Compost as a Soil Additive

What is the best way to build a safe, healthy lawn? Start from the ground up. Healthy soil is a necessary foundation for lush, weed free grass. Continued application of synthetic fertilizers and pesticides deplete beneficial nutrients in the soil and create a toxic environment for earthworms, fungi, bacteria, and beneficial nematodes through radical changes in pH and the buildup of toxic salts and other compounds (including heavy metals) often found in fertilizers. Compost application on the other hand will enhance populations of soil micro-organisms by providing them with an excellent source of nutrients.

Improve Soil Quality

When soil organisms use and decompose compost, they form slimes, gels, and filaments that bind soil particles together into soft clumps called aggregates. Soil aggregates improve conditions for turf growth by increasing soil pore space, which then allows for less restricted root growth and easier flow of water, nutrients, and air through the soil and to plant roots. The low density of compost helps increase soil softness or friability, while its high surface area and chemical activity increase the water and nutrient holding capacity of soil. Several turf specialists recommend applying high rates of compost to improve degraded soil. The NOFA Standards for Organic Lawn Care recommends applying one inch or three cubic yards of compost per 1,000 square feet for marginally good soils. For very sandy or low-organic-matter soils, they recommend a two-inch layer or six cubic yards of compost per 1,000 square feet.

Compost as a Fertilizer

In addition to improving soil structure, high quality compost is a great natural, slow release fertilizer. Unlike soluble, synthetic fertilizers that immediately release all available nutrients into the soil, the organic residues used to form compost must decompose before their nutrients are available to plants. Good quality compost contains both readily available and stored nutrients. Once compost is added to the soil, weak acids secreted by plant roots release the available nutrients from compost, and over time soil organisms break down and mineralize additional nutrients. Characteristics of a high quality compost include light, crumbly texture, dark brown to black in color, earthy aroma, and 30-50% moisture content.

Fully mature compost can also effectively suppress some turf pathogens. You can make a preliminary assessment of compost maturity by filling a plastic bag with moist compost, sealing it, and letting it sit in the sun for a few days. If the compost has an earthy smell when you open the bag, it is mature; if it smells of sulfur or ammonia, it is still immature. If you are purchasing compost, make sure that it is from a reputable source and that it is mature, nutritionally well balanced, and does not contain heavy metals, pathogens, or other toxic substances.

Compost can be tilled into the soil during turf renovation, used as a topdressing, or sprayed on as compost tea. Solid compost can be applied as an unmixed material or mixed with sand for easier handling. The method of application depends on whether you want to improve soil quality, enhance soil fertility, or control pests and diseases.

When fertilizing turf, base the compost applications on the nutrient needs of the turf. To accurately calculate the amount of compost needed, sample the soil and test the nutrient content of the compost. For maintenance of existing turf, apply compost as a topdressing. To best mix compost with the soil in existing turf, core aerate the turf, broadcast the compost, then run a drag chain over the ground to sweep the compost into the aeration holes.

What’s the best time to apply compost?

Spring or fall. Compost applied in the spring provides nutrients during the main growing season while compost applied in the fall helps prolong the growing season, strengthens root growth for the dormant season, and promotes early spring growth. Compost mixed with cool-season grass seed in the fall will facilitate effective overseeding.

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