Clay

Clay soil is common in the Bay Area, and it can feel like concrete on a hot day. In fact, clay particles have electrical charges that hold them tightly to their neighbors.

​Electron microscopic view of clay (WSU)

The science of clay

Clay is just one type of soil. Soil is made up of varying combinations of sand, silt, clay, air, water, minerals, microbes, earthworms, and more. All those ingredients are arranged into soil structures called aggregates, which contain solids and spaces. [This is different from soil texture, which refers to the percentage of sand, silt, and clay found in a sample.] The particles of various minerals found in soil are measured in micrometers (μm), or microns [one micron equals one one-millionth of a meter]:

• sand - unreactive quartz, shells, obsidian and other minerals; larger than 50 μm
• silt - boxy or spherical particles of quartz or feldspar; 2 to 50 μm
• clay - flat plates of feldspar that hold together using electrostatic forces; less than 2 μm

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[For my Burner readers, playa dust can be 0.3 μm, or three-tenths of one one-millionth of a meter.
That's why you will never get it out of your car or your tent.]

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Relative particle sizes

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The spaces between soil particles are called macropores and micropores. Macropores are greater than 0.08 mm and they hold air and water. Because the spaces are larger, water moves passively, pulled by gravity. Micropores are less than 0.08 mm and mostly hold air. Macropores are so small that the surface tension of water molecules means active suction must be used to pull the water out of these tiny spaces. Clay soil has far more micropores than macropores, so water and nutrients are held tightly, which means it is less porous.


Porosity, or permeability, refers to the ability of air and water to move through soil. Soil that is rich in organic material tends to have better porosity. Porous soil allows roots to find water and nutrients, and allows for healthy gas exchanges. Being flat, clay particles lie on top of each other like a deck of cards. This is why clay soil is so susceptible to compaction.


Clay and soil compaction

Compacted soil can create a barrier to roots seeking water, nutrients, and stability. It can even alter nitrogen, making it unavailable to plants (denitrification). This is especially true next to streets, driveways, buildings, and other heat islands. [If your unimproved clay soil ever feels spongey, it may indicate a masked chafer infestation.] On the other hand, leaching of nutrients is far less common in clay soil. More often, we end up with a super abundance of certain nutrients that creates an imbalance for our plants.


Clay soil and plant nutrients

Clay is made up of many negatively charged secondary minerals. That negative charge loves to attract and hold on to cations, or positively charged particles, such as potassium, zinc, and nitrogen. [That’s why most Bay Area soils have an abundance of potassium.] This attract-ability gives clay soil a high cation exchange capacity (CEC), which is a fancy ways of saying clay can hold onto 6 to 8 times more water and nutrients than sand. If you get a soil test (and I urge you to do so), you will also see a base saturation figure. To illustrate, CEC can be seen as the number of electrical outlets in your home, while base saturation is the number of those outlets currently being used. On average, sand has a CEC of 5-15, silt has a CEC of 8-30, and clay has a CEC of 25-50.


In the same volume of soil, clay has 100,000 times more surface area than sand, so there are plenty of places for attachments to occur. You can improve your clay soil’s base saturation by monitoring and correcting soil pH. Alkaline soils may need acidification, while acidic soils may need the addition of lime to bring the pH into a range suitable for plant growth (6.0 to 7.0). A proper pH can make important nutrients, such as iron, available to your plants.


Safety note: When planting trees around your home, keep in mind that root systems of plants growing in clay tend to be smaller, because so many more nutrients are available closer to the tree. This can result in a smaller in-ground support system for your tree, which makes it more likely to fall. Just sayin’…


Clay and drainage

When soil is extremely dry, it can’t absorb water because it becomes hydrophobic. Like a dry sponge, the water simply rolls off. Clay soil can act the same way. The rate at which water can enter soil is called its infiltration rate. Infiltration rates are given as millimeters of water absorbed per hour:

• sand - 30 mm/hr
• loam - 10 to 20 mm/hr
• clay - 1 to 5 mm/hr

Pooling water under nectarine tree indicates drainage problem in clay soil (Kate Russell)

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Because clay drains so slowly and can hold so much water, poor drainage can lead to fungal disease. This is particularly true for beans, stone fruits, and cucurbits. Proper irrigation of plants grown in clay soil means watering slowly and gently. Overwatering clay soil can drown your plants. Also, to avoid smooth dinnerware-like edges and compaction, avoid walking on or working wet clay soil or mud.


