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Soil Improvement Information Series

Building Soils With Compost and Soil Amendments

Compost is a natural organic material produced when microorganisms break down organic residue. This process occurs continually in nature, resulting in a sweet, earthy smelling brown material called compost. Compost adds food for many organisms and an enormous diversity of organisms to your garden soil when you use it as a soil amendment. It is a richsource of organic matter. Although compost contains plant nutrients, it is ypically characterized as a soil amendment rather than as a fertilizer because most of the nutrients are not readily available and only become available slowly, over many years.

There are several ways to acquire and use compost to benefit the soil and plants in your own yard and garden:

The best way to improve the soil is to mix in plenty of compost or other organic matter before planting. Thoroughly mixing these materials deep into the soil helps retain water and air, and provides nutrients to the plant roots.

When: Mix in organic matter with existing soil before planting perennials, lawns, trees and shrubs, each time annual beds are replanted, and when dividing perennials or repotting container plants.

How: Use a shovel or digging fork to mix amendments into the top 6 to 12 inches of soil. Amend large planting areas not just small holes for each plant.

How Much: Amount of compost recommended to be tilled into each 100 square feet of planting area. Use these following guidelines.

Soil Testing: For a list of local laboratories who can test the amount of sand, silt and clay in your soil call your County Extension Service.

What: Different types of organic amendments may provide special benefits for certain plants or soil types, as the chart below shows. Any clean organic amendment will improve the soil. The best advice is to use what is reasonably priced, plentiful and easy to get. Use as Mulch for Vegetable and Perennial Gardens - Mulch means placing compost (or another material) on the soil’s surface around a plant. Mulching with compost is especially beneficial because earthworms and other soil life move through the mulch and help carry these materials down into the soil. Placing mulch around plants also helps to limit weed growth, reduce evaporation, moderate fluctuations in soil temperature, and reduce soil erosion.

Use to Top-dress and Renovate your Lawn - Spreading a thin layer of compost on your lawn can do wonders for the soil underneath. This technique works best if you first aerate the lawn.

In the spring or fall when the soil is moist, use an aerator tool (you can rent one) to remove plugs of soil and thatch from your lawn.

Next, using a spreader tool or a rake, spread a thin layer (about ½ ") of compost over the lawn. The grass should be standing up through the compost after application, not bent over or buried.

You can also mix grass seed in with the compost to encourage new growth and fill in bare spots where weeds might take hold.

Consider a Retrofit. If the soil under your lawn is unhealthy, one of the best actions you can take is to retrofit your lawn. Grass thrives in 8-12" of healthy soil. In addition, seed-grown lawns are more sustainable in healthy soil than sod-grown lawns because roots grow directly into the soil rather than through layers of potentially different soil types. Retrofitting a lawn with healthy soil and reseeding may be worth it-in fact, several local studies have indicated that a lawn grown on a deep layer of healthy soil pays for itself in 5-7 years based on water savings alone!

What Does Compost Do? Improved soil structure creates passageways in the soil for air and water. In heavy clay soils, the addition of compost enhances the physical make-up of soil which improves soil structure, porosity and bulk density to create a better environment for plant growth.

Supplies Slow-Release Nutrients to Plants without using fertilizers Compost is a good source of nutrients including nitrogen, phosphorus, potassium, sulfur, and microorganisms essential for plant growth. Since compost is made of relatively stable organic matter, nutrients are slowly made available for root uptake and. In this way, nutrients are less likely to be lost through leaching.

Holds Moisture and Reduces Erosion Compost has a large capacity to hold water-many times its own weight. This reduces water loss and leaching in soil. The soil-binding properties of compost result from its humus content which acts like a glue, holding soil particles together, making soil resistant to erosion and improving moisture retention.

Immobilizes and Degrades Pollutants Compost has the ability to bind heavy metals, pesticides, herbicides and other contaminants, reducing both their leachability and absorption by plants. The soil microorganisms that compost supports also help break down pesticides, fertilizers and hydrocarbons. This same binding effect allows compost to be used as a filter for storm water and other runoff.

Provides Organic Matter Compost supplies organic matter to the soil which increases the presence of microorganisms. The activity of microorganisms promotes root development and assists in the extraction of nutrients from the soil. It also encourages the growth of earthworms and other macro-organisms, whose tunneling increases water infiltration and aeration.

Suppresses Soil-Borne Diseases and Plant Pathogens Plant disease is influenced by both the level and type of organic matter as well as the microorganisms present in soil. Detrimental organisms like oot-eating nematodes, a number of specific plant diseases and several lawn diseases are suppressed by microorganisms found in compost.

How good is the compost? The quality of the compost used for soil enhancement is important. Commercial compost should have a sweet, earthy smell. Compost should meet your Department of Ecology’s guidelines for Grade A compost.

Suggested Reading: Soil Biology Primer, Soil and Water Conservation Society, Revised 2000. See web site for ordering information -

The Rodale Book of Composting, Deborah L. Martin and Grace Gershumy, editors, 1992.

Worms Eat My Garbage, Mary Appelhof, Flower Press, 1982. Recycle With Earthworms: The Red Wiggler Connection, Shelley C. Grossman, Toby Weitzel, Lucy Warren (Editor), and I. Donnabella, Shield’sPublications, 1997

Let An Earthworm Be Your Garbage Man, Home, Farm and Garden Research Associates, Shield’s Publications, 1954.

The Worm Book, Loren Nancarrow and Janet Hogan Taylor, Ten Speed Press, Berkeley, CA, 1998. Browser Boxes

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This website page was first published February 24, 2002.

This page was last updated March 03, 2018.