Compost

Compost is organic material that has been broken down by microbial activity into a stable, dark, crumbly material called humus. The raw inputs - vegetable scraps, leaves, grass clippings, straw, coffee grounds, paper - are unrecognizable in finished compost. What remains is a biologically active material that improves nearly every soil property when added in sufficient quantities.

What Composting Does

Decomposition is accomplished primarily by bacteria and fungi, with larger organisms - nematodes, mites, pill bugs, and earthworms - playing supporting roles. Thermophilic bacteria drive hot composting: temperatures in an actively managed pile reach 130-160°F, which kills weed seeds and most pathogens. Mesophilic bacteria and fungi do the work in slower, cooler passive piles.

The end product contains:

• Stable organic matter that improves soil structure
• A slow-release nutrient package (typically 1-3% N, 0.5-1% P, 1-2% K by dry weight, plus trace elements)
• A diverse microbial community that continues decomposing organic matter in the soil
• Humic acids that improve nutrient retention and soil aggregate stability

Carbon-to-Nitrogen Ratio (C:N)

The rate of decomposition depends on the ratio of carbon to nitrogen in the pile. Microbes need carbon for energy and nitrogen to build proteins. An ideal C:N ratio is roughly 25-30:1 for fast decomposition.

High carbon (brown) materials: Dry leaves, straw, cardboard, wood chips, sawdust. C:N ratios of 60:1 to 500:1.

High nitrogen (green) materials: Fresh grass clippings, vegetable scraps, coffee grounds, fresh manure. C:N ratios of 10:1 to 25:1.

A pile that’s too carbon-heavy decomposes slowly and stays cold. Too nitrogen-heavy generates ammonia odors and stays wet. The practical fix: roughly 2-3 parts browns by volume to 1 part greens.

Hot Composting

Active management accelerates decomposition to weeks rather than months. Requirements: a pile at least 3x3x3 feet (to retain heat), adequate moisture (squeezed material should release a few drops, not drip), proper C:N balance, and regular turning to introduce oxygen.

A properly managed hot pile can produce finished compost in 4-8 weeks. Turn every 3-7 days when pile temperature drops below 130°F. The pile will rebuild temperature after turning