This week we will be tackling one of the most common frustrations of gardening in our area… CLAY!
Q: My yard has clay in the soil and even though we have turned it over many, many times and have added bagged soil to it, the ground still hardens up. What can I do to improve the soil for my plants? – Carrie B.
Unfortunately for us and our gardens, the native soil in most of the SF Bay peninsula regions (bordering the bay) is very fine, bay-mud clay!
Proportionally, most ideal garden soils are made up of only a very small part clay. Good garden soil is mostly sand, silt, and organic material and only about 10 to 20% clay.
Continual “amendment” of your clay –that is, the addition of about 6 inches of organic materials (like composted Redwood Chips, Organic materials, etc.) tilled or turned into the top 8 inches of clay, will likely have to be done every spring for at least 3 years until you will begin to notice an improvement in the condition of your soil from year to year. You do not have to evenly distribute the organic material into the clay –in fact, most experts caution against that. You can use a motorized tiller the first year and turn the soil with a spading fork thereafter.
Another very important step that you can take is to keep a thick (3 to 4 inch) layer of mulch on the surface of your planting bed. The mulch aids in amending the soil by providing organic material for the worms and other insects to incorporate down into the soil profile. The insects and the organic material can help to change the texture of the soil –making even a clay-based soil drain better- by making it crumbly (friable), and allowing air into the root zone. The mulch also prevents the rain and irrigation water from pelting the surface of the soil and further compacting the surface layer. This layer of mulch also keeps the moisture in the soil, prevents weed seeds from germinating and makes your soil “healthy” by giving it the beneficial microorganisms that are so vital for plant nutrition and competition against plant pathogens.
A side note: If you are attempting to grow plants that need “sharp” drainage (very fast draining soil) it will probably be necessary to plant them in a raised bed –with your root zone elevated above the natural grade.