Clay soil is a frustrating growing medium. It has an extremely fine texture, which allows it to hold onto nutrients that leach through other soils, and it can also hold a great deal of water. It’s fairly resistant to erosion, and the pH rarely fluctuates.
And yet, the only thing that seems to grow well in clay is dandelions and burs.
So, what causes plants to die in clay soil? Plants struggle in clay soil because they lack one crucial ingredient: oxygen. The fine soil particles of clay make it impossible for oxygen to penetrate, which affects multiple processes necessary for healthy plant growth.
Luckily, improving clay soil is simple, albeit time-consuming. But first, you need to be able to recognize exactly what is causing your specific plants to have an issue.
Wrong Plant, Wrong Place
The most common reason for plants dying in clay soil is that they violate the right plant, right place rule.
Many nurseries base their warranties on the right plant, right place rule. Basically, they only cover plants that die if it was the right plant for the space and the right space for the plant.
If you are planting flowers and shrubs that need well-drained soil, or have very fine root systems, they will not perform well. In wet climates, clay soils hold on to too much water. In dry climates, clay soils won’t allow water to penetrate.
Both conditions make it difficult for plants that need consistent moisture but also good drainage. Also, plants with dense, fine root systems will suffer in clay because if it is too dry, they will not be able to grow down aggressively to water. And if it’s too moist, they will quickly succumb to root rot.
Before you plant in clay, make sure you are planting the right plant in the right place.
Clay soils tend to be extremely stable, so it takes a great deal of work to change or manipulate the pH value. This is good news if your soil is in the sweet spot (~6.5), but not so great if your soil is too high or too low.
The poor man’s test for usable clay soil is checking if it’s easy to till. If it’s sticky and difficult to work, your pH is off.
A soil’s pH value will determine how much of each nutrient is available to plants. Cation exchange capacity (CEC) determines how well a soil holds on to nutrients, and how freely plants can steal them away by absorbing them.
When gardeners talk about available nutrients, they’re talking about how much of the total vitamin or mineral that is present in the soil is free to be used by plants.
At a neutral pH (7), most vitamins and minerals are available in healthy amounts for most plants. As the pH drops, macronutrients become less available, and micronutrients become too available. This can result in nitrogen or phosphorus deficiencies, or iron and copper toxicities.
As the pH rises, most macro-nutrients become freely available, with the exception of nitrogen. However, the micronutrients quickly become tied up and only trace amounts are free to be absorbed. This results in deficiencies that are more difficult to diagnose, but just as harmful if not treated.
It’s important to test your soil’s pH before using any fertilizers or amendments. If a plant is showing signs of iron deficiency, and the soil is alkaline, an iron amendment will only add more unavailable iron to the soil.
The problem is not that iron isn’t present, it’s that it’s not free to use. Lowering the pH will make all trace minerals more available and will treat deficiencies.
Nutrient deficiencies/toxicities can have a variety of symptoms, but they also act differently than over or under-watering.
Common symptoms of nutrient deficiencies and toxicities include:
- Yellow leaves with green veins
- Light green veins with deep green leaves
- Crinkled leaves
- Blossom drop
- Fruit drop
- Mottled leaves
- Stunted growth
If you notice these symptoms, do a soil test to determine the pH value. Adjust to read 6.5, and then wait to see if the plants respond. If they still show symptoms, test again and amend for the lacking nutrients.
Clay soil is made up of very fine particles, which makes it incredibly difficult for them to absorb water in dry climates, and incredibly difficult for them to drain water in wet climates.
Over time, the deeper roots will wither and die, while concentrating new root growth in the top few inches of the soil. This will leave the plant constantly stressed for water and depleted of nutrients.
If you live in a wet climate, chances are your clay soil will hold too much water. This will lead to a soil that doesn’t allow oxygen to permeate, which will prevent roots from being able to breathe (source).
Over time, the deeper roots will rot, and new roots will form in the top few inches of soil. This will leave the plants unable to hold themselves firmly in wet soil, while dead and dying roots attract pests and rot.
Dry climates cause fewer problems than wet climates because it is much easier to add water to soil than it is to take it out. Regardless of your climate, adding compost will help your soil both hold more moisture and drain easier.
Never add sand in an effort to help with drainage. Sand plus clay equals concrete, and it’s impossible to fix.
Drip irrigation is best for clay soils. It allows you to thoroughly soak a targeted area while leaving the majority of the surface area dry and able to absorb some oxygen. This, combined with a compost topdressing and mulch, should help alleviate some of the common problems with clay soil.
Plants That Thrive in Clay Soil
Unfortunately, most of the plants that thrive in clay are weeds. However, there are some tried-and-true landscape plants that will perform well in poor soils, and they are available at most nurseries.
For clay soils in a dry climate, try these:
- Black-eyed Susan
- False Sunflower
- Big Bluestem
- Butterfly Weed
- Compass Plant
- Indian Grass
- Sea Holly
- Perennial Sunflower
- Balloon Flower
- Russian Sage
For clay soils in a wet climate, try these:
- Japanese Painted Fern
- Ghost Fern
- Ostrich Fern
- Heartleaf Brunnera
- Bee Balm
- Goat’s Beard
- Blue Star
- Cup Plant
- New York Ironweed
- Sweet Flag
Many vegetables can be grown in clay as long as the soil is prepared and maintained correctly.
To manage clay soil for vegetable gardens, do the following:
- Till the soil when it is moist and crumbly. Too wet or too dry will cause compaction.
- Create beds no wider than 4’
- Do NOT walk on tilled soil. This will cause compaction.
- Plant a variety of deep-rooted and shallow-rooted plants in each bed. This will prevent hardpans from forming.
- Mulch beds to retain moisture and add organic matter to the soil. This will also encourage earthworms.
Water at low pressure for longer periods of time. This will allow water to saturate the soil.
Till every 2-3 years. This will keep the clay from becoming overworked and blocky.
Clay soil can be difficult to manage, but it is one of the best foundations for a gardener who is willing to put the time and effort into making improvements. Compost is the most crucial ingredient in unlocking the potential of clay soil.
Visit our other articles for more information on building a compost pile, using compost, or adding compost straight into your clay soil.