“Upland soils” on the Central Coast are composed of wind-deposited materials, i.e., the soils are “sand dunes.” In their native state, sand dunes are essentially lacking in the nutrients needed to sustain healthy plant life. Sand also tends to compact, and in the presence of alkaline conditions, the particles may cement together to form sandstone layers, which may be a few inches to several feet thick and be found a few inches to several feet below the soil surface. If you put these conditions together, you may have soils lacking in nutrients and which have very poor drainage.
I have recently been made aware of other factors, which exacerbate the already bad conditions described above. As you know, virtually all the new housing on the Central Coast is built on slabs. What this means is that the builder prepares the building site, pours a concrete slab and erects a house. To prepare the site for the slab, the soil must be made flat. This usually requires moving soil. The soil must also be compacted, so it will hopefully not expand or settle causing cracks to develop in the slab. Streets, water lines, sewage systems, and utilities must also be installed. BOTTOM LINE, there is much movement and mixing of soils by heavy equipment, resulting in two things of concern to the home gardener. First, not only is the slab site compacted, but the area that will be yard and landscape is also compacted. Second, the soils are thoroughly mixed and rearranged. The small amount of poor topsoil that was present may be mixed with even poorer subsoil or may be gone altogether, moved to another location in the leveling process. All the above may potentially leave the proud, new homeowner with yard and garden soil that resembles a sidewalk and is about as productive.
What to do? First, the gardener must begin rebuilding the topsoil. The surface eight to ten inches of soil should be loosened up by ripping, rototilling, spading, digging fork – whatever it takes to get the job done. While the loosening process is being done, organic matter should be incorporated into the soil. Wood chips, saw dust, leaves, straw, animal manures, hay, compost, silage, bark, peat moss, and/or bio-humus. These products will help keep the soil loose and aerated and help improve water and root penetration. Loosening eight to ten inches of the top layers of soil is a minimum for lawn establishment. For the garden area, and for tree and shrub sites, the soil needs to be loosened to eighteen to twenty-four inches. If the soil contains compacted or sandstone layers at deeper depths, “stove pipe” holes should be augered several feet deep at the planting site, and back-filled with pea gravel so water can drain. The gardener should also consider planting flower beds and other non-turf landscaping on berms, which are eight to ten inches above the general contour. Placing plants on berms is very useful for fruit trees, especially citrus and avocados, which require well aerated soil and good drainage to develop a healthy root system.
After all this effort, the gardener must continue to care for his/her plants by regular additions of fertilizer and organic matter. Watering practices must also be monitored to make sure the plants get adequate water but are not being flooded. “Tight” soils that are loosened and aerated may over time tighten up again, so the effort to keep these soils friendly to plant roots must be ongoing. Building productive, healthy soil is an ongoing, slow process. In some of our new subdivisions on the Central Coast the homeowner is essentially “starting from scratch!”