Get the dirt on your soil
by Nan Sterman
Do these phrases sound familiar when you're reading about plant descriptions: "prefers well-draining soil," "tolerates clay soils," "requires rich soil," "avoid compacted soils," or "adapted to basic soils?"
These terms sound confusing, and with 2,300 types of soil in California, it's no wonder! However, understanding the basics of soils can help you interpret how well a plant will do in your garden.
Heavy soils and clay soils are nearly synonymous. They have soil particles that pack closely, leaving little air space in between. As you irrigate, water fills those air spaces much like water fills holes in a sponge.
With heavy or clay soils, the water fills the holes slowly and stays a long time. Since plant roots need air as much as water, when you overwater, the air spaces are filled for too long and your plants can drown.
Well-draining soils are the opposite. They're comprised of large, irregular particles, often sand or decomposed granite. Water flows through quickly, washing down, past the root zone, taking valuable organic matter with it. The challenge here is keeping water and organics in the soil long enough for roots to get what they need.
California has both heavy and well-draining soils, often in the same garden. Test your soil's drainage by digging a hole 2 feet wide and 2 feet deep. Fill with water, let it drain, then fill again. Well-draining soils will empty within a few hours or overnight. With heavy and clay soils, water sits.
Wondering what kind of soil you have?
Have it soil tested at one of these California laboratories:
A and L Western Agricultural Laboratories, Modesto
Soil and Plant Laboratory, Inc., Anaheim
Rich soils are rare in our coastal cities. Instead, these soils tend to be in wet canyons and in valleys where organic matter washes down from surrounding mountains. Drive Highway 5 or 99 through the Central Valley to see deep, rich brown soils. Add water and you have, quite literally, the nation's breadbasket.
For coastal urban and suburban gardens, though, avoid plants that require rich soils.
Poor soils, or "lean" soils, are the opposite. Most of us garden in these nutrient-poor and often alkaline, or basic soils. Native plants evolved in those soils.
Plants from South Africa, Australia, the Mediterranean, and coastal Chile evolved in similarly poor soils, so they all make good garden plants for many Californians. Plants that require acidic soils do not.
Compacted soils are most often the byproduct of construction. Because we live in a geologically active (read "earthquake") region, soils beneath structures are compacted to stabilize them. Unfortunately, the surrounding land is usually compacted, too, in the process.
Add to that the cut-and-fill approach for carving developments from hillsides and you are left with hard, mineral subsoils that have no air spaces for plant roots, no organic matter to support beneficial soil microbes and no nutrients to keep the soil food web going.
If you inherit such hard, pale soil, layer it generously with organic mulch now and renew at least annually. Eventually, enough organic matter will incorporate into the soil to support plants. Until then, however, it will be a battle.
In fact, high quality, organic mulch improves every type of soil, so that plants not just survive, but thrive under even the most difficult soil conditions.
Nan Sterman is author of "California Gardener's Guide Volume II." She's a gardening expert, communicator and designer who has long grown an organic garden of plants that both feed her family and beautify her yard.