When soil gets compacted, water can pool on the surface and your plants can be suffocated. Ideally soil has air pockets to allow moisture, worms, roots and microbes to pass through. When soil is compacted, these air pockets get squeezed out and it’s harder for water and nutrients to reach down to the plant’s roots.
What causes it?
Soil compaction can be caused by heavy machinery, too much foot traffic, digging soil when it’s wet, or even over-working the soil. Clay soils are particularly prone.
01:07 How do I know I’ve got it?
A quick test is to push a steel rod or long screwdriver into moist soil – if you can’t push it very far you may have compacted soil. To confirm, dig a 30cm-deep hole and slice a layer off the side for a closer look. Looser soil separates easily, while compacted soil is smoother and more solid – and often has visible horizontal layers. You might even see plant roots growing horizontally along the lines, unable to break through.
How to fix it
The best fix is to avoid compaction in the first place, so avoid driving on lawns and make vegie beds no wider than two arms’ lengths across, so you can reach the middle from either side. If you can’t avoid having to step onto garden beds, then put in stepping stones to mark out where people should step. The soil will still compact a little, but not as much because the weight will be spread out, plus only that small area will be affected.
Another option is to create paths across a vegie bed with a plank of wood or some bricks.
To fix compacted soil, there are slow and fast versions, depending on how much effort you want to put in and whether you’re growing lawn or vegies.
02:06 1. Aerator: Whatever other method you use, this is a good pre-treatment. Rather than digging over the soil and risking compacting it more, using an aerator is a good way to instantly get air, water and any top dressings of gypsum and organic matter into your soil. Aerators remove a small plug of soil, but if you don’t want to buy another tool, go over the lawn or bed with a garden fork. Spring and autumn are good times to do this, but you can do it at any time of the year, so long as the soil isn’t waterlogged, frozen or bone dry.
02:40 2. Gypsum: If your soil is local clay (i.e. not introduced top soil) it can be useful to apply some gypsum, then water it in. The gypsum reacts with the clay and encourages the tiny clay molecules to clump together into larger particles that allow more air pockets to form. However, it’s a chemical reaction that will eventually wear off so long-term you need to get organic material into the soil as a more permanent way to aerate it and encourage microorganisms and worms.
3. Organic matter: There are different ways of introducing organic matter depending on whether you’re working on a garden bed or a lawn.
For a lawn, after aerating, many turf managers will top dress (i.e. spread) a layer of sand and cow manure over it; never add so much that you can’t see green. You can also spread worm castings or compost tea to encourage microbes and other wee beasties in the soil, which will help aerate it.
03:18 In a garden bed, the best way to introduce organic material without digging (which can damage compacted soil as well as being hard work), is to apply layers on top and let nature do the work for you. Simply pile your garden bed high (at least 15cm) with organic material – it can be wood chips, shredded paper, straw, chopped leaves, lawn clippings – whatever you can lay your hands on. Then leave it for up to a year! The material will decompose faster in hot, wet conditions, and take longer in colder or drier areas, but the material will eventually decompose and be drawn into the soil via worms, beetles and other organisms, creating lighter, more friable soil. This is an especially easy way to reactivate soil around a newly built home, where the soil has been disturbed, there’s been lots of heavy machinery, and you may even have been left with just clay subsoil. It might be slow, but the wait will give you time to get to know your land before you plant.
04:18 A slightly faster option is to cover the soil with material that is full of soil microbes, as these will speed up the process. This includes compost, worm castings, and manure – and less of the dense, harder materials such as wood chips, which take longer to decompose. Make sure to cover this ‘active’ compost etc with a layer of straw mulch to avoid it drying out as this will kill the useful microbes, which don’t like hot sun. Deep watering once a week will also keep things moving. After about a month you’ll notice a difference and, if you keep feeding your soil every spring and autumn, it will soon be full of life and all your plants will thrive.
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