Many gardeners look at clay soil gardening and decide that they’d much rather garden on more open, organic soils.
Then they try to figure out how to do this. 🙂
Or, they accept the clay and learn how to garden with what they have.
So, here’s the current wisdom on changing soils from clay to more open, well-drained soils.
Soil Particles and What You’re Trying To Do
There are three basic sizes of soil particles with sand being the largest and clay being the smallest. Clay soils have a high proportion of clay particles. Some garden writers recommend adding sand to clay to change the structure. This can work if you add only a very small bit every year and thoroughly dig/mix the sand and clay together. I’m told that no more than one quarter inch of sand spread equally over the area is the upper limit on a yearly basis if you thoroughly mix the two.
What most often happens though is that gardeners tend to ignore this and add too much sand. Then what happens is there’s too much sand to mix and you wind up with pockets of clay and pockets of sand and one unholy mess. You have neither clay soil, open soil or some hybrid. You have poorly draining sandy soil. If you thought clay soil gardening was a problem – wait until you try gardening in this stuff.
#1 Soil Replacement
Some gardeners simply dig up and replace all the soil to a depth of eighteen inches. This works as long as the slope of the underlying soil is away from the garden area so when the water percolates down to the underlying clay, it continues running away from the garden.
If you’re in a bowl or the underlying slope is the wrong way (towards the house and not away from the house) you can create a separate problem (do you want a poorly drained garden or a poorly drained basement?). This is the big-money method of eliminating clay soil gardening.
#2 Digging In
Some gardeners dig in copious amounts of organic matter every year. The theory here is that the organic matter will separate the clay particles and increase the drainage. This works well although the soil is only as well-drained as the depth of your digging. Double digging or trench digging will work quite well.
Some gardeners swear by adding gypsum to the soil. This works by inserting itself between the clay particles (aggregation actually).
The recommendation is to add 20 to 30 pounds per 100 square feet to establish a new garden and a yearly top up thereafter of 50 pounds per 1000 square feet.
Gypsum does not change the pH of the soil so it is safe to use around the acid-lovers like Rhododendrons. Gypsum is a “hydrated” product and you may have trouble finding it in garden centers.
Go to a home building store and purchase “pure” builders plaster. Make sure it has nothing in it other than plaster (some contain cement). It is not hydrated but it will work just the same.
Note that simply “spreading” the gypsum isn’t going to do anything. You have to dig this stuff into the soil so it will mix with the clay particles.
And do note it doesn’t work on clay soils that are limestone based
Ah, digging. Here’s that word again – and you can’t get away from it in clay soil gardening.
The addition of compost will increase the bacterial count of the soil. Bacteria are on reason that soil “aggregates” or forms little clumps. The more bacteria, the more little clumps and the better the drainage is. This is another way of saying that you’ll get better drainage if you use compost regularly.
None of these recommendations comes without a price. They all demand some intervention on the part of the gardener and most of that is called “digging”. You’ll find it you combine two or more of these steps, your soil will improve much more. Dig deeply, add gypsum, a little sand and lots of compost to establish new beds. Maintain the beds by a yearly top dressing (and cultivate it in) of gypsum and compost. Mulch to maintain the organic matter level and spray with compost tea monthly.
Excavate the entire garden to a depth of 24 inches, regrade, install drainage tiles and refill with excellent soil.
Sell the property, find a house where some other gardening fool has done the above to eliminate all problems with clay soil. 😉
If you’re interested in how gypsum works, here’s a link to the Missouri turf management website that summarizes it nicely.