What you never knew about clay and never dared ask.
Clay particles are the smallest mineral particles in the soil. So light and small that if you disperse them in water the tiny grains float. Indeed without interference the smallest might float for ever. Clay particles are defined in soil science as the mineral component in soils less than 0.002mm. This crude definition distinguishes clay which is both chemically and physically different from the the larger mineral particles, silt and sand.
All three minerals, sand, silt and clay, are unchanged in properties and size by anything the gardener might do.
Only on the smallest of sites is it possible to change the mineral make up of soil. This normally unchangeable characteristic is described as soil texture. To change the proportions of sand, silt and clay by incorporation of imported minerals is fraught with problems and is beyond the scope of my article today.
Clay is described as a secondary mineral. It has been produced over the millennia by physical and chemical geological change. It bares little resemblance to any obvious mineral bedrock as it has often been deposited from water over many thousands of years. There are hundreds of clays all subtly different - even within your own garden. There are of course well defined and classified broad clay categories each with significantly different characteristics.
Ask any gardener and he will tell you that his is the worst clay in the country. On the contrary It is important to realise that when a soil contains a proportion of clay it is a very good thing!
|Gardeners often imagine when they dig down to the subsoil it is pure clay. On this ‘heavy’ Oxford soil there is plenty of clay but sand and silt too.
Soils with plenty of clay are described as heavy. This is a traditional measure of a soil’s workability and nothing to do with its weight! My sandy soil here would have once been described as a ‘one horse soil’. A sticky clay might have needed four horses to pull the same plough!
Concentrated clay tends to occur in what gardeners and farmers call subsoil. Topsoil which descends gradually down to the subsoil is often the result of mixing of sand, silt and clay components by historic cultivation. The nearer the surface the darker and more fertile the topsoil will be. In my opinion it is usually a bad thing to bring sticky clay subsoil to the surface.
It is important to recognise that subsoil is not usually just clay and will also contain silt and sand. Some subsoils are very similar in constitution to their topsoil and merely lack the magic of organic matter.
Pity the poor gardener who inherits a sticky clay subsoil stripped of its topsoil!
A study of clay
Tiny particles of clay have a platelike composition. Water is absorbed within its volume as well as on its very large capillary surface. Most clays are expandable and swell - but not all. Some clay soils expand several inches between dry summer and wet winter and the whole garden rises or sinks.
Differential expansion and contraction induced by the presence of tree roots can cause considerable damage to buildings.
The surfaces of clay particles have negative electrostatic charges. This enables clay to hold positively charged ions. These include plant nutrients such as calcium, potassium, magnesium and sodium. On acid soils hydrogen ions displace some of these nutrients. Negatively charged nutrient ions such as nitrate are NOT held by clay and are liable to leach from a soil in winter Some negative nutrients such as phosphate are held in the soil by other mechanisms.
Clay does not hold its nutrients so tightly that they are unavailable to plants.
Clay particles stick together and its called flocculation. The significance of this is frequently confused when lime is claimed to improve soil structure. There is a classical schoolboy experiment when lime water is added to dispersed clay particles shaken up in a test tube of water. In a modern miracle the particles stick together and flocs of clay rapidly settle out.
In 1953 many square miles of Southern England were flooded by sea water. Sodium displaced calcium on clay on large tracts of land. The clay became deflocculated and became an absolute mash up. The soils were eventually restored by adding significant quantities of calcium sulphate that ‘knocked out’ the sodium. (Calcium sulphate is the only form of lime that does not make a soil alkaline). The soil was saved by re-flocculation!
Ever since then soil textbooks have been paranoid about deflocculation!
I argue that the ‘default position’ for clay in normal soil is flocculation. If it is flocculated, lime cannot flocculate soil even more.
Even though I claim that this is not the reason why lime improves clay soil structure, secondary effects of lime might do so and gardeners will continue to add lime to clay soil to improve it’s condition. But don’t add too much and perhaps use calcium sulphate.
The wonderful thing about clay is that the particles do mix and stick together with the help of organic and mineral ‘glues’ such as glomalin, humus and iron compounds. Intimate mixtures with organic matter and sand and silt form soil crumbs. Crumbs in clay soils are much more stable than any in other soil types but are likely to be destroyed by heavy rain or irrigation and especially by excessive cultivation. The formation of crumbs and other loosely bound larger soil structures is called aggregation. It is rather confusing, as such aggregates are NOT what you buy at B&Q to make concrete!
It is a gardener’s ambition for his soil to be made up of ‘water stable aggregates’
Good things about clay soil
1. Assuming your soil to be some favourable mixture of sand, silt and clay then clay does many fine things to your soil. (Any pun is deliberate).
2. Clay holds many nutrients that are readily available to plants. Sand and silt do not although some gardeners do effectively add finely crushed granite as a slow release fertiliser. Mineral chalk and limestone also release soluble calcium and raise pH.
3. Clay is water retentive and significantly improves soil water holding capacity. En passant, I might mention that the plant is unable to ‘suck out’ the last half of its fully wet content. Nevertheless that’s still a lot of available water.
4. Clay readily forms fairly stable aggregates with organic matter and contributes hugely to good soil structure.
5. The repeated expansion and contraction of clay as it freezes and thaws in Winter helps when making a ‘frost mould’ seedbed in Spring. This temporary improvement is not exclusive to dug soil!
Farmers and gardeners recognise that there are very narrow ‘windows of opportunity’ after rain to make a conventional seedbed tilth in Spring. This window might be as short as 24 hours between being too wet and sticky, or lumpy hard dry.
|A clay profile exposed to drying in a local wood. Note that it cracks in an approximate hexagonal pattern
Bad things about clay soil
1. Damage to buildings! My ‘lunatic’ friend Po Simpson used to be a house damage insurance assessor. He did not last long - he is too kind. He sent me this link. Apparently if there was a tree on a clay soil it was always to blame. Even when it wasn’t!
2. Too much clay in a soil leads to a sticky plasticene structure when wet - and a hard impenetrable surface when dry. You get the same effect in a ‘normal’ clay soil when natural structure and aggregation is destroyed by excessive cultivation. (In ‘my book’ most cultivation qualifies to be excessive!).
3. Loosened clay soil is seriously damaged when walked on when wet.
4. If clay soil is ‘puddled’ by compression when wet it will impede drainage. I have even seen water stand on the soil surface when it is dry below!
You cannot change clay.
Clay will always be clay – well unless you wait thousands of years! I know this comment is insulting to the knowledge of most of my readers, but when I was a kid new to gardening I did not ‘get’ this fact. I think new gardeners sometimes think that by working the soil they will change the nature of the soil mineral particles. They won’t.
Cultivation will change the overall soil condition – called structure: usually for bad, sometimes for good. It will not change the physical and chemical nature of the clay particles.
Clay soil is ‘improved’ when the particles stick together with each other and sand and silt. Organic matter and natural glues are fundamental in this weak and intimate bonding together.
Soil is also ‘improved’ when natural channels, cracks and spaces are undisturbed
Where clay particles are torn apart from crumbs disaster follows.
|A hundred and fifty year old clay spade. Perhaps it was used to dig out the clay at Brickyard Farm down the road?
I have puddled in clay before.