By Martha Proctor >> IJ correspondent

Good garden soil contains 30% to 50% sand, 30% to 50% silt, 20% to 30% clay and 5% to 10% organic water. The bacteria, fungi and worms in the soil produce substances that act like glue, binding all these different parts together to form groupings. These groupings determine the size of the soil pore space. In healthy soil, approximately half of the pore spaces are filled with water, and the other half are filled with air.

This environment promotes beneficial microorganisms and earthworms, which break down plant residue into organic matter, ultimately producing healthy soil. Healthy soil holds water and nutrients like a sponge, making them readily available for plant roots to take up. It also suppresses pests and weeds that may attack, sequesters carbon from the atmosphere and cleans the water that flows through it — which ends up in rivers, lakes and aquifers.

Good soil is coffee-colored, has a good smell, is rich in organic matter and can hold water but drains well. Poor soil can smell bad, is compacted and drains either too well or holds onto too much water. Soil that drains well is brightly colored, whereas soil with poor drainage and low oxygen content may have dull grayish tones.

Much of the ground in urban areas and around houses is lifeless dirt. Topsoil is typically removed during construction, and low-quality fill dirt is added to replace it. Dead dirt results from the removal of topsoil, the overuse of chemicals, other harmful practices and compaction.

Unhealthy soil induces poor plant performance and growth. Marin County gardeners must frequently amend their soil with mulch and organic matter to compensate for compaction, poor drainage, erosion, lack of air, too much or too little water, and pH issues.

Plants grow best when the air spaces between soil particles are maintained. When soil is compacted, the pore spaces inhabited by soil microbes collapse, limiting the movement of air, water and nutrients through the soil and, ultimately, compromising the growth and expansion of the plant’s roots. Avoid walking on or working wet soil to de

crease the likelihood of compaction. Soil should not be dug until it reaches 50 degrees and is sufficiently dry enough to crumble.

Compost, organic mulches, living plants and cover crops — a crop, usually a legume, planted in the off-season to add nutrients — are examples of organic matter that, when added to the soil, can improve pore space, drainage, available organic carbon, the population of earthworms and microorganisms, water-holding capacity, nutrient retention and plant root growth. Plants in well-drained soil are often less stressed and less susceptible to disease.

A good rule of thumb is to add compost to 25% of the planned depth of the plant’s roots, e.g., work 2 inches of organic matter into a depth of 8 inches. In late spring or early fall, spread 2 to 3 inches of mulch over the soil; avoid piling it up against tree trunks.

The soil’s pH is an important factor to check because it influences the availability of nutrients to plants growing in the soil. Most plants grow best in soil with a pH between 5.5 and 7.5, or slightly acidic to neutral, as this is the range in which all plant nutrients are most readily available. Soil microbes are most active in this range also. Lime can be added to the soil to raise pH or sulfur to lower it.

Add compost and topdress with mulch to promote healthy, productive soil in your garden. These amendments help provide the necessary oxygen and moisture to boost the number of microorganisms, beneficial fungi and earthworms, encourage nutrient recycling, improve soil texture and, ultimately, enhance fertility.

Correct soil deficiencies in your garden before you plant. Evaluating your soil and making adjustments prior to planting saves time and money and results in a healthier, more beautiful landscape. The health of your soil determines the health of your plants.

Sponsored by UC Cooperative Extension, the University of California Marin Master Gardeners provides science- and research-based information for Marin home gardeners. Email questions to helpdesk@marinmg.org. Attach photos for inquiries about plant pests or diseases. Please call 415-473-4910 to see when a master gardener will be at the office or drop off samples 24/7 in the sample box outside the office. To attend a gardening workshop or subscribe to Leaflet, a free quarterly e-newsletter, go to marinmg.ucanr.edu.