Job number one in improving clay soil is to open up the pore structure to fix the drainage issues, so excess water can drain through, and air can penetrate deeper into the soil.
The tiny, pancake-shaped particles of clay soils pack tightly, leaving little pore space. Clay soils drain poorly, and the air spaces between particles are often flooded or anoxic. Clay soils don't breathe.
Most plants need air around their roots, not just water. Without air, roots sicken and die, while plant and soil pathogens thrive.
Making annual additions of good garden compost or composted manure is one way of improving clay soil—the slow way. It will take 4-5 years, but it will eventually turn a clay soil into a nice clay loam.
You can get to the same tilth in a year if you add coarse materials—like perlite, 5/16” lava rock, fine gravel, coarse sand, and wood chips—along with a large amount of compost–the first year.
Nitrate Depletion and High-Carbon Materials
Be cautious adding organic matter when improving clay soil. A Nitrate Depletion Period ensues whenever high-carbon materials are mixed into vegetable garden soil. Soil bacteria multiply to assimilate the new carbon food source, sucking up soil nitrates to fuel the growth of their populations. This deprives your plants of the nitrogen they need to grow, and lasts for a few days to a few weeks, depending on temperature, soil moisture, and how much carbon was added.
Eventually, the bacteria use up the added carbon and start dying off. The nitrogen bound up in their bodies is then released back into the soil, where plants can use it. But the damage may already be done if your vegetables are stunted from lack of nitrogen at a critical time in their growth.
Warning: Do NOT add coarse materials to clay soil without adding garden compost, composted manure, or other organic matter. If you add sand or fine gravel to clay without adding organic matter, you get concrete when it dries out.
I used to make a “chinking mix”, a soil mix to fill in around boulders and between rocks. It had to drain quickly, but retain some water for grasses, herbs, and other plants around the rocks.
The best mix I came up with used equal parts coarse sand, small horticultural lava rock, coir (rehydrated coconut husk fiber), and compost, and it worked great.
It drained quickly, and the grasses and herbs I planted rooted through it into the clay below, and thrived. But because I laboriously mixed it one batch at a time in the wheelbarrow, I only used it around rock clusters, and not for improving clay soil. Then I found out that the soil yard where I got some of my raw materials made their own, similar mix.
Except they did it with bulldozers, rolling, turning, and mixing small hills, instead of loading it in bags, and mixing it one wheelbarrow load at a time.
The compost they used wasn’t nearly as good as mine, but as a structural soil amendment for improving clay soil, it had HUGE bang for the buck. I used it when I put in my first tomato bed at my community farm plot, adding a 2” (5cm) layer to the lower layer soil amendments, and a 2” (5cm) layer to the top.
I topped it off with a 3” (7cm) layer of my own compost, mixed it all together, and the next year, when I deep-dug the bed, it had the same loaf-like structure and tilth as a 5-year-old bed in my last garden.
Is Coir Necessary? No, the mix will work with other organic soil amendments instead of coir. Just be careful adding fine-textured, high-Carbon materials (like sawdust)—see Nitrate Depletion Sidebar. Coir has many advantages for improving clay soil, though:
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