Clay Soils Are Very Nutrient Rich But Can Be A Blessing Or A Curse. In this Article I Show How To How To Improve Clay Soils In The Garden.
People often want to know how to improve clay soils. That’s understandable as clay soils are made up of very small particles, so closely packed together that water has a problem flowing through the clay soil. And what water there is tends to “stick” to those fine particles making it harder for the plant to get the water. this makes the soil “heavy” and hard to work.
Worse still, once the clay dries out it cracks into big lumps that are extremely hard to break down into a seed bed. Dry clay is also very hard to re-wet. The rain just flows around the big lumps or, if the surface isn’t cracked, will flow away down any slope. If you don’t have a slope it will just sit on the surface and not soak in.
In fact, if your land has a dip in it, the result of heavy rain is a pond.
Village ponds made use of the impervious nature of clay and in areas where the soil didn’t helped water would be lined with clay to ensure they did. A foot or so of clay would be added and to make sure it was water tight it would be “puddled”. Puddling means to stamp down the clay so that a puddle is formed.
Puddling can be done with our feet and in the past people would sometimes do it in bare feet. When the pond is very big it would have been very tiring to stamp down all the clay so villagers and farmers had a tick to use. They used farm animals. A herd of cows or a flock of sheep would be repeatedly driven across the pond so that the clay became throughly puddled and water tight.
There’s more on the characteristics of clay soils in the last sections of this post.
The Problem With Clay Puddling
When you want a pond puddling is a great way top achieve it. But what if you walk or dig clay soil when it’s wet? It starts to puddle. And that’s why it’s a really bad idea, to work wet clay soil. Ploughs cause enough problems when used on clay farmland, but the bigger evil is to use a rotavator on wet garden and allotment soils. The rotating blades cause a smearing action which is far more effective than puddling with our feet. It makes the clay soil completely impervious to water in the same way as a potter makes a pot and smears the clay into shape. Once that clay dries out it becomes totally impervious to water. And even if it doesn’t dry out not much water can run through the clay once smeared.
Is Puddling Ever Good For Crops?
There is only one situation where I know puddling will benefit a crop. It’s when you grow rice and want water to hang around. Rice farmers puddle their fields so that water doesn’t drain away. I don’t know anyone growing rice near me in Devon! In fact I don’t know anyone anywhere in the UK growing rice.
How To Recognise Clay Soil?
Clay soils are easy to recognise as when the wet soil is rolled into a sausage shape it retains its shape without cracking. It will feel soft and silky whereas sandy soils crumble and feel gritty in comparison.
Clay soils also easily puddle.
Clay soils are defined as those with at least 30% clay content. They can be pure clay though, or with virtually no other constituents.
The more clay there is in the soil the harder they it is to cultivate.
The Practicalities of Clay Gardening (& How To Improve Clay Soils)
Clay is very nutrient rich and crops can do well on clay soils provide they are managed correctly. My grandfather’s garden and orchard were clay and were eventually bought by the brick company that quarried clay of brick making. The clay went down several hundred feet and the quarry was huge.
And that brings me to the first practicality of clay gardening.
The first question to ask is how deep is the clay you are going in. Is it a thin layer over, say, sand or loam? Or does it go down hundreds of feet?
The only way to tell is to start digging …. or look for a clay quarry over the hedge!
In some senses it doesn’t matter how deep it is but in others it’s important.
For example it is possible to improve a deep clay soil and make the top few feet easier to grow in, but the soil is never going to be easy to drain. I often see people say dig in some gravel or sand and it will cure drainage problems in clay soils. It doesn’t if the clay is tens or hundreds of feet deep! It’s gardening myth.
But let’s assume you have a shallow clay layer. Then it is possible to improve the soil and make the clay more amenable to growing veg crops. But I wouldn’t start with adding sand or gravel. All that happens if you do is that each piece of sand or gravel gets surrounded by clay and the water is still prevented from draining through.
But if you add organic matter, such as compost it starts to break up the soil and allows drainage.
Some people recommend adding gypsum to improve clay soils. Gypsum is calcium sulphate, it works by chemically breaking up some of the lumps. But it is short lived and my experience is that organic matter works far better.
