Catherine StewartHow soil drainage works: it’s not what you think

‘Good drainage’ or ‘well-drained’ soil is used to described the favoured growing conditions of about 80% of what you want to plant in your garden. But do you know really know how soil drainage works, so you can get that fabled ‘good drainage’? Drainage myths and half-truths have been repeated for years by gardeners and even experts. The trouble is that water in soils just doesn’t behave in the way that intuitively you’d think that it should.

OLYMPUS DIGITAL CAMERAAlthough most gardening writers, horticulturists, landscapers and home gardeners have got past the notion that adding ‘crocks’ or broken bits of pot to the bottom of a planting container will ‘improve drainage’**, many don’t really understand why that is so, or how that also relates to garden soil drainage. They talk about it reducing the available root zone area for the plant, and it does, but that’s not really why crocks are a no-no.

[**except, it seems, the so-called ‘experts’ on many very popular sites who are still doing it, like the Huffington Post Home section: http://www.huffingtonpost.com/hometalk/the-8-best-gardening-tips_b_5563873.html or the very popular Apartment Therapy: http://www.apartmenttherapy.com/dont-throw-it-away-3-things-you-can-do-with-your-broken-pots-and-planters-203379, or LifeHacker: http://lifehacker.com/5824643/use-styrofoam-packing-peanuts-to-line-a-planter-for-improved-drainage. Honestly. Just goes to show how little you can trust what you read about gardening online – except GardenDrum of course!]

This problem came up recently when my friend and colleague Helen Young sent me a question she’d received through her gardening column in the Weekend Australian newspaper called ‘Twig’. Helen gets an amazing array of questions and puts a huge amount of time into researching the right answers. However her tiny column space means sometimes she can’t develop an answer the way she’d like to, so she asked me to expand on this one on GardenDrum.

The question came from Richard, who asks:

Hello! I was hoping to get your advice on fixing the drainage under our turf – the ground is so soggy it squelches underfoot! We laid the turf over approx 150mm of clean topsoil. Under this I spread weed matting, then about 50mm blue metal [crushed basalt aggregate – Ed]. The blue metal sits upon pavers – we wanted some grass in our courtyard for our young son to play on.

I’m unsure what is causing the poor drainage, but it must be either the topsoil or weed matting – the blue metal is free draining and the original pavers never pooled even in the heaviest of rain.

The topsoil is quite dense and seems to hold a great deal of water. Digging into the soil it is wet, and has a faint rotten egg smell.

The weed matting is that black woven stuff you buy from Bunnings – its quite porous and I thought would be ideal for separating the topsoil from blue metal whilst allowing water to pass through. There did not appear to be any water pooling on top of the weed matting when I dug to it.

Would adding sand to the topsoil improve its drainage? Is there a more appropriate geo-textile which might be used instead of the weed matting? Do you have any other suggestions to improve the drainage?

Although your first thought might be – “grass on top of paving, no wonder it doesn’t drain”, that’s not the main issue here at all. Lots of gardens have impervious surfaces, like bedrock, quite close to the surface and still manage to grow good quality turf. Richard’s quest is not impossible.

There’s lots to learn about soil drainage in Richard’s question. At the heart of this issue is understanding how water moves through soil – or, more to the point, how it doesn’t.

 

Surface tension and gravity in soil

It seems logical that rain or irrigation water should seep down through the soil, and then flow through the woven weed mat into the open, fast-draining blue metal, hit the paving and then drain away to wherever the paving always drained before. But this logic misses one important fact:

the forces of adhesion in a soil can be stronger then the force of gravity

That means that water can stick so tightly onto the soil particles that gravity is not strong enough to pull it off.

