Soil Care & Improvements

Soil is where your plants grow. It is not dirt! Dirt is what gets under your fingernails or on your child’s face.

The soil in which your plants grow provides all of the raw materials (except CO2) which your plants need for growth. If any of the raw materials (minerals, water and oxygen) are missing, your plants will not grow properly.

Our Bay Area soil contains a great percentage of clay which, in some areas, is denser than in others. Where the clay content is particularly high, gardeners will refer to their soil as adobe. Gardening books would refer to our soil as a heavy soil. It swells and is both sticky and slippery in the winter; and shrinks, cracks and can become almost rock hard in the summer. Consequently, clay soils can be difficult for gardening. On the other hand, our clay soil is among the richest soil in the world because its origin was ocean bottom containing all the minerals plants might need.

The gardener’s first concern is to put the soil in a more easily handled condition. A common error is to make a hole in the native soil using a post hole digger or similar setup and then drop a one gallon plant in the hole. (Some commercial landscapers have been known to do this). This doesn’t work because the soil in the container is formulated for optimum root growth and dropping the plant in a small adobe hole is almost the same as putting it into a clay pot where its roots cannot move out. Under such conditions, the plant dries out, wilts, and may die.

The gardener’s soil has to be made suitable for plants. The professional will refer to tilth, porosity, O.M. ratio and so forth. The home gardener needs to “loosen” the heavy clay soil. She/he needs to amend the soil by adding organic matter. When the soil is still moist but not sticky-wet and before it gets hard-dry, three to four inches of organic matter are spread over the surface of the soil to be amended. The organic matter could be steer or horse manure, sawdust, wood shavings, redwood or fir compost, rice hulls, or even dry hay or straw. Gypsum at the rate of 10 pounds per 100 square feet, and Sulphate of Ammonia at the rate of 2 pounds per 100 square feet is also spread over the area and then all plowed, dug or roto-tilled to a depth of eight inches. Horse and steer manures have practically no mineral content and almost no fertilizer value. However, they still contain undigested plant material which is an excellent source of organic matter.

Gypsum reacts chemically with the clay particles and pushes them apart making the soil more porous.

Sulphate of Ammonia provides Nitrogen to the soil bacteria enabling them to break down the cellulose in the plant material.

The organic matter works its way into the clay soil separating it into small globules, which increases the overall porosity of the mix. Porosity (air spaces) permits air to get to the roots of the plants and allows excess water to drain. The soil becomes more sponge-like. Each year another inch of organic matter is added to the soil to replace that which is lost through bacterial action.

When all the plants have been installed, the whole area is mulched with 2 to 4 inches of organic matter which will gradually in corporate itself into the soil. The mulch can be Forest Blend, Gold Rush, Bumper Crop, one of the products listed above, medium sized (1” – 1 1/2”) fir bark or even dry leaves or pine needles.

Mulching the soil has 4 main benefits: 1) It adds organic matter to mix into the clay soil, 2) it reduces evaporation and cuts down on watering, 3) it stabilizes soil temperature especially in the summer by preventing the soil from over-heating, and 4) it creates a more attractive landscape. When an entire area cannot be amended, the hole into which the plant is to be placed is amended. The hole is dug three times the diameter of the plant’s container and about one inch less deep than the height of the soil in the container. This native soil is then mixed with one-third its volume of organic matter such as Forest Blend, Gold Rush or Bumper Crop. The plant is removed from its container, placed in the hole and backfilled with your mixture (2/3 native soil plus 1/3 amendment). Never backfill with straight planter mix, organic amendment or potting soil. When finished, the root ball should be resting on a firm base and be just a bit higher than the surrounding soil. It has been planted in a transition zone of soil and its roots can eventually work their way into the native soil. (See our Care Guide on Planting Trees and Shrubs.)

Never add sand to clay soil as an amendment because the sand is heavier than the clay and will sink below the clay level and form two separate layers. Some of the sand will also bind to the clay forming a brick-like mass.

Neither should you add a layer of “top soil” over the native clay soil and then plan to do your gardening in the “top soil”. “Top soil” invariably has thousands of weed seeds per square meter which don’t sprout until you have planted your lawn or shrubs and then two or three months later see the weeds coming up throughout your landscaping. Additionally, the top soil forms a soft layer over the clay native soil and water does not move downward in a uniform manner. Water does not or cannot move freely between soils of two different textures.

If your soil needs amending, the only material that works is organic plant material and this must be mixed with the native soil. Gypsum is of some benefit but lasts only a year or so. Numerous products are advertised on television or in written format and promise to “cure” all kinds of soil problems. Most of these claims are nonsense and a waste of money. The only product guaranteed to improve the texture of clay soils is organic plant material; and it must be supplemented yearly.

A moisture meter is a good investment. We have seen plants wilt when gardeners assure us that adequate irrigation is taking place. What happens is that the native soil is moist but the root ball of the new plant is dry because the two soils are completely different and no water is moving from the ground into the root ball. Thus, it is critical that the soil be amended as described to create that “transition zone” for the new plant’s roots. Your moisture meter should test the water content in the native soil area and in the root ball of the plant.

If weeds have been a problem or “top soil” has been added to the mix, it is worth sprinkling a pre-emergent weed killer such as Concern Weed Prevention Plus over the area before the mulch is applied.

The second soil problem we have to contend with is soil alkalinity. All substances, (water, soil, coffee, grapefruit, apples, leaves, butter, and everything) is acidic or basic (alkaline) or neutral. Chemists have devised a number scale to measure acidity or alkalinity in a substance. The scale runs from 0 to 14 where zero is the most acidic which anything can be. Fourteen (14) is the most alkaline (basic) which anything can be. Seven (7) is neutral (neither acidic nor basic).

The number is always preceded by the letters pH. Anything with a pH less than 7 is acidic and those with a pH greater than 7 are alkaline. Going from one number to the next (pH7 to pH6) is a tenfold increase in acidity. If you went from pH7 to pH9 it would be a one hundredfold increase in alkalinity (10 x 10 = 100).

Most plants grow best at a pH between 6.5 and 7 (slightly acidic). Some plants such as Camellias and Azaleas grow best at a pH between 6.0 and 6.5. Blueberries need a soil pH between 5.0 and 5.5. Our Peninsula clay soils are alkaline (pH 7.5 to 8.0) and Hetch Hetchy water has a pH between 8.5 and 9 (very alkaline). Many of the minerals plants need for good growth (Iron, Phosphorus, Potassium, etc.) need to be in an acidic environment in order to dissolve. If minerals do not dissolve, plants cannot take them up. Even if there are adequate amounts of Iron, Phosphorus or Nitrogen in the soil, they will not dissolve if the soil is alkaline. One of the most effective ways to lower soil pH is by adding Sulfur or Iron sulfate. The yellow leaves of citrus trees are a good example of plants which may suffer from Iron or Nitrogen deficiency. When adding more Iron and Nitrogen doesn’t seem to help, it’s because the soil is not acidic enough to dissolve the Iron. Adding Iron sulfate makes the soil more acidic, which dissolves the Iron and greens up the plant.

Similarly, when selecting fertilizers for your garden, look at the back of the bag or box to be sure that Iron and Sulfur are included in the ingredients. All Master Nursery fertilizers are formulated for our soil and contain Iron and Sulfur plus other micronutrients.