Less-than-ideal soil types, a harsh summer climate and alkaline water quality in the central San Joaquin Valley create adverse conditions for growing crops or maintaining lush, green gardens.
Our natural landscape is a hot, arid savanna plain. The Fresno/Clovis area receives on average only 11.6 inches of rainfall every year which is not enough to thoroughly soak the soil and wash away (leach) salts from the soil. Our soil and water are highly alkaline with pH levels well above neutral, 6.5 to 7. Soil and water samples often indicate pH levels of 8 and above. Soil type varies widely, with heavy clay and hardpan beds lying right next to sandy strips. There are few expanded areas of sandy loam soil, the ideal soil type.
Before planting, we should attempt to correct and amend soil conditions in our gardens. We have two full growing seasons here in the Valley and our next planting season begins in just a few weeks. It can take several years of biannually amending the soil as we plant each season to reduce soil pH levels, to improve soil structure and water-retention capacity, and to increase the amounts of beneficial fungi and bacteria in the soil.
High soil pH levels interfere with roots’ ability to draw up minerals and nutrients, especially iron. That’s why so many gardenias here show yellow leaves with green veining, a symptom of iron deficiency; without sufficient soil sulfur, the roots don’t absorb iron. We can’t change the pH levels in our water, but we can amend the soil with sulfur which lowers soil pH levels. Depending on plant size, a cup or two of sulfur granules added when planting will reduce alkalinity. Digging in a cup or two of sulfur granules twice a year around the root systems of established plants will quickly (two to three weeks) change yellow leaves to green. Sulfur granules are available at most garden centers and local nurseries.
Heavy clay soils can be lightened by adding gypsum (calcium sulfate). Gypsum helps prevent clay molecules from binding together and it also helps lower soil pH levels.
Plants grown in well-amended soils need much less fertilizer; well-amended soil absorbs and holds water much better than non-amended soil. Compost and humus are excellent soil amendments. They contain beneficial micro organisms and fungi that attach to plant roots, increasing the root surface area and greatly enhancing the capacity to draw up nutrients and water.
For best results, add 30 percent compost by volume to native soil in planting beds. Previous recommendations suggested laying down a two- to three-inch layer of compost or humus and then turning it in twice a year when planting. Adding a three- to four-inch layer would be better, but it can get expensive. When buying bagged compost or humus, try to find a garden center or nursery that restocks regularly. The beneficial micro organisms die in the bag or box after being stored for several weeks, especially in our summer heat. But you can always take a shovel full of well-amended soil from your garden or your generous neighbor’s garden and add it to the store-bought compost or humus where the transplanted beneficial micro organisms and fungi will quickly reproduce.
Send Elinor Teague plant questions at firstname.lastname@example.org.