A Simple Drainage Test + How to Transform Heavy Clay

heavy claggy clay is best left alone until it dries outtion

Winter is a telling time in your soil health journey – from now through early spring you get to see your drainage in action. Those of you at the basemap phase of gathering info and learning your land – perk your ears on up! How wet are your soils?

Drainage is absolutely key for plant health. Our edible gardens must have oxygenated soil to be at peak performance. Water displaces air, you see, so if your soil is soggy, it is also airless. It’ll feel heavy, sticky, be pale or grey-ish in colour and most likely be sour smelling. Soil life thumbs it’s nose at conditions such as these, and herein lies the deep reason for the drainage – without soil life, there cannot be above ground abundance.

There are, of course, degrees of wetness. From vegie patches that are underwater for long periods – a loud cry for drainage to be resolved, to soil that’s simply a heavy clay – awaiting transformation to a delicious vegie-growing loam.

Lets begin at the beginning, with a simple drainage test. It’s great to do this when soils are at their wettest so you can understand your worst case scenario and shape your land to cope with it.

A simple drainage test

measuring the drainage test to assess how fast the water is draining

Do this test in a few different spots about your garden/ land.

  • Dig a 30cm by 30cm (or there abouts) hole.
  • Fill with water and let it empty.
  • Fill with water again right to the top, as soon after the first empty as poss.  
  • After 15minutes, measure how much water has drained away. Times this by 4, to find your hourly rate.

If your hourly rate is less than 2.5cm – you’ve got drainage issues that need sorting. Sorry about that my friend. Its not ideal, but at least you know and can now surge forward + get set up for success.

If its 2.5cm through 7.5cm, your soil gets the gold star! You are good to grow.

More than 10cm an hour is too quick and your soil will need to be built up with lots of compost and organic matter before growing vegies or fruit trees successfully.

You’ll have different drainage times in different parts of your land. Mark them all down on your basemap and where possible, use the well drained areas for food and the poorly drained/ dry areas for plants that suit those conditions. Working with is always the easiest road, and the first option to explore. Its not always possible though – there are times when we need to muck in and change things up a bit.

Solutions for poor drainage

natives along the side of the driveway help reduce water run off
Densely planted natives alongside our driveway go along way to soaking up our high rainfall.

The pertinent question here is – where is the water coming from? Scout it out and get to the root cause. Some fixes are super easy – sorting the overflow from your tank or gutters can be all it takes. Can you capture this water in another tank and store it for summer usage? Another common source of too much water is water shed off roads or slopes or driveways.

Our mantra, regards water is to slow it, sink it, store it.

Water flows from high point to low. The harder the surface, the faster it flows and the faster it flows the less opportunity there is for it to sink into the landscape. Intercept it as close to where it enters your property as you can, then turn its attention sideways by catching it in a drain or ditch or swale and redirecting it to use elsewhere. This sideways pause is so valuable – slowing the flow down and allowing it to seep through natural filtration processes, which cleans it before it arrives in our rivers and streams , protecting them, and the end point of all our runoff – the ocean.

Trees are amazing at soaking up water. Not only holding it in their massive root systems, but capturing and slowing much of it in their tops as well. Densely planted areas are the simplest and best way to sink and store water. To this end, plant the roadside, slopes and driveway up – anywhere there is watershed. Every bit of water held makes a big difference. As trees grow you’ll be amazed at the difference. Bear this all in mind if you remove trees or clean up fallen trees – leave the roots behind!

If you redirect your water, be careful not to off load it onto your neighbours. There are loads of groovy, simple ways – perhaps send it through a french drain through your paths, ending in a low point at a pond or wetland, planted up with water loving flaxes + cabbage trees + watercress + taro + frogs. Create a hum of biodiversity that’ll benefit your whole property, and infact the world. You could do this in community with your neighbours – all send your water to one magnificient wetland! Imagine the life!

Raised beds on top of a poorly drained area may seem like a great hack – and they can for sure grow you some vegies, but they wont ever reach garden nirvana because the soggy, unhealthy soil beneath informs the health of the soil in the raised beds above. Everything is connected you see.

