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Washington State University

Using Compost As A Soil Amendment

Hands holding healthy soil

If your soil lacks organic material, is compacted or doesn’t retain water, compost makes a great amendment. Different soil types will need different applications of compost to create the same increase in organic matter in the final mix. For example, since sand is heavier than clay, the approximate amount of organic amendment required to increase the organic content of 5,000 sq. ft. of soils at a depth of 6″ for a clay loam would be 9.5 cubic yards of compost-for a sandy soil much more is needed, about 14.5 cubic yards.

For your backyard vegetable garden, compost application rates are often measured in inches. For example, if your garden is 100 sq ft., and you wanted to incorporate a one-inch layer of compost, you would need 8 cubic feet of compost. One and one half five gallon buckets equals approximately one cubic foot, so you would need 12 buckets of compost. You can see why gardeners are always looking for more compost! Some gardeners buy compost in bulk by the “yard”. There are 27 cubic feet in one yard of compost.

Compost can also make excellent mulch. Different kinds of compost are better for different types of vegetation, and application rates differ for various uses. For example, for established shrubs and trees you would want coarser, fluffier more woody compost, which would not have to be as finely composted. For your vegetable garden, you would want a finer material.

Here are some suggestions:
Top Dressing for Lawns: ¼” thick. Compost should be fine. It can be sifted through a ½ ” screen. Apply after aerating or re-seeding, then water.

Annuals and Perennials: 1 – 2″ thick. You want to avoid applying too thickly in the fall, as it could promote diseases and rodent activity.

Trees and Shrubs: 1-3″ thick. Coarse compost is best. Be sure to keep mulch at least 3-6″ from the trunks to avoid fungal diseases and rodent damage.

Erosion Control: 2 – 4″ thick. Coarse compost is best. Put 2-4 ” over any area without cover. Put 3-4″ on slopes.

There are certain plants that are sensitive to over mulching. Take care to mulch lightly when mulching these plant families: azaleas, rhododendrons, dogwoods, mountain laurels, hollies, cherry trees, lindens, and spruces.