Garden Soil in the Valley

Dirt is what you play in. "Wash your hands! They're dirty!" Soil is a living, breathing thing. The success of many plants depends on how you dig, turn, and amend (which means 'mix stuff in') the soil before you plant. Spring is a great time to start the process, even when the nights may be too cool for young seedlings to go in the ground.

Plants can grow entirely without soil: hydroponics is the practice of growing plants in a soilless medium, providing all of the nutrients in fertilizer solution. At the opposite end of the spectrum, organic gardeners build soil with lots of compost and add slow-acting organic fertilizers. Their motto: feed the soil, not the plants. Double digging and Square Foot Gardening are two techniques that rely on mixing large amounts of organic material deep into the soil. Though both give good results, it isn't necessary here--we live on some of the best ag soil in the world.

Good garden soil anchors the plant, and stores moisture and nutrients for the plant's roots to take up. It is a mix of tiny rock particles, compost, beneficial fungus and bacteria, worms and other living things. The living part of the soil is important. There are even good nematodes (microscopic 'worms') which attack harmful insects and other nematodes. Recent research has focused on the presence of beneficial mycorrhizal fungus which grow on the plant's roots and enhance the uptake of nutrients, and fertilizer manufacturers have added these to specialty plant foods.

Soils vary regionally and locally.

We live near a young, growing mountain range. The steady pounding of rain on steep mountains sends lots of little tiny rocks--sand, silt, and clay--tumbling down into this great river valley. The big rocks--boulders and cobbles--land in the foothills. The gravel ends up further down--hence the gravel industry near Woodland. The tiniest particles of silt and clay travel the furthest, eddying and settling into our neighborhoods. Those of you living along the creeks tend to have sandier soils, because sand is larger than silt or clay.

Decaying leaves and grasses provide natural organic material typical of soils in the midwest or eastern states. It takes 20- 30" of rainfall annually to support perennial grasslands, and 30 - 40" to support a forest. We only get 17" average here, so we have very little naturally occurring organic material (usually less than 10%).

What difference does organic stuff make? The smaller the soil particles, the slower you need to water. Water spreads out on clay soils, soaking in very slowly, so runoff and flooding are likely if it is applied too quickly. Chunks of bark, leaves, twigs, and compost are much bigger than clay or sand particles. One of the reasons we don't use much peat moss here is that it is an unusually fine organic material, so it provides fewer benefits than coarser products. The drainage of mineral soil can be greatly improved by the addition of organic material.

Know your neighborhood!

Our soils vary from moderately sandy in south and east Davis (Willowbank residents are entitled to gloat…), through good loam soils in the older central neighborhoods, to heavy clay soils in west and north Davis. The heaviest soils occur in Binning Tract, North Davis Meadows, and Stonegate. In some subdividions, soils were mixed with subsoil or large amounts of other soil was brought in, so there are odd drainage problems in Stonegate and Covell Park (north of Covell Blvd.).

There is no actual hardpan in this area, but years of plowing have packed a layer of clay a foot or so underground. This can really slow down the development of a young tree, although their roots will penetrate it eventually. Landscapers in the heavier soil areas sometimes drill through this 'plowpan' with a soil augur to improve drainage. Digging a wide planting hole is another option so the young roots establish more quickly.

You CAN make great garden soil.

Soils with a lot of clay can be disconcerting if you moved here from a rainier climate. They shrink when dry, and cracks form on the surface--conjuring images of the dust bowl. There may be plenty of moisture down an inch or so, but the top looks DRY. Dry clay is also hard as a rock, and wet clay is sticky and unworkable. So here's a hint: soak your soil a few days before you plan to try to turn it, and don't try to work your soil when it's wet.

The good news is that we can create good, workable soils to garden in. Clay plus organic material makes some of the richest soils in the world: they retain moisture and nutrients, so plants don't need to be watered or fertilized as often. You can also amend the lazy way: a thick mulch applied to the surface will be eaten and digested by earthworms, slowly improving the soil structure. Good soil comes from happy worms, and happy worms like mulch!

The purpose of turning stuff into the soil is to make the soil looser, improve drainage, help feed the plants, and keep moisture near the roots. It doesn't make much difference exactly what you turn in--mulch or compost products from decomposed sawdust, worm castings, manure, fine bark, lawn clippings, leaves, etc.--as long as it isn't too fresh.

Why? Things that are decomposing use nitrogen, so they should be left to stand and compost for a few months before you mix them into your garden. If in doubt, just add some extra fertilizer at the same time. At the other end of the spectrum, fresh manure can burn plants. Let it rot a while for best results. Though it needn't be elaborate, a good working compost pile is a boon to any gardener.

Not everything needs soil amendment.

Large woody plants such as trees and shrubs don't need to have anything added except a small amount of 'starter' fertilizer. So the common practice of rototilling compost into the entire yard is unnecessary where these will be planted. But an amended soil allows smaller roots to grow, helping young plants establish more quickly. It is beneficial for flowers, new lawns, vegetables, and plants with limited or fibrous root systems such as roses and young Citrus trees, and most shade plants.

Amending the soil enables us to provide special plants with the things they need.

Some plants need to have the soil acidity increased (i.e., they need the soil pH lower): Camellias, azaleas, Japanese maples, and blueberries are popular examples. Much of the problem is from the alkalinity of our water, not our soil. Adding products containing sulfur reduces the pH, and organic material helps the process. Other plants must have perfect drainage: Daphne odora, grown for the fragrant winter bloom, will succumb to fungus unless it is planted in very porous soil (contrary to popular belief, though, Daphne does not prefer acid soil--don't treat this like an azalea!).

The record warm temperatures this spring [March 2004] have people itching to plant! Although many flowers will be fine, the nights need to be consistently warm before we put summer vegetables in the ground. But you can build your soil now and be ready to go in April.

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