10mm aggregate, or blue metal'. Photo by Billbeee

10mm aggregate, or ‘blue metal’. Photo by Billbeee

Water moves in soils by surface tension, so where one soil particle touches another, the water will spread. If below the soil particle is a huge void – like the air outside the bottom of a pot, or, in Richard’s case, both the layer of weed mat and the layer of coarse blue metal – which have huge voids compared to the much more finely textured soil above them – the water won’t ‘fall’ into the void. Only when that soil is completely saturated, which means that all the air spaces in the soil are completely filled with water, will the excess water be pulled by gravity into the void. Once that excess water has ‘fallen’, each soil particle will still remain completely covered by the water that clings to it by surface tension.

This permanently wet zone is called the capillary fringe. But how does that affect Richard’s drainage? Rain water moves down through the soil profile and, assuming it’s still getting through the weedmat OK (and that’s another issue), it hits the blue metal layer and stops until that soil is completely saturated, at which point only the excess water will then fall into the drainage layer. As the spaces between the soil particles are mostly filled with water, it stops air getting into the soil, creating squelchy, wet soil, and also smelly, anaerobic conditions around the plant roots.

tough soil digging with forkAnother way of understanding this is to look at a real soil profile. Soil (usually) becomes more finely textured with depth. In other words, the soil is coarser and more open near the surface and has more clay fines in it as you dig down. This means that water isn’t just pulled downwards by gravity but also by surface tension, as clayey soil has a lot more surface area in it than lighter organic or sandy soil. If water builds up in the lower clay layers because it can’t drain away when the soil becomes saturated, then you’ve got drainage problems, but otherwise it should all work to keep water moving downwards from the topmost layers of your soil profile.

The greater the difference between the texture of the two layers, the more the upper layer will hold on to it. Blue metal (that’s a coarse gravel) has huge voids compared to even a sandy topsoil.

 

Soil depth

But that’s not the whole story, as it’s also the depth of topsoil in Richard’s lawn area that’s the second part of the problem. If the topsoil were deep enough, the saturated capillary fringe would be down low enough in the soil profile that the grass roots would be above it, and could even use it as an extra reservoir of water in drier times. This is called a perched water table. Many plants are adapted to growing over perched water tables where there is a shallow dry zone over a very wet subsoil. Constructed sports turf also uses perched water tables created by having a coarser sand layer under the main root zone. This holds more water at the bottom of the upper zone just under the grass roots, which it can access during drier times.

This short video shows how a saturated zone can extend to the top of a soil profile.

What depth of soil stays saturated is dependent on the texture of the soil – how clayey or sandy it is, with fine textured soils having a deeper capillary fringe. A loamy soil could easily have a capillary fringe of about 200mm (8 inches). As Richard’s soil depth is less than that, the saturated zone in the soil is deep enough to extend right to the surface, so the whole soil profile stays permanently wet.

Deeper soil or even a taller pot will allow more water to drain away from the root zone than a shallow one, which is why you shouldn’t put ‘drainage crocks’ in the bottom of a pot [yeah Huffington Post, get it right]. If you do, you’re making the soil profile shallower, and that saturated capillary fringe extend higher in the pot. Even if a pot has lots of drainage holes, the saturated zone can extend up into the plant root zone. For any one type of potting mix, the capillary zone will be the same height, no matter what size or shape the container. If it’s, say 100mm, then a 200mm high pot will have a clear 100mm above the saturated zone, but a shallow 100mm high bowl will have none. If you want to plant in shallow bowls, you need to use a very coarse, sandy mix so its capillary zone is reduced to a few centimetres (1″) deep – or choose plants that prefer boggy conditions, like reeds.

You can see this for yourself with a simple experiment. Water a potted plant until you can see water running out the bottom. Hold it up in a level position and wait until no more water is draining out of the pot. Now tip it 45 degrees and……more water drains out of the pot! Tip it back to upright and it will stop again.

What’s going on?

When you tip the pot, you temporarily make the soil column in the pot taller. This means that the capillary fringe, which we know is always a standard depth for that mix, will suddenly have more draining soil above it. The column of water held in the potting mix is more affected by gravity, which pulls out more water.

