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What is ‘Well-Drained Soil’?

September 2, 2019

Written by Dave G.

We have all seen it, and some gardeners have even come to dread it – those words in the description of that tree or shrub you are aching to have – ‘Grow in fertile, moist, well-drained soil’. The first two parts are not so bad, after all, if we enrich our soil, and we water regularly, that is surely achievable. But what about this ‘well-drained’ bit? How do I know if I have it? Can I change it if I don’t? Why is it needed? It sometimes seems that almost every plant wants this, but what, exactly does it mean?

Why Do I Need Well-Drained Soil?

While it is true that ‘well-drained’ is about water in the soil, the truth is, that is secondary. What this need is about is air, specifically oxygen and carbon dioxide. A good way to think about the ideal soil is like this. Every spade-full of soil should be half solid, and half spaces. The solid part is a mixture of broken rock of different sizes, plus pieces of decomposing plant and animal material. The rest of the soil is the spaces in between those solids – a system of interconnected spaces and channels of all sizes, from clearly visible to spaces with atomic diameters. In soil-jargon, these are called ‘pores.’ After a period of steady rain, those pores will be completely filled with water. the soil is ‘saturated’.

Plant Roots Need Oxygen

Isn’t that a good thing?’ you might be thinking right now – ‘After all, water is good for plants, right?’ Well yes, plants certainly need water, but the roots that carry water into the plant need something else too – oxygen. Some people make the mistake of thinking that plants are fundamentally different from us. After all, they make food from sunlight and carbon-dioxide, and give out oxygen, in the process called photosynthesis, while we do the reverse, using oxygen to burn the energy in food, and releasing carbon-dioxide. If we turn off photosynthesis, plants are just like us – their cells are very similar, and use oxygen to burn stored food, releasing carbon-dioxide. Plant roots are in the dark, so they can’t use sunlight. Instead they use food provided by the leaves up above, and they live just as we do, taking in oxygen, and releasing carbon-dioxide. That oxygen comes from the air drawn down into the soil to replace the water that gravity removes. It is that simple. Good drainage is about water flowing down and out, while fresh air is drawn in. Not only does this keep the plant roots happy, it stimulates all kinds of microbes to work in the soil, releasing good nutrients, and discouraging root diseases. It takes tough plants to survive with very little air at the roots, and most are happier with ‘well-drained soil’, that brings them plenty of fresh air to breathe.

Why Are Some Soils Well-Drained?

As soon as those passages in the soil – the pores – are full of water, good-old gravity gets to work. It draws the water down through the pores, deeper into the soil, eventually taking it away from where plant roots are – mostly in the top foot or so – and down deep, to eventually drain away into surrounding streams, rivers, lakes and on to the ocean. But as well as gravity, there are other forces at work. There is an atomic attraction between the water and the walls of those pores, called ‘adhesion’. There is also a force holding one water molecule to another, called ‘cohesion’. The smaller the spaces, the more important these forces become, relative to the force of gravity. If the pore spaces are small enough, these forces overcome gravity, and keep the water inside the pores, instead of draining away. The soil remains close to saturated, and plant roots suffer from a lack of air and oxygen.

It is easy to see that if the soil has big solid pieces, then the spaces between them must be big too – picture a box full of basketballs. If the pieces are smaller, the spaces are smaller too. Picture a box of golf-balls – smaller spaces between them, right? Real soil is a mixture of big pieces – sand – and small pieces – clay – in different proportions. The bigger the pieces, as we see in sandy soil, the faster the soil drains. The more smaller pieces there are, as we see in clay soil, the slower it drains.

Check Your Soil

A simple test to find out your soil type – sandy, loam, silt or clay – is always a good idea. Unless your garden is low-lying, or along water, if you have a sandy or loam soil, your natural drainage is almost certainly fine. Also, after a heavy rain, how soon can you go out and dig? If it is within half a week, then you are fine. If after 4 or 5 days your soil is still wet and sticky, clinging to your boots, then you don’t have ‘well-drained soil.’ For more details, check out our blog on testing your soil.

How Can I Make My Soil Drain Better?

Now we have seen the importance of good drainage, how can we help our soil, whatever it is, to drain better, so that those plants that need ‘well-drained soil’ come within our reach?

One way is to raise the soil up. Raised beds, terracing, slopes, berms, or just simple mounds to plant our trees on. All these things will change the levels in our garden, and make water move faster, like a stream running downhill. These things are usually part of the overall landscaping of a garden, although planting trees on mounds of earth, at least 6 inches above the surrounding area is a great way to do it on a small scale, for a single tree, or a few trees.

The second way is NOT to add sand. Adding sand sounds like an easy answer, and some people try it, but it isn’t. First of all, you would have to add a lot – as much sand as you have soil, so you had a 50-50 mixture, which is obviously not practical except in a small spot, like a rock garden. More important, think of those basketballs and golf-balls again. If I mix a bunch of basketballs – sand – into a box of golf-balls – clay – do I have more spaces? Not at all. In fact, I have simply filled the spaces that were between the big balls with small balls, and the result is a solid mass – cement.

Instead, the best way to improve the drainage of any soil is to add lots of rich organic material. Compost, rotted leaves, manures, all these things are very coarse, and full of spaces. Adding them to soil creates big channels and allows the water to flow through. Even better, gums and resins in this material glues the particles together into bigger lumps, like gluing those golf-balls together into big basketballs and creates semi—permanent big pore spaces for rapid water movement. Regular additions will usually turn even the worse, wettest soil, into ‘well-drained’ soil after a few years.

Wow, that was a lot! The solutions may seem easy, but it certainly helps to understand why some things work, and some don’t. The more we know about the life of our plants and soil, the better we get at growing them.