Three weeks ago I attended a workshop on soil health presented by the Center for Urban Horticulture at UC Davis. Very interesting new research was presented on what makes and keeps soils healthy and how plants react to being grown in healthy and unhealthy soils. Healthy soil provides anchorage for plant roots. You will have noticed that roots can be easily pulled out of sandy soil or that compacted or clay soils prevent root systems from spreading. Amending clay and sandy soils with large amounts of organic materials including compost and humus improves the soils’ structure by creating space between clay molecules or by filling the spaces between sand granules. Creating or filling large spaces between soil molecules also leaves room for air which prevents roots from drowning or rotting.
In healthy soils, nutrients are consistently available to plant roots so that fertilization can be reduced or even eliminated. Nutrient-rich organic materials and soil supplements that contain micro-organisms and beneficial bacteria and fungi can replace commercial fertilizers. Research is proving that when beneficial bacteria and fungi attach themselves to roots, they increase the roots’ capacity to draw up water and nutrients. Research is also showing that overfertilization, especially with high nitrogen fertilizers, actually kills off beneficial microorganisms.
When soil pH is above or below neutral (pH 7), roots are less able to draw up essential nutrients, especially nitrogen. The alkaline water and salts-laden soil here in our Valley have very high pH, 7.5 or even higher, commonly causing nitrogen deficiencies that show up as yellow leaves with green veining (most noticeable in gardenias) as well as zinc and manganese deficiencies. Correcting soil pH levels is a crucial step in creating healthy soil. We can lower our high pH levels by adding sulfur granules each time we amend our soil and each time we plant.
Rapid water absorption with little or no runoff and an increased ability to hold water for longer periods of time are characteristic of healthy soils. Water runs right through unamended sandy soil and forms puddles on top of compacted or clay soil. One experiment at UC Davis irrigated furrows in two plots-one of which has been organically farmed for twenty years and the other which has been conventionally farmed. Water in the furrows on the conventionally farmed plot ran rapidly to the end of the row without soaking into the soil. In the organically farmed plot which was regularly amended with compost, it took several flushes for the water to reach the end of the row because the healthy soil absorbed and held water as soon as it touched. We can reduce water usage in our garden by improving drainage and water retention in our soils.
Plants raised in healthy soils are more productive, need less water and fertilization, and are more resistant to disease and pest insects-requiring fewer applications of fungicides and pesticides. Conserving our planet’s resources and promoting natural means of pest and disease controls begins with creating healthy soils in our gardens.
Send Elinor Teague plant questions at firstname.lastname@example.org or email@example.com (“plants” in the subject line).