Sunday 18 August 2013

Benefits of clay soil

What are the advantages of clay soil? Clay soils can be the most fertile of all

I am frequently irritated when gardeners tell me that my ideas about minimum cultivation are very interesting but on their own heavy clay soil my methods will not work. If they read more carefully they might be persuaded that minimum cultivation methods have a greater contribution to make to the structure of clay soil than to any other. It’s just that the real and substantial merits of ‘no dig’ take a year or two to come through. Gardeners with heavy clay soil put the clock back to zero every time they dig. The damage they do by digging  - gaining apparent and very short term improvement -  repeatedly recreates the conditions that lead to of a rock hard soil in a dry summer and a podgy mess when very wet.

This is not to deny that some gardeners have inherited a site with little topsoil and with an almost pure clay subsoil and conditions at depth where drainage water has nowhere to go. Such unlucky people really do have a problem but digging will not help. 

My intention today is to proclaim the benefits of clay when it occurs as a constituent of soil.

A smidgeon of soil science 
Most soils are ‘mineral soils’ and are composed of mixtures of horticulturally indivisible particles of sand, silt and clay.
Soil classification systems define sand, silt and clay by the particle’s size. A clearly visible particle of sand is some order of magnitude larger than clay. Although the identification of a  mineral particle is based on size, particles of clay are physically and chemically different from sand, For example unlike sand, clay absorbs water and holds plant nutrients. 
A desirable mixture of particle sizes occurs in a ‘loam’. A loam is loosely and not very precisely described as a soil  composed of one third of each of sand, silt and clay. In practice these values are skewed and there is a range of soil textures which are correctly classified as loams.
Texture is a very precise scientific term that refers to a soil’s mix of particles. In normal practice it is an unchangeable characteristic of soil. Adding small amounts of sand, silt or clay makes very little difference to texture and if concentrated in pockets added minerals can have unfortunate cultural consequences. To change soil texture vast amounts of a foreign material needs to be imported. It is not only impracticable, it can easily lead to horticultural disaster!

Difference between soil texture and structure.
Please excuse a small indulgence. As a former lecturer teaching horticultural students the meaning of these two terms could be difficult. It was an easily predicted exam question but never-the-less a few students never quite got it!  Similarly much of the horticultural press  which is written by unscientific journalists don’t get it either and misuse the term. Worse I sometimes read garbled explanations….

Structure defines a soil’s condition. Structure can be easily changed by the gardener. For good or for bad!  Walking on wet soil loosened by cultivation compacts it. Compaction  is just one of many examples of poor structure. My previous posts are littered with examples of how good soil management - often by leaving it alone - improves soil structure.

Benefits of clay. 
Clay particles hold water within their porous matrix. Being very small, plenty of water is also held on their surface by capillarity. Other than non-mineral  reclaimed peat soils, clay soils can hold a greater reserve of water than any other soil. 

Provided that clay occurs in suitable proportion with other minerals and is well managed it helps to give soil a stable structure. For example soil crumbs hold well together and worm and root channels do not easily collapse. 

Clay intimately mixes with organic matter. Decayed organic matter forms relatively stable mixes with clay. Natural uncultivated clay soils have a very high level of organic matter.

Because the internal and external surfaces of clay have a negative electrostatic charge it hold positively charged nutrients such as potassium, calcium, magnesium and most trace elements. This is very significant as these nutrients are held against leaching and yet at the same time are readily absorbed by the plant. It is not for nothing that clay soils are often proclaimed ‘the best’ for roses and many other fine plants.

Provided clay soils are not compacted, an unlikely occurrence in the none diggers garden, they absorb water very well. I frequently curse my own sandy soil which is quite repellent to water when it has become very dry. I go to Bolton Percy and my watering immediately penetrates in...

Two major disadvantages of clay
Contrary to the last stated advantage of clay If heavy clay soil is broken up by cultivation and later subsequently compacted when wet it can become impermeable to water. Really wet  surfaces subjected to heavy pressures when wet can be so bad that water stands on the soil surface even though it might be perfectly drained below. The same principle is seen in action when ponds are lined with puddled clay!

Some, but not all clays (there are many different types of clay) swell when they absorb water. Your building insurance will cost more if your soil texture is clay!

I get stuck again in 2015


  1. Our plot is a clay solid and having lots of paths between the beds means we don't have to walk on where we plant.

    We don't dig in the garden which is also a clay base but over the years it has had lots of old potting compost added to it. As this is planted up with perennials we couldn't really dig if we wanted to and have few weeds growing - presumably these are lurking underground waiting for day light to spark them into action

    1. Thank you Sue for more of your very helpful comments that add something to the blog.
      I noticed on your recent post about plastic mulches that the soil at the margins looked as if it was clayey

  2. Interesting read Roger as our soil is very clay dominant. We've added quite a bit of top soil and mixed it into the upper 12-18 inches of soil.
    Hopefully we haven't ruined it! So far, most shrubs and plants seem happy, except my smallish Frasier fir which appears to have dried out too much in the July dry spell. Not sure whether to see if it comes back or pull it out to be honest!

  3. No, I recommended it myself in an earlier post provided it is mixed in rather than being a separate layer. Although I wanted to deter folk from trying to change their soil texture by adding inappropriate materials or creating sumps where there are pockets of changed soil etc I think you have done the sensible thing

    As to your fir if it looks very dry it might be a goner, but I always like to wait until I am sure.

  4. Your point about clay preventing leaching of nutrients brought up a question. We're constantly told now that if we put compost down in autumn the nutrients will wash out over winter. This has always sounded like twaddle to me, especially on my clay soil. Is it more the case that nutrients are more easily lost from compost mulches if put down on sandy soil, whereas with clay they'll amalgamate with the clay particles?

  5. Hi Mr Weeder do you come across any of my blogweeds?
    Good question
    The main nutrient that might leach from compost is the inorganic nitrate that has been released when the compost has been acted upon by bacteria in the warmth of the summer.
    After a hot dry summer soil is particularly rich in soluble nitrate and as it has a negative electric charge it is NOT held by clay or compost, nor sand of course. This is why it is a good idea to grow plants in autumn and winter so that they will absorb the nitrate and convert it back into organic matter that does not leach.
    Clay does hold all the positive charged nutrients such as potash, magnesium and calcium that might be released from the organic matter- although organic matter has its own negative electric charge and holds positive nutrients in a similar manner to clay.
    I would add organic matter to the soil at ANY time of the year

    1. Thank you. Another reason to put down green manure, I guess.


Note: only a member of this blog may post a comment.

Related Posts Plugin for WordPress, Blogger...