Gunky clay is no one's idea of wonderful garden soil. When wet it's heavy and slimy, and it dries into chunky slabs that crack into pieces. Like other extreme types of soil, tight clay can be radically improved with regular infusions of organic matter and thoughtful handling. I have been growing vegetables in clay soil all my life, and we have come to an understanding. I respect clay soil's needs and quirks, and it pays me back with a fun and fruitful garden.
Understanding Clay Soil
The most remarkable thing about clay is its density. Clay particles are tiny, many times smaller than sand grains, so wet clay feels slick rather than grainy between your fingers. When the tiny particles become coated with water, slight pressure can cause them to glue together. If you can easily shape a handful of moist soil into a ball that holds together nicely, you probably have clay. Another test is to mix soil with water in a jar and then look at how the sediment settles. Clay soils will settle into layers of fine sediment that feel like gloppy mud, and the water will take hours to clear.
Because of its fine texture, clay soil tends to pack down, which limits the amount of soil oxygen available to plant roots and soil microorganisms. Here mulches can help, because they act as shock absorbers during heavy rains, and host earthworms, which replenish air to tight soils with their constant tunneling activities. When growing vegetables in clay soil, mulches slow down natural compaction and promote natural soil ventilation, which in turn increases the comfort of plants' roots.
The footsteps of people and pets can compact clay soil, too, so it's important to improve the soil in permanent beds that are never walked upon. There is a catch, in that there is very little you can do in clay soil when it is wet – you must be patient and wait, because attempts to move wet clay always result in a sticky mess.
There are various tests for when clay soil is dry enough to work, and here's mine. Throw a spadeful of soil into the air and catch it with the spade. Gardening activities are on if it shatters, but not if it cracks into big chunks. When I must venture into a dripping garden to harvest veggies for dinner, I walk on broad boards placed in the pathways. The boards distribute my weight so even the pathways get a little protection from extreme compaction.
Good Things About Clay
Because of its density, clay soil does a good job of holding onto both moisture and nutrients. You can take advantage of the nutrient retention talents of clay soil by using slow-release mineral fertilizers such as rock phosphate and gypsum (calcium sulfate) to build soil fertility. Many gardeners think gypsum helps to loosen the tight texture of clay. Gypsum is not a substitute for organic matter, but it certainly works as an easy, sustained source of calcium.
Promoting ventilation is a constant goal when growing vegetables in clay soil, which is easily accomplished by mixing coarse forms of organic matter into the soil between plantings. Chunky types of organic matter like garden compost, weathered sawdust, or chopped leaves will enhance the structure of clay better and longer than very fine materials like ground peat moss or screened compost. When mixed into tight clay, large particles of organic matter become life rafts for microorganisms, which are major players in the transformation from compacted clay to fertile clay loam.
After organic matter has been added to clay soil for more than three seasons, you will see dramatic changes in the soil's texture, or tilth. It will dry faster after heavy rains, crack a little less in dry weather, and won't require as much digging to keep it aerated. Instead, you can start using a long-tined broadfork to restore air to the root zone when renovating planting beds. Clay soil is heavy, so using a broadfork is less work than digging and turning a bed. Once a bed is perforated with deep holes from a broadfork, compost and organic fertilizer spread over the surface can be raked into the holes.
Best Vegetables for Clay Soil
One of the best approaches to growing vegetables in clay soil is to stick with veggies that like clay during the first few seasons of soil improvement. Lettuce, chard, snap beans and other crops with shallow roots benefit from clay soil's ability to retain moisture, and broccoli, Brussels sprouts and cabbage often grow better in clay soil than looser loams because their roots enjoy firm anchorage. Mid and late season sweet corn are a good choice, too, but some of the best vegetables to grow in clay are squash and pumpkins. As long as they are grown in planting holes that have been generously enriched with compost, summer squash and small pumpkins seem to do well no matter where they are grown.
Or perhaps you should try rice. Clay soil is perfect for growing rice because it holds water. The world would quickly starve without it.
By Barbara Pleasant