Where organic matter is added to a clay based No Dig garden the impact is almost miraculous. Adding a 4-5 inch layer of compost in year one, followed by 1-2 inches each year thereafter makes a huge difference. Ensuring that the plant roots are left in the soil, where as they decompose they form drainage channels also helps a lot.
How To Test How Well Your Clay Soil Drains
If you dig a shallow hole in the soil and fill it with water you’ll soon see the water disappear if the soil is sandy. But in clay soils it takes much longer. And if after 24 hours the water is still there then you have a very poorly draining clay soil. It might be that a clay pan has formed at plough or rotavator level. Or maybe the clay is very fine and naturally extremely poor draining . But it wil dictate how and what you grow in future.
In very poorly drained soils I would favour deep rooted edimentals and perennial veg plants. Some trees do well in the conditions. And certainly my grandfather’s land, sold to for brick quarrying, grew very good apple trees.
The land was never quarried as the brick company ceased operations. It is now a wild natural place full off abundant growth and wildlife.
How To Improve Clay Soil Drainage
Imagine a thin layer of clay soil over a wet draining loam or sand. When it rains the clay will inhibit drainage into the soil and much water will run away to stream and rivers. In fact, when it rains hard the pounding scion of large raindrops will puddle the soil surface and drainage will worsen.
Now add compost to the surface of clay soil and sow deep rooting plants in it. The roots will penetrate and form channels in the soil. Worms will incorporate the compost into the soil and produce drainage channels lined with your compost and their worm compost.
Congratulations. You’ve just “fracked” you first piece of clay soil.
But be careful. If you walk on it when it is wet, or dig it when wet, it will loose the carefully built up structure and you’ll be back to square one.
You need soil porosity to allow the water to drain away. And you need it so that water can penetrate deep into the soil and act as a water reserve in dry weather.
Manage correctly clay is some of the most fertile soil we can grow in.
Now imagine a deep call soil. Perhaps it is like my grandfather orchard with clay going down hundreds of feet. You can’t just “frack” the clay and get down into free draining soil. It’s going to be much harder to manage.
But don’t despair. There are ways to improve and manage it.
Add compost and deep rooting plants as you would with a shallow layer of clay soil. And accept the limitations of that soil. You can still grow winter and summer crops but you need pathways from which to work and you need to ensure you never walk on the soil , or dig it, when it is wet. Treat it well and it will provide you with excellent high yielding crops.
The added bonus, when you are not digging, is that the soil surface will remain friable, even in wet and dry weather.
It your clay soil slopes the excess rain will flow away.
Managing Clay Soil In a Dip!
If your plot is clay and low lying it is potentially a pond. The clay holds water from draining even if not puddled. And if you have no lower land to drain it to you have a problem.
No end of soakaways are going to solve your problem simply because there is no where for the water to go. A deep soakaway will soon fill with water, however much rubble and grit you put in it.
And just adding sand, grit or gypsum isn’t going to help when there call goes down tens or hundreds of feet.
How To Improve Clay Soils Fertility
Clay soils are naturally fertile. But crop yields can be further improved by growing leguminous crops, cover crops or green manures.
Adding compost also helps as it improves porosity and available nutrients as well as making the soil more manageable.
What pH is Clay Soil?
I see a lot of disagreements over this simple question. Some websites claim clay is acidic. Others claim it is alkaline.
The reality is it can be either, but is more likely to be around the neutral mark.
Clay Soil Characteristics
Clay soils feel very sticky when wet and very hard when dry.
Roll clay soil into a sausage shape when wet and it will not crack.
Clay soils can hold more total water than any other soil BUT only about half of it is available to plants.
When clay soils get wet they swell. When they dry out they shrink and crack. Dry clay soil can have cracks in it big enough to put your fingers or hand into.
Clay soil smears very easily when wet. Once smeared the surfaces become impervious to rain or water penetration.
In spring clay soil warms up very slowly. this is due to the large volume of water in them. Clay soils are never as early growing as sands and loams. Clay soils are late soils.
Clay soils are nutrient rich with plenty of potash. Though phosphate levels can sometimes be low.
The Science Bit
If you are interested in the science behind clay and How To Improve Clay Soils, check out this link.