By Nan Sterman
Soil is one of the most challenging aspects of Southern California gardening. We have myriad soil types, in part resulting from the region’s historic seismic activity and eons-long erosion. We garden in soils that include clay, sand, cobble mixed with clay, and decomposed granite, to name just a few.
Unlike soils in other regions, ours have very little organic matter. Since rainfall is limited, native plant leaves tend to be small, leathery, even needlelike. They just don’t have large-leaf biomass that falls on the ground, creating a thick layer of natural compost that builds soil.
As a result our soils tend to be basic rather than acidic. Our clay soils drain very slowly, making it easy for gardeners to overwater and drown plants.
To further complicate matters, if you live in a subdivision, chances are you garden in subsoil. Subsoil results from the cut-and-fill process commonly used to create building pads. The surface soil gets scraped away, leaving sterile, deep-layer soils lacking organic matter, beneficial insects, and beneficial soil microbes. Developers write homeowner association rules requiring homeowners to plant gardens within months of moving in. Yet the soils are completely unsuited to supporting the plants that go into the ground.
So, what’s a gardener to do? First test your soil’s drainage. The kind of soil you have is less important than how fast or slow it drains.
- Dig a hole 2 feet wide by 2 feet deep, or as close to that size as you can get.
- Fill the hole with water and let it drain.
- Fill it again and note how long it takes for the water to completely drain out. If it drains in a few hours or overnight, that’s fairly well-draining soil. If the water stands for more than a day, that’s heavy soil.
Choose plants accordingly. With a little research you can find out which plants prefer well-draining soils and which tolerate clay or other heavy soils. Plants native to your community are always the best adapted, followed by plants native to your region of California, then plants native to other Mediterranean climate regions. So, for example, California lilac, rosemary, agave, and Spanish lavender (shown) tend to do very well in Southern California gardens and require no soil amendments at planting.
If you move into a brand-new house, carpet the soil with thick layers of compost and mulch before you plant. If you can wait a year to plant, so much the better. In that time your soil will be colonized with the tiny web of life that supports plant growth.
To grow acid-loving plants and those that prefer rich soils, such as azaleas, blueberries, and hydrangeas, your best bet is to grow them in containers. Use an acidic potting soil, acidic fertilizers, and water them amply; those are thirsty plants.
Vegetables do best in raised beds filled with enriched soil and amended with worm castings and compost. Water generously and don’t forget to fertilize!