Marin Master Gardeners
University of California
Marin Master Gardeners

How to build healthy soil

The dirt on your soil

Soil, the loose upper six to eight inches of earth, is teeming with microorganisms (animal), the residue of live and dead plants (vegetable) and the decomposed rock from which the soil originated (mineral). Soil supplies plant roots with a source of air, water and nutrients plus insulates them from extreme changes in temperature. Topsoil is the layer most influenced by climate and most enriched by the addition of organic matter. It is the cornerstone of a healthy garden. 

Here are the key components of healthy soil and the steps you can take to get there.

Soil texture and structure: working toward loam

Soil texture is grouped into three categories: coarse (sandy soils), medium (loamy soils) and fine (clay soils). Very coarse, sandy soil dries out rapidly and is difficult to maintain at a high fertility level. It feels gritty, doesn't form clods, and can appear somewhat dusty when dry. Clay soil, which is common in Marin County, retains more water, has slower air and water movement and holds more mineral nutrients than sandy soils. Clay soil feels smooth between the fingers and is sticky when wet. Clay soils amended with organic matter are some of the most productive farmland soils on earth.

Loamy soil has roughly equal proportions of sand, silt and clay. It allows roots to penetrate easily and it drains well.  The majority of plants -- and gardeners -- enjoy the benefits of loamy soil.  

Good-quality topsoil has a crumb-like structure. The tiny pores between soil particles allow water from the soil surface to infiltrate so water is retained but excess amounts drain down to allow space for air circulation.

Understanding soil nutrients and pH

Ideal soil contains sufficient quantities of approximately 20 essential plant nutrients. Carbon, hydrogen and oxygen are drawn from air and soil water; the other nutrients are dissolved in the soil water and absorbed by plant roots. Nitrogen (N), phosphorus (P) and potassium (K) are the elements plants need in the largest quantities.

Soil pH is a measure of the soil's acidity or alkalinity. Soil pH affects the solubility of soil minerals, the availability of nutrients to plants, and the activity of microorganisms. The midpoint of the (1 to 14 point) pH scale (7.0) is neutral. Numbers below 7 are increasingly acidic as the number decreases; numbers above 7, increasingly alkaline. Most plants prefer a pH ranging from 5.5 to 7.5 or slightly acidic to neutral, the range in which all plant nutrients are most readily available. Soil microbes are most active in this range also.

Gardener's to do list for great soil

Test. The most accurate way to determine the nutrient content, pH, structure and texture of soil is to send a sample of your soil for analysis to a soil-testing laboratory. Contact the Master Gardener Help Desk at 473-4204 for more information. Do-it-yourself soil testing kits are also available at garden centers but do not produce as detailed information.

Tread lightly. Many gardeners, anxious to get into their gardens in early spring, work their soil when it is still too wet. Soil should not be dug until it is sufficiently dry enough to crumble when worked and has reached a temperature of 50 degrees. Soils high in clay content are easily damaged if worked when wet. Foot traffic and heavy equipment crush the soil's pores, which limits plant's roots access to nutrients, air and water.

Amend. Amend soil with compost. A good rule of thumb when incorporating compost as a soil amendment is 25 percent of the planned depth, i.e., two inches of organic matter worked in to a depth of eight inches. Animal manure is another good soil amendment. Follow application instructions posted on the bag. Fresh cattle manure should be applied in fall or winter. Work the manure in right after spreading as this helps decompose the manure and lessen odors.

Mulch. After amending with compost, add a layer of approximately three inches of mulch every year. Mulching reduces moisture evaporation, suppresses weeds, moderates soil temperature and helps prevent soil compaction. In addition, mulches decompose slowly thereby adding organic matter over a longer period. Use bark in perennial beds if possible, since it decomposes more slowly.

Weed. Weeds crowd plants, compete for nutrients and moisture and often harbor pests. Eradicate as needed.

The health of your soil determines the health of your plants. The mantra of Master Gardeners -- "compost, compost, compost, mulch, mulch, mulch" -- still rings true. The best way to amend your soil's structure and to provide oxygen, moisture and nutrients is to add moderate amounts of compost and top dress with mulch. Begin by testing your soil for pH and nutrient content. Follow these recommendations and with time, your garden soil and the plants you grow will reward you with beauty and bounty.

Contributors: Marie Narlock, Martha Proctor

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