
I wanted to know how long clay lasts in soil and so I asked Dr. Petra Marschner, School of Agriculture, The University of Adelaide, who has been studying sand clay mixtures. He said, “Movement of clay into the lower horizons will depend on clay clod size and stability. Movement will be low when the clods (aggregates) are large and stable. ”
What makes clay aggregates more stable? “Organic matter stabilizes clay.” Clay that is rich in calcium is also more stable. Sodic clay which is clay that contains a lot of sodium is very unstable and will quickly disperse and filter through the sand.
Bentonite is available in at least a calcium form and a sodium form. The sodium form swells more and is commonly used to seal the bottom of ponds and in cat litter. Make sure you use calcium bentonite for amending clay soil.
The 30 year study mentioned above showed that bentonite remains in the sandy soil for a significant amount of time with most of it being in the upper 1 foot (30 cm). There was very little clay found at the 2 and 3 foot depths, suggesting that “the migration of the finer particles of bentonite into deeper soil layers was not substantial.”
Measuring Soil Stability
There is a simple way to measure the stability of any type of soil – the soil stability test.