Friday 15 May 2015

Understanding clay soil

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. 

Notorious Foggathorpe clay! Genuine clay subsoil!  Blue tones in clay indicates  poor drainage and lack of oxygen when it is saturated. More rusty colours in clay  indicate the presence of oxygen which converts  blue/green ferrous iron compounds into the ferric condition.

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.


  1. I would welcome your thoughts on something we are about to do with our clay. We have a 'lawn' which is very badly drained area of clay. We need to dig some new foundations, and the plan is to use the spoils to extend a terrace over this badly drained area. Height gain about a metre. I plan to top it with about 25cm of bought in topsoil, as the topsoil that is there is not really worth the effort of trying to re-use. I'm expecting that we'll have to wait until that small window day when the clay is workable, and then level the subsoil. I'm guessing maybe rotovate it? So in theory the topsoil goes over a level base that has had the surface, which may get compacted, broken up.

    Do you think it will work? Is 25cm topsoil enough? At the moment the area gets very wet, but I'm a bit worried that it might go to far the other direction, and create something that is very dry, rather like a roof garden with only a thin layer of soil over a solid base.

    I'd appreciate your thoughts, as neither the contractor nor I really know what we are doing!

    1. Oh you are a challenge Sarah!
      I assume that the ‘spoils’ are a mixture of topsoil and subsoil and are not like the picture of Foggathorpe clay? They are likely to make a water retentive base into which roots will eventually grow. I don’t see much point in rotavating unless it requires serious mixing.
      25cm of topsoil should be ok. As you I think know, I am not too keen on very sandy layers lying on top of clay because that causes problems of its own – but even then nearly a foot is quite deep and it is thin sandy layers that cause problems.
      My comment about a small window of opportunity applies to seedbed preparation. I don’t expect your contractor will have any problems moving your subsoil.

    2. Yes but I'm on topic...understanding the clay :-) Agree with Sue, he might have problems levelling it. That's why I thought it might need some mechanical help when it's dry enough.

      Actually it might be more like the Foggathorpe. It's definitely dark purple brown not rust coloured, but I thought that might be organic matter. It does leach oily looking water sometimes, but it's not peaty. I'll have to dig some up and look for blue. It squelches in the winter, and only rushes, moss and creeping buttercup are happy in it. Which is why I thought building it up to a terrace might be the best thing to do.

    3. Don't get me wrong, I think building it up is a great idea.
      And congratulations being on topic! ;-)

  2. The two things I would add as problems with clay soil are in wet weather it can be too claggy to work with and in dry weather too hard.

    1. claggy is a great northern word - I should have used it!
      I had a very close look at your own clay soil surface in your recent post about your strawberries

  3. Another good article Roger. I have heavy clay in my area as a result of glacier activity long ago. I also have both gray and red clay very near the surface in my garden due to the stone bed that is under the property making up the Niagara Gorge - cut by those glaciers. It is very difficult gardening in this locale, but a few towns over has the best soil around minus the bedrock. All courtesy of the glaciers.

    1. The soils around York have been influenced by glacial action. My old college Askham Bryan is on a raised terminal glacial moraine and the soil is a bit of a mixture and includes clay and limestone debris

  4. Very good post. I have gardened in clay soil, in fact we were near an old clay pit that was used by a brick factory. The main thing I learned was that some plants can handle it better than others.

  5. and some plants such as roses Jason, I am sure you will agree, thrive on clay soils.
    Interestingly the clay at brickyard farm near me - like my garden - is under 5 foot of sand. Bricks were made in the 19th century and the site is now a lake.


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