Good clay soil has great structure!

What is good soil? Many gardeners would jump in and say loam, that elusive perfect mix of clay, silt and sand. In the real world, however, our urban soil is less than perfect. What can we do to make our soil sing?

Brampton Garden

Clay soil rules!

Since I am writing a blog, not a book, I need to limit myself to one type of soil. Today, I am focusing on clay soil, because, hey, I, garden in clay soil. But I am not alone. Over 52 percent of homeowners in North America also garden on clay soil. This one is for the clay gardeners!

 Clay soil characteristics

  •  Poor aeration and slow drainage
  • Difficult to till and warms up slowly in spring.
  • High capacity for holding water
  • High capacity for nutrients, however, most nutrients are “locked-up” and therefore unavailable for plant uptake.


Texture – the proportion of sand, silt and clay – is a fixed feature for soils. Trying to change our clay soil to loam by adding sand is an ineffective and expensive exercise. It takes adding an enormous amount of sand to clay soil to change it to another type of soil. Soil structure, on the other hand, is malleable. Good soil structure can compensate for less than ideal clay soil.

Good Soil Structure

  • Not only absorbs more rainfall, but rain also infiltrates the soil faster
  • Excess water drains quickly away
  • Roots and soil organisms grow and move through the soil with ease
  • Soil is easier to dig.


Good soil structure, when soil clumps together to form crumbs, ensures enough pore space regardless of texture.

Soils with good structure: have crumble, porous, fine, subangular and subrounded aggregates with no or little clods.

Soils with moderate structure: High rate on both course clods and crumble fine aggregates. Clods have few or no pores and are subangular or angular in shape.

Soils with poor structure: High proportion on coarse clods and very few fine aggregates. Clods have few or no pores and are subangular or angular in shape.

Improving soil structure

But how do we improve structure in clay soils?  The traditional answer is “add organic matter. “ But we need to do more than that if we want long-lasting results. We must test our soil.

Calcium:Magnesium ratio

Dr. William Albrecht, Michael Astera, Steve Solomon and Erica Reinheimer all suggest first checking the calcium (Ca) to magnesium (Mg) ratio. They recommend a base cation saturation ratio of 68% Ca to 12% Mg.

“Magnesium’s effect is amazingly powerful when the soil has a lot of clay in it, even ten percent clay and too much magnesium can make soil become rock hard and airless, even if it has had heaps of organic matter put into it,” says Steve Solomon & Erica Reinheimer, The Intelligent Gardener. Growing Nutrient Dense Food.

The relative amounts of calcium to magnesium in our clay soil “determines if the soil is open, airy and loose or if it is tight and airless. It determines if the clay portion of the soil clings tightly to itself or if it opens up and separates – flocculates is the technical term for this. The ratio of Ca to Mg has as much or more effect on the soil’s air supply as the level of organic matter does,” says Steve Solomon & Erica Reinheimer. The Intelligent Gardener. Growing Nutrient Dense Food.

Use dolomite lime only if soil tests show a huge deficiency of magnesium. Yearly supplementation with dolomite lime (CaMg(CO3)2  yielding 22% calcium and 11% magnesium, a 2:1 Ca to Mg ratio.) is a no-no in soil already sufficient in magnesium. It tightens up clay soil.

Instead use calcitic lime (CaCO3, calcium carbonate) rather than dolomite if soil tests show excessive magnesium. Calcite lime has a 6:1 calcium to magnesium ratio, which has 30% to 40% higher calcium content compared to dolomite.

Gypsum, (CaSO4, calcium sulfate), which used be used to break up clay, has come under much criticism. However, even Dr. Linda Chalker-Scott, The Horticultural Myth garden prof, says, “Gypsum effectively changes the structure and fertility of heavy clay soils, especially those that are heavily weathered or subject to intensive crop production.”

If your soil has more than enough calcium but too much magnesium, wait a couple of years. Excess magnesium will naturally leach out of the soil.

Organic matter

Once we have a balanced Ca:Mg ratio in our clay soil, then we can add organic matter. Adding organic matter increases the biological activity of the soil, which in turn improves clay soil structure and makes clay’s locked-up nutrients more available for plant uptake!

Soil organisms — mycorrhizae, earthworms, and other soil microorganisms –as well as plant roots exude the “glue” that sticks sand, silt and clay particles together. Organic matter also provides the environment where fungi and soil bacteria can feed and thrive, and in doing so, busily manufacture those stores of easily released nutrients for plant uptake.

We don’t need a ton of organic matter to make this work. If the soil tests for 7% organic matter on clay soil, it is doing very well. Over 10% organic matter may be excessive.

Clay soil makes good gardens. It is all in the structure.


Sources of organic matter


Plant residues and by-products

  • Lawn and garden clippings
  • Chopped up seaweed (if you live near the ocean)
  • Apple/grape pomace* (if you live near a cider factory/winery)
  • Wood shavings
  • Straw

Animal manure

  • Horse
  • Cow
  • Sheep
  • Chicken

Green manure

Nitrogen Fixers:  alfalfa, beans, clover, soybeans, vetch

Grass: barley, buckwheat, ryegrass, oats, wheat

*pomace: the skins, pulp, seeds, and stems of the fruit.

Written by Cristina da Silva
Wednesday, February 12, 2014 in Building Soil

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  1. Jeavonna Chapman says:

    Great information. Now I’m curious about the problems with gypsum. It’s my go-to instead of lime.

    • Thank you for your kind words, Jeavonna! This blog has a curious effect of having an ever increasing spiral effect! One answers spirals into more questions. That is why the blog is so long… 900 words!

      One issue:Gypsum is not a liming agent, It does not neutralize hydrogen ions or change soil pH like lime (calcium carbonate).

      Because gypsum is very soluble, too much added to soil quickly pollutes the waterways. Got to apply it carefully!

      Great pdf on gypsum’s use in Ag. Gypsum is very useful!

      Dr Linda Chalker-Scott has more about potential problems of gypsum, like using it on sandy soils! Here is the link: The myth of Gypsum Magic

  2. This is very authoritative advice.
    A tip. Crushed egg-shells don’t take the place of lime, but if you have them they all help. V slow release.

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