 

Compaction

Ball play. Photo by Andy

Ball play. Photo by Andy

If Richard’s children are playing regularly on the lawn that’s growing on this wet soil, they will have caused the soil to compact, as wet soil is much weaker than dry soil and packs down quite easily. This means that there are fewer air spaces in the soil and water will be held even more tightly by those villainous adhesive forces, causing it to be even wetter and the capillary fringe to be even deeper.

 

The weedmat

Richard has no doubt used weedmat as he wanted to stop fines from the soil moving through the gravel and into the drains, which could clog them up. The trouble is, that’s exacerbated the texture difference between the topsoil and the base, stopping water moving from one to the other. I’ve also yet to encounter a weedmat that doesn’t block up, so I’d remove it immediately.

 

The underlying pavers

These are not the main problem. However, in heavy rain, the fall on the pavers to their drainage point may not be sufficient to allow the everything to drain quickly enough.

 

So what can Richard do to fix his squelchy soil and ailing lawn? Will added sand or a different geotextile fix the problem?

The short answers are probably not, and no. Adding sand will reduce the water-holding capacity of his top soil, and slightly reduce the height of its capillary fringe, but it won’t be enough to get rid of the saturated zone surrounding the grass roots. It’s most likely effect would be to cause the grass to wilt in dry times as well.

So what would I do? With all the usual provisos and cautions that I haven’t seen the site and don’t know all the circumstances, here are 2 possible alternatives:

1. Lift the turf, remove the blue metal layer and the geotextile, and use that depth to bring in more soil before relaying the turf. Increasing the soil depth may be enough to reposition the saturated area to below the grass roots.

2. Lift the turf, remove the weedmat and blue metal, replacing it with a 20-30mm (1″) layer of coarse sand over which he spreads a light, sandy topsoil and then relays the turf. Then there will be less texture difference between the two layers, allowing water to move more easily between them and reducing the height of the capillary fringe.

I’d go for #2. I certainly couldn’t guarantee it it, but it’s much more likely to grow turf over those pavers than what Richard has now.

POSTSCRIPT JANUARY 2016: Richard has sent in an update about the state of his lawn

Over a year ago I asked a question about the grass in our backyard and you provided some excellent advice on how to improve the drainage. I’ve been meaning to send an email for a while now to once again say thankyou, your suggestions have worked and the grass is still alive and going strong!
In the winter months I had to seed the areas which did not get enough sun for the buffalo but this has been the only issue.  I’ve attached an image of our tiny patch (taken a while ago now just after those dreadful hailstorms which shredded most of our plants!)
Thanks again!
Richard

Rehabilitated lawn with improved drainage Photo Richard Harrod

Rehabilitated lawn with improved drainage Photo Richard Harrod

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Catherine Stewart

About Catherine Stewart

Award-winning garden journalist, blogger and photographer; writer for garden magazines and co-author of 'Waterwise Gardening'; landscape designer turned landscape design judge and critic; compulsive networker and lover of generally putting fingers in lots of pies. Particularly mud pies. Creator, curator and editor of GardenDrum. Sydney, NSW.

18 thoughts on “How soil drainage works: it’s not what you think

  1. helen mckerral on said:

    Great solid explanation, Catherine!

  2. Eugene on said:

    Wow…….did not know any of that. Completely done my head in. All these years doing the wrong thing. Very news worthy bit of info there Catherine.

    • Thanks Eugene. No doubt there are still lots of things I’m doing wrong too!

  3. Bruce on said:

    I’ve been gardening for seven years. I’ve taken several semester-long classes at the local Adult Schools. I’ve had at least five experienced gardeners tell me to put natural or styrofoam ‘crocks’ in the bottom of my containers. (I think there was even a hot tip in Martha Stewart Magazine about the styrofoam crocks trick).

    But only now, in July 2014 have I finally learned what’s going on in the soil when it comes to drainage. And not just for pots. Now I know how to think about the soil out in the yard too.

    I’m sure I’ll make more mistakes, but now I will be learning from my mistakes (as well as from my now more frequent successes).

    Thanks so much for giving us the facts.

