Finished compost, often called “black gold,” is a dark, crumbly substance resulting from the decomposition of organic materials. It has an earthy aroma and a fine texture. This stable, biologically active soil amendment holds immense value for any gardener or landscaper. Finished compost works primarily by improving the physical structure of the soil, rather than acting as a fast-acting fertilizer. It introduces beneficial microorganisms and slowly releases essential plant nutrients over time, supporting long-term soil health and fertility. The specific application method depends on your gardening goals, such as preparing a new bed or maintaining an established area.
Mixing Compost into Existing Soil
Incorporating finished compost directly into garden beds is the primary way to achieve lasting soil improvement. This technique physically blends the organic matter with the native soil, which is particularly beneficial for correcting problematic textures. For instance, in heavy clay soils, the compost helps aggregate fine particles into larger clumps, improving aeration and drainage.
Conversely, when added to sandy soils, the sponge-like qualities of compost increase the soil’s capacity to hold both water and dissolved nutrients, preventing them from rapidly draining away. The general recommendation is to spread a layer of two to four inches of compost over the surface. This layer should then be mixed or tilled into the top six to eight inches of existing soil for thorough incorporation.
The preferred time for this deep soil amendment is before planting, either in the spring or in the fall. Blending compost into the rooting zone helps establish a strong soil structure and a vibrant biological community that sustains plant growth throughout the season. Applying this amount of organic matter annually helps continually build a healthy, resilient foundation for productive gardens.
Applying Compost as Top Dressing and Mulch
Applying finished compost to the soil surface acts as both a mulch and a slow-release nutrient source without disturbing the soil structure. For established garden beds, a thin layer, typically one-half to one inch deep, can be spread around plants. This layer functions as a protective blanket, suppressing weed germination by blocking light and moderating soil temperature fluctuations.
On lawns, a fine, sifted layer of compost is used as a top dressing to improve turf health. A layer no thicker than one-quarter to one-half inch should be spread, ensuring the grass blades remain exposed to sunlight and air. This practice introduces organic matter and beneficial microbes directly to the root zone, helping to fill in bare patches and increase turf density.
Trees and shrubs also benefit from a top-dressing application of compost. The material should be spread in a layer about one-half inch thick around the plant, extending out to the drip line. It is important to keep the compost a few inches away from the trunk or stem to prevent moisture buildup that can lead to rot. This surface application allows nutrients and beneficial microorganisms to gradually leach down into the soil with irrigation or rain.
Specialized Uses and Preparations
Finished compost is used as a component in various specialized growing media and liquid preparations. For container gardening, compost is frequently combined with inert materials like peat moss, coir, or perlite to create a custom potting mix. A common ratio involves using approximately 25% to 50% compost by volume, mixed with other ingredients to ensure proper drainage and aeration. This mixture provides a stable environment and a consistent nutrient supply for plants in confined spaces.
When preparing a mix for starting seeds, the use of compost requires a cautious approach. Compost must be fully aged and finely sifted to prevent nutrient overload that can harm delicate seedlings. It should be used sparingly in seed-starting mixes, often constituting less than one-third of the total volume, or used only when potting up seedlings after their initial true leaves appear.
Processing compost into a liquid form, known as compost tea, is another specialized application. Compost tea is created by steeping finished compost in water, sometimes with added nutrient sources and aeration, to extract beneficial microorganisms and soluble nutrients. This liquid can be used as a soil drench to inoculate the root zone with microbes or applied as a foliar spray to help suppress plant diseases.