Transforming heavy clay

My winter soils in the early days after a few years of composting and mulching. Starting to change colour but still a bit soggy in winter and early spring.

If your drainage gets the gold star, but your soil is heavy and wet at this time of year – take heart!, its simpler than you think to transform it. You will, one winter not too far away, be able to plant and sow.

Meantime, sticky clay should be left well alone – like a grouchy teenager, playing with it in this state only makes it worse. Cover it with lots of mulch for now and grow your crops elsewhere in containers.

3 ways to kick start a clay transformation

A fadge covers the compost at Edible Backyard
A compost pile is the best beginning!

Once the soil’s dried out (no puddles, no sound effects), sprinkle on gypsum to help break up the clay, then choose one of these 3 ways. Dont agonise, just go with the one that draws you, suits your budget and energy levels. They are all awesome.

  1. Cover the ground in cardboard and build a compost pile on top. This is my fav.
  2. If on top of grass, cover the ground in thick newspaper or cardboard and spread about 15cm of compost on top. If buying it in, work out how many cubic metres you need with this math – length of area x width of area x 150mm (or however high you are going).  Sprinkle on a dose of mineral fert. – mineral balance is your gardens strong foundation. 
  3. If on top of existing garden beds, spread a 1 or 2cm layer of compost and sprinkle on a dose of mineral fertiliser. If the beds are weedy, lay wet newspaper down first and let the weeds melt back. Only weed them out if they’re pernicious. If the soils compacted or slick clay, wield your forksta and aerate the ground first, before spreading the compost.

When using bought compost, my go to first crop is always a greencrop, to settle the compost in. Nothing calls out to microbes like a bunch of roots – sugar exchange a go! Then before planting out the first vegies, sprinkle on a full spectrum mineral fertiliser and saturate the soil in a biological brew.

In time, all you’ll need is a bit of compost. But for now, these extras are awesome to boost the transformation along.

Drive the transformation home

Buckwheat, meadowsweet, yarrow homemade mulch

The reason your clay holds water so, is that it is made up of very small particles. There’s no room for air down there – perfect for hanging onto water! By regularly adding organic matter, you’ll alter this tiny particle situation, joining them together bringing space and in this space along comes biology, and humus – the glue of life.

You’ll notice there is no digging in my list, nor the addition of sand. Go on top, and leave the rest of it to the soil life.

living soil looks like this edible backyard nz

Little and often, keep at it – and as your soil improves, you’ll need to add less and less. Stay in touch with your soil regularly to keep up with the changes – this habit is at the heart of being an awesome gardener.

Soon you’ll see a change in colour and an increase in earthworms, you’ll feel a loosening of the texture, smell a sweeter smell and for the brave, the soil will loose it’s tangy taste. And above ground, all your garden dreams will be coming true cos as above, so below. 

Take heart and keep on keepin’ on. Loam wasn’t built in a day!


  1. Absolutely loved this post Kath, thanks heaps! I’m not sure if I’m brave enough to taste soil yet but I’m tempted after your post!

    • Go on Sophie! You can do it! Just a little on the end of your tongue – don’t gulp it back, just taste it and spit it out. It’s amazing how good (oh lord am really putting myself out there now) good healthy soil tastes (the opposite is true as well :)) A bit of parsley afterwards is the thing…..

  2. We installed a drainage ditch with a couple of sumps a couple of years back. We still have boggy soil in winter, but it’s an improvement on standing water!
    Our plan (long-term, alas, due to energy constraints as much as anything), is loads of organic material and as many more trees as we can find room for.

  3. Brian Gibbons says

    Thanks for sharing your knowledge.
    One process we use for clay soils in production gardens is to mulch in the winter with crusher dust from the local quarry. Crusher dust is rock sifted down to 1 – 5mm.
    It is better if it is from granite or basalt as they have good mineral content as well.
    This also keeps the soils warmer in winter compare to an organic cover.

    Apply compost material and mulch after the soil warms up in mid spring.
    Do the same the following winter with the crusher dust.
    Gradually the crusher dust blends with the clay and compost to create different particle sizes in the soil to become a ‘friable’ soil.

    Other drainage processes are required when there is too much overland or underground water.

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