In areas prone to heavy rains, rain gardens can be used to offset the risk of flooding and fungal disease. Rain gardens divert excess water into sunken areas, away from buildings and vulnerable plants, filtering that water and allowing it to be absorbed slowly, without causing runoff or pollution.


Transplants and bare root trees

Because clay particles fit together so tightly, your soil can become as hard as a piece of pottery, making it difficult or impossible for transplants and bare root trees to get established. When installing stone fruit crops, such as nectarines, almonds, and apricots, you will need to be sure to rough up the edges of the planting hole to make it possible for new roots to work their way into the surrounding soil. In fact, roughing up these edges is a good idea for all your transplants.


Plants that must have good drainage to avoid fungal problems, such as crown rot, are often planted in mounds. This added elevation keeps moisture away from the crown. This practice is common when installing avocados, olives, squash, and melons.


Improving clay soil
Mulching and composting are the best ways to improve the structure of heavy clay soil. As the organic materials break down, they increase the number of spaces between the clay particles. This allows air, water, and roots to move through the soil more easily. It also improves drainage and permeability. Other ways to improve soil structure include aeration, sprinkling coffee grounds on the soil, incorporating perlite, and cutting plants off at ground level, rather than removing them, roots and all. Those roots play host to millions of soil microbes that help maintain healthy soil. As you work to improve your clay’s structure, you can still garden using raised beds, vertical gardens, keyhole gardens, and containers. Whatever you do, do not add sand.


The Sand-Clay Myth

Our intuition tells us that we can lighten heavy clay soil by adding sand. It sounds right. Sand has plenty of spaces, right? Putting the two together should give us a nice, happy medium, right? Wrong. Instead, the tiny clay particles fill in all the spaces around the sand grains, creating a soil that is even heavier than before!


Clay and plant choice

While dandelions' taproots seems able to penetrate concrete, many plants have a difficult time getting established (ecesis) in compacted clay soil. Some plant families, such as the sunflower family, need a regular top dressing of aged compost to perform well in clay soil. These plants include artichoke, lettuce, and tarragon. Other plant families, such as the parsley family, simply cannot thrive in clay and are better grown in containers. This group includes carrots, celery, parsnips, and fennel. The allium family of onions, garlic, and chives can be grown in clay soil, but the addition of organic material will help them thrive. The same is true for lilacs, and members of the mint family, such as lavender and salvia. Beets and Swiss chard prefer loose soil, but can be grown in amended clay soil.


Berries are shallow-rooted plants that really prefer loose soil. If you have clay soil and want to grow blackberries, raspberries, blueberries, or strawberries, you will want to work a lot of aged compost into the soil before planting.


Some plants are so rugged that they can help break up compacted clay soil. These plants include cilantro, cowpeas, and fava beans. Other plants, such as germander, yarrow, and sage, seem to barely notice hard-packed clay soil, as long as they get a little water during the peak of summer, which makes them excellent ground covers. Olive and plum trees, like many herbs, seem to thrive in less than ideal soil.


The “other” clay

The finely textured clay used to make porcelain, called kaolin clay, can be used to protect many crops from damage by insect pests such as codling moth, stinkbugs, cucumber beetles, squash bugs, cabbage loopers, aphids, cutworms, and armyworms, just to name a few. Apparently, insects don’t like getting kaolin clay on their feet, so they go elsewhere. Your watermelons, apples, and peaches will thank you!


Did you know that clay, sand, animal dung, and straw are used to plaster woven stick fences? It’s called pleaching!