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Improving Clay Soil FAQ

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Clay/Heavy Soil Improvement FAQ

The following FAQ outlines the basic procedure for improving the quality, texture and fertility of a heavy garden soil. The methods described can be applied to the planting of a single plant, a group of plants, or over a larger area, such as a bed, border or veg patch.

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Just how bad is the top-soil?

Do you remember this classic piece of 'science' from your primary schools days? Take a milk bottle, or other transparent bottle, and fill to about one-third of its volume with your garden soil. Soil in Milk Bottle
Fill bottle to about one-third full with soil
Shake vigourously
Add water and cover opening of bottle
before shaking vigourously to
disperse all the soil particles
Then, fill the bottle to two-thirds full with water, and shake vigourously for a minute or so, remembering to keep one hand over the top. Place on a level surface and allow to settle. After a few minutes, the grits and heavier sands in the soil will sink to the bottom of the bottle, with the finer sands, silts and clays taking much longer to settle. The organic matter, sometimes referred to as 'humus', will float at the surface of the water.

This simple experiment will give you a visual indication of the nature of your soil, showing the approximate proportions of sands, clays and humus. An 'ideal' soil would contain roughly one-third humus, one-third clays and silts, and one-third sands and grits. After a few hours or so, it should become apparent which constituents are lacking in your soil. Settled soil
Soil constituents settle out to
form separate distinct layers
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IMPROVING THE SOIL

A garden is only as good as its soil, and soil improvement is the single, most effective technique to create a better garden. The simple milk-bottle test described above will give a good indication of which constituents of a good soil are under-represented in your patch, most likely to be sands/grits and humus.

Adding Sands/Grits

Working a sharp, grit sand into the soil, will not only improve the drainage characteristics of the soil, but will also improve the aeration of the soil, render it easier to work, and encourage strong root formation by your plants.

Grit Sand
A typical grit sand - penny used for scale
Choose a sharp or gritty sand, not a building or a soft sand, which contain too many 'fines' and clay particles. If the sand is from a marine source, it should be washed to remove all salts. A good grit sand will contain a reasonable amount of tiny pebbles in the 3-6mm size range, along with smaller sand grains, but less than 3% clay particulates. Your local Builders' Merchant will most likely stock a grit sand, sometimes called a 'concreting' sand, in 40kg bags that can be carried in a car, and in 1 tonne 'agg-bags' that can be delivered to your home by the merchant's delivery wagon. Garden Centres also stock 'horticultural sands' in 25kg or 40kg bags, although they are often more expensive than Builders' Merchants.

Adding Organic Matter

The addition of organic matter will also aid drainage, improve aeration and add to the fertility of the soil by the release of valuable nutrients from the decomposing matter. Suitable organic matter includes well-rotted garden compost, spent mushroom compost, used compost from pots, tubs and hanging baskets, well-rotted manure, well-rotted leaf-litter or, as a last resort, bought-in multi-purpose compost or peat (environmentally naughty). You will note that the key phrase is "well-rotted" - partially-rotted or 'green' organic matter feeds from the soil to aid decomposition, rather than feeding the soil, as is intended.

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WORKING THE SOIL

Now that we've determined what is going to be added to the soil to improve it, and we have the requisite materials on-site, the real hard work can begin. This task is best undertaken in winter or in early spring, before the growing season begins, but it can be done at any time if you don't want to wait.

The area to be improved should be cleared of vegetation. If particularly precious plants are situated within the improvement area, and you don't wish to disturb them, then they can be left in-situ and worked around, provided that care is taken to avoid excessive root damage. If possible, it is better to dig plants out and store them somewhere suitable for the duration of the works, and re-plant them once work is complete.

Using a garden fork, break up the soil by driving in the fork and levering back to lift the soil. You need to loosen at least the top 200mm of the existing soil, and you may want to consider a double-dig system for deeper improvement. There is no need to turn the soil a great deal at this stage, as you'll be doing plenty of that shortly.

Once the ground is broken up, scatter the sand and/or organic matter over the surface to a depth of 75-100mm, and then, again using a fork, turn the soil over, mixing in the added material as you work. Try to break up any 'clumps' of soil with the tines of the fork, so that there are no 'clumps' bigger than about 40-50mm. Keep turning the soil until it is reasonably well mixed, and there are no obvious patches of all-soil, or all-sand, or all-humus.

You can repeat this process as many times as you feel is required to work a generous amount of sand/grit or humus into the topsoil, although 75-100mm of sand/grit and 75-100mm of organic matter will probably be adequate for most soils. The deeper you work the soil, the better the results will be. 200mm is a minimum depth to improve, but you may need 300mm or even 450mm depth for best results, depending on the soil-type and on what you intend to grow on the improved land. The now-improved soil can be planted immediately, if required, although it is often best to leave it to 'settle' for a week or so, and then re-work the top 100mm with a combination of fork and rake to break up any more 'clumps' before planting-up.

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AFTERCARE

Congratulations! You've done the hardest bit of the work now, and your garden will reward your efforts over the coming season. However, soil improvement is not a once-in-a-lifetime operation, and needs to be done on an annual basis to keep the soil in tip-top condition. Bear in mind that the soil will now be easier to till, thanks to the previous efforts, and in future seasons, you will only need to work in 50-75mm of organic matter, which can be applied as a mulch in mid-season, and worked into the soil at the end of the year. If further sand/grits are required, these should be dug in over the dormant winter or early spring season.

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WHAT NEXT?

Ok. So you've done the soil improvement routine, and your garden is still damp or waterlogged. It may be that your garden is in a natural hollow or low-spot, and it may be impossible or impractical to properly drain the garden, and so, the only option is to improve the soil and to make a feature of the damp conditions by planting a bog-garden, or by creating ponds with marginal plants. Details on ponds and bog-gardens are given in other FAQs or on the newsgroup as replies to questions from subscribers.

The other alternative is to install a land-drainage system, if feasible, which is covered on cormaic's paving website.

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FURTHER QUESTIONS

Q - What about proprietary 'Clay Breaking Compounds' that I've seen in my local garden centre?

These products do not seem to be highly rated by experienced gardeners.

Q - What about using lime and/or gypsum?

Both lime and gypsum work by aggregating the clay particles and make the clay more open and permeable. Gypsum does this without raising the pH and is therefore suitable for use where acid-loving plants such as rhododendrons and azaleas are growing.

This is a much fuller explanation of the uses of gypsum, written by Chris Hogg...

Peter Cox in his book in his 'Dwarf Rhododendrons', published by Batsford in association with the RHS, 1973, recommends gypsum for improving the soil where Azaleas and Rhododendrons are grown.
Another advantage of using gypsum rather than lime for improving clay, again because it's acid, is that it can be used at the same time as ammonia-providing manures and fertilisers, because it won't volatilise the ammonia and waste it. Otherwise it should work in a very similar way to lime, although don't use gypsum if you're actually trying to raise the soil pH and decrease its acidity.
Gypsum, (AKA plaster, plaster of Paris, calcium sulphate) is slightly soluble in water (a similar solubility to lime, roughly about 0.2%). If I had a clay problem and I wanted to keep it acid, I would get a few bags of plaster from a builder's merchant and work that in, assuming it's free from other additives (cement?) that would raise the soil pH. I imagine this would be a lot cheaper than buying a 'clay breaker', knowing most garden centre prices. Builder's plaster is unhydrated, while gypsum is the hydrated form, but that would soon change when it came into contact with damp soil. But, like other clay treatments, little will happen if you just sprinkle it on and stand back. The clay would still have to be broken up and worked with all the other recommended additives (compost, manure, grit, etc.) to get the best results, as with using lime.

I have no idea as to how much needs to be applied; as much as it takes, within reason? Can you use too much? To quote from another rhododendron book (Rhododendrons and Azaleas, Sunset/Lane Books, California, 1973)

"Gypsum applications on sodium soils make pronounced, prompt effects. Calcium causes flocculation - grouping of particles into small grains with air spaces between. Soil becomes more workable, more favourable for root growth. Applying gypsum indiscriminately to soil won't harm most plants since very few species are affected by even saturated solutions of gypsum; it's one of the safest soil conditioners you can use".

Sounds wonderful, doesn't it! So why isn't everyone using it? There must be a reason. A significant point here may be the mention of sodium soils (how do you tell? I don't know). A marly clay will already be full of calcium from the chalk, so adding gypsum, or lime for that matter, won't make much difference. On the other hand, marly clays, by implication, may be less of a problem anyway.

Q - I want to prepare about 50sq feet of very heavy clay soil ready for lawning (seed) next year would this method be suitable?

It's probably 'overkill' for a lawn, but that doesn't mean you can't use this sort of approach. If you do, be sure to let the tilled earth 'settle' for at least a month before sowing or turfing.

Q - As heavy digging is out of question, would rotavator be o.k?

Rotavators can sometimes be harder to handle than digging, especially in the hands of a novice, but there's no reason why you couldn't use one, although the depth of operation is limited, and they can be difficult to use on sloping ground.

Q - I have heard mention of lime and sawdust for breaking up clay, is this feasible and if so what sort of quantities?

Lime alters the way clay particulates coagulate, but still needs a few seasons to work fully. The typical recommendation for lime application is 100g per square metre, but this will obviously depend on the type of clay being worked.
Sawdust is just a variation on the 'humus' addition, although it will deplete the soil at first, as it decomposes, and there may have been harmful preservatives used in the original timber.

Q - What are the cheapest options as I've just spent all my cash on this computer?

Grit sand in tonne bags - around 25 quid incl delivery and VAT - and your own garden compost, are the cheapest options.

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FAQ written by cormaic on 19th April 1999 from the thread in uk.rec.gardening
Thanks to Peter Maughan, Victoria Clare, Sarah Dale, Chris Hogg, Eileen and Malcolm for their valuable input to this FAQ.

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