Compacted soil, what it does and how to fix it

Compacted soil, and what it is doing to your garden

If water sits in puddles on top of a bed instead of soaking in, and the plants in that bed never really get going no matter what you feed them, the problem is very often underneath rather than anything you have done above ground. Healthy soil is roughly half solid material and half pore space, and that pore space is where the air and the water live. Squeeze it out and the roots have nowhere to go, which is the whole of it.

How to tell

Push a screwdriver or a thin stake straight down into the bed when the soil is damp. In decent soil it will go in 20cm or more with hand pressure. If it stops dead after a few centimetres, that is your compacted layer, and you will usually find the roots of whatever is growing there running sideways along the top of it rather than down through it.

The other tell is water. Pour a bucket onto the bed and watch. Soil that drinks it in a minute is fine, and soil that holds it on the surface for several minutes has a problem.

What causes it

Weight on the ground, mostly, and weight on wet ground in particular. Walking the same line across a bed, parking a trailer on the lawn, a load of soil dropped and barrowed across the same route, or a builder who was there before you all do it. Rain and irrigation do their bit too, by washing fine particles down into the gaps and settling everything tighter over time.

Working soil while it is wet is the fastest way to cause it. If a handful squeezes into a sausage that holds its shape, leave it alone and come back in a few days.

Keeping it from happening

Stay off the beds. That is the single biggest one, and it is why beds are sized so you can reach the middle from the path. Stepping stones, a board to kneel on, or beds no wider than about 1.2m all do the same job.

Keep the surface covered. Bare soil takes the full force of rain and a hot sun, and it crusts over and seals. Mulch spreads the impact, keeps the surface open and feeds the worms that are doing the aerating for you.

Raised beds and no-dig methods avoid most of the problem entirely, because you build upwards instead of working the compacted layer at all.

Fixing soil that is already hard

Start with a garden fork rather than a rotary hoe. Push it in, lever it back a little to crack the soil open, pull it out and move along. You are not turning the soil over, you are opening it up, and that distinction matters because over-tilling smashes the structure and the soil sets harder than it started within a season.

Then get organic matter into it. Compost and aged manure forked into the top 15cm feed the worms and the soil life, and they are what turn a cracked-open layer into a genuinely open one that stays that way. This is slower than it sounds and it is the only fix that lasts.

Mulch on top afterwards, and keep off it while it recovers.

A word about gypsum, because it is not a general fix

Gypsum gets recommended for any hard clay and it only works on some of it. It helps sodic or dispersive clay, where the particles repel each other and the soil goes to slop when wet and sets like concrete when dry. On other clays it does very little.

There is a simple test. Drop a dry crumb of your soil into a glass of clean water and leave it overnight without stirring. If the water goes cloudy and the crumb spreads out into a haze, the clay is dispersive and gypsum will help. If the crumb stays sitting there and the water stays clear, save your money and spend it on compost instead.

Gypsum does not change soil pH, which is the other thing worth knowing, so it is not a substitute for lime and it will not fix an acid bed.

Cover crops

Deep-rooted crops sown over a fallow bed will break up a compacted layer for you and leave organic matter behind when you chop them down. It is slow, it costs almost nothing, and it works on a bed you are not going to use for a few months anyway. Cut them off at ground level before they set seed and leave the roots in the ground to rot, because the holes those roots leave are the point of the exercise.

Shop related products

• Brunnings Ultimate Dig It In Compost 30L. Forked through the top 15cm of a hard bed.
• Naturally Good Cow Manure 25L. Aged manure to feed the soil life doing the work.
• Naturally Good Mushroom Compost 25L. Another source of bulk organic matter.
• Martins Organic Sugar Cane Bale 11kg. Mulch to stop the surface crusting again.
• Eco Boost Liquid Compost Soil Rejuvenator. A liquid option where you cannot dig material in.

Did you know we keep gypsum in the shop in a 10kg bag and a 20kg bag, along with garden forks, and none of that is listed online. Give us a ring on (08) 8536 4333 to check what is on the shelf.

Bagged soil and compost are heavy, so if you are getting a few of them, our local delivery is $5 around Strathalbyn and $10 on the regional run, on goods bought from us. The rest of the range is in our garden centre.

Keep reading

• How to build a lasagne garden bed, layer by layer
• How to stop soil washing off your garden beds
• How to aerate a lawn, and when it is worth doing

I am the reason there is a worn track straight through the middle of one of my beds, and I have a stepping stone on the shopping list about once a year.

Candeece
Strathalbyn H Hardware