    • Thanks Bruce. Although garden ‘experts’ in the UK and Australia seem to have moved on from advising wrong things about soil drainage like crocks in pots, I’m amazed at how persistent it still is in the USA. So much advice is based on what people think should work, rather than what fits with the science and close observation. It sounds like you’re a real thinking gardener, which means you’ll be a successful one.

  4. Lisa on said:

    Thanks Catherine, now I get surface tension!. Garden Drum provides what many garden sites overlook – deeper and comprehensive explanations of gardening phenomena in order to problem solve. Great post.

    • Great to hear that you now ‘get’ it. Lisa. I know exactly what you mean – you sort of think you understand until you have to explain it to someone else, or problem solve, and then it all seems to melt away. It’s taken me a lot of reading and observing to get it too!

  5. Surprisingly, the issue of soil drainage is not caused directly by aggregate size and compaction alone, but a dysfunctional soil biological network (soil food web) – culminating in good earthworm activity – unable to build structure deep into the soil profile. In many situations, using microbiology in the form of advance microbial compost or actively aerated compost teas can rapidly restore soil structure and aeration. Check out Tree Preservation Australia for details. This could well be the third alternative.

    • While I agree that soil biology is important in binding soil particles into stable aggregates, and channelling earthworms will improve the way water can move through the soil, introducing microflora and microfauna into an environment in which they cannot sustain themselves will not provide any benefit. Until the soil texture and interface problems are sorted and the saturated zone that currently extends to the soil surface is reduced, only anaerobic bacteria will survive for long, rather than beneficial actinobacteria. This is a soil physics problem of fine textured material over coarse, not one of a deficient soil biota.

      • Eugene on said:

        I am likewise highly suspicious of compost teas after engaging in hours upon hours of fruitless search trying to find any decent peer reviewed papers that back up the sometimes outrageous claims made by the adherents.

          • Eugene on said:

            James is spot on in that article Catherine. In my research on the topic I kept coming back to the same conclusions he has reached. Mainly found it all here –

            http://puyallup.wsu.edu/~linda%20chalker-scott/Horticultural%20Myths_files/Myths/Compost%20tea%20again.pdf

            Dr Linda Chalker-Scott has convinced me that while it may not actually do much harm (as opposed to the …shudder…’black water’ that is generated by anaerobic methods) the good it does is marginal and, in the case of powdery mildew, not replicable often enough to be worthy of serious consideration.

            I was initially swept up in the enthusiasm and spent $200 and a bunch of man hours on a sweet little brewery for the garden. Then I did my research……sigh……….

  6. Great post. Thanks Catherine. Learnt a lot from it, the least of which is why my plants do better in shallow pots than in deep ones – and I am digging out some crocks this weekend, too.

  7. Joel Vntrs on said:

    What does 20-30 mean in your statement “…, replacing it with a 20-30 (1″) layer of coarse sand over which he spreads a light, sandy topsoil..”?

    Thanks

  8. Deb on said:

    I have had quite a bit of paving removed and I am trying to put in a garden in its place however the base of crushed road base or blue metal is still there. I have forked it over to loosen it up and tried to mix it a bit with other soil. Other than mulch should I add anything else to counteract it and create better soil for my plants. Will it be too acid?

    • Hi Deb – Road base is more likely to be alkaline than acidic. From a soil physics point of view, the main problem with road base is spreading a more finely textured soil over it, which then won’t drain into the coarser road base. However there’s also the soil chemistry to consider so, before you do any planting, buy a pH test kit (about $17) so you know what you’re dealing with. It’s a good investment – if the pH is way out you could lose all your plants. I have found that alkaline soil is harder and slower to change the pH than acidic soil. It requires the addition of sulphur in warm, wet conditions as it’s soil bacteria that convert the sulphur to sulphuric acid.

      • Deb on said:

        Thank you for your reply. So if too alkaline. What if I dug in compost to it. i.e. composted horse manure and straw or, a pine cone/needle compost. I have access to plenty of those things.

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