Planting time for cool-season vegetables and flowers begins in mid-September here in the central San Joaquin Valley. Soil preparation should begin two to three weeks ahead of planting time to allow amended and tilled soil to settle a bit and to allow any excess nitrogen from urea in manure-based amendments to dissipate. Pull out non-productive summer vegetables and heat-stressed summer flowering annuals now to make room for cool-season leafy greens, root vegetables and spring-blooming annuals. (But wait until temperatures are a little cooler in early October to plant heat-hating pansies.)
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Even though you may have turned in a bag or two of compost or humus or a product labeled as a soil amendment into your planting beds last spring, you’’ll want to repeat that process biannually for both the fall and spring planting seasons. And plan on increasing the amounts of amendments you add.
Amending any type of soil improves water retention and drainage, can lower or raise pH levels, improves root development and, when organic matter is added as an amendment, provides sources of macro-, micro- and trace nutrients that reduce the amounts of fertilizer needed for vigorous plant growth and flower production. Well-amended soil is friable, meaning that slightly moist soil will hold shape when lightly squeezed in your hand and then crumble apart easily when poked with your finger.
Here are a few guidelines for amending your soil taken from a September 2016 lecture I attended given by Richard Evans, director of Plant Sciences at UC Davis, for the California Center for Urban Horticulture. The complete transcripts of “The Latest Dirt: Research-Based Innovations in Soil Health” are available on the CCUH website, ccuh.ucdavis.edu.
Compost is still decomposing and provides a great source of living micro-organisms and beneficial fungi; humus has finished the decomposition process and contains far fewer living, active micro organisms and fungi, but humus lightens and restructures soil composition thereby improving drainage and water retention.
Evans recommended adding 30 percent organic matter by volume to amend the soil. That percentage translates to 5.6 cubic yards per 1,000 square feet if you’re tilling to a 6-inch depth or 8.3 cubic yards per 1,000 square feet to a depth of 8 inches. That’s a lot of compost or humus but the results are well worth the effort and cost.
Our soil and water is highly alkaline with high pH levels and it’s also full of salts – caused by our low rainfall amounts. To lower soil pH by 1 unit, Evans advised adding 20-40 pounds of sulfur per 100 square feet. To lower high salts, he recommended adding 20 pounds of gypsum (calcium sulfate) per 100 square feet and thoroughly soaking it. Gypsum also helps break up heavy clay soil. Those amounts may seem a little daunting to the average home gardener without a rototiller. Adding a cup or two of sulfur granules per bush or tree when planting or spreading a 2-inch thick layer of gypsum over planting beds each time you turn the soil is a lot easier and will eventually result in lower pH levels and salts content.