The Dirt
The Sandpiper, July 2005

Although the weather in Del Mar is about as close to perfect as one could desire, the soil needs a lot of help. The two main soil types in Del Mar are sand (large particles) and clay (fine particles). Although these soils have very different structural attributes, both are lacking in organic content (humus). Also, sand allows water to run through too quickly for nutrients to be absorbed; clay holds water without releasing it, thereby starving the plants for air. Both types benefit from amending before planting.

In an existing garden, it may be enough to dig a large hole, place the plant, and amend only the soil that is returned to the planting hole (the backfill).

This amended soil will provide the micronutrients and beneficial organisms that plants need for growth. At the same time, as the compost breaks down, the soil will become better aerated and less alkaline; the pH will drop to a more neutral range where nutrients can be metabolized more easily. Furthermore, the compost will increase the water-holding capacity of the soil. NOTE: this step should be skipped when planting California native plants; they are adapted to our native soil and will not grow as well in a nutrient-rich environment.

Next, I suggest using a pelletized time­released fertilizer such as Osmocote at planting time (in the backfill mix) and every April 1 and October 1 thereafter (on top of the soil). Fertilizer formulas contain three numbers, e.g., 10-10-10. The first number represents Nitrogen, which is needed for green growth. Accordingly, turf grass fertilizers usually have a high first number. The second and third numbers represent Phosphorus and Potassium. Both are necessary for flowers, fruits and strong root growth. For plantings other than turf, a balanced formula (all num­bers approximately equal) is the safest bet. These fertilizers depend on being dissolved and washed in by irrigation or rainwater. They are not effective with drip irrigation.

Finally, a 3"-4" layer of mulch around plantings will help to keep moisture in the soil by preventing evapotranspiration; it will also keep weed seeds from sprouting and keep the soil cool. As a bonus, when mulch slowly decom­poses, humus is added to the


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soil. Mulch should be kept a few inches away from plant stems to prevent rotting. I replenish the mulch under my plantings every fall. I prefer to use finely-shredded bark mulch because it decomposes relatively quickly, thereby emiching the soil. It also looks beautiful.

The first objective is to add organic material to the soil. In general, 25-30% of the existing soil should be replaced with compost. If the

soil has never been amended, as is the case in a new subdivision, the top 3-4" of existing soil should be removed. Compost is then spread over the entire area to be planted and tilled into the top foot of soil.

Agriservices, in Oceanside, and Miramar Wholesale Nursery, in San Diego, both offer wonderful, weed-free compost and mulches that are made from our recycled garden waste. These amendments are sold by the cubic yard and cost much less than the bagged varieties that are available at local nurseries.

This is a very abbreviated description of amending thesoil. For a more in-depth description, I recommend Pat Welsh's Southern California Gardening. Pat is a Del Mar gardener and garden writer; her excellent book is available in most San Diego bookstores.

In this article, I will discuss how you can improve your soil so it can support the plants you want to grow. First, however, my non-gardening husband tells me he needs a few definitions, so let's start with that. Compost is a mixture of wholly or partially decayed plant material. Mulch is a protective covering of plant material, usually bark, that is applied to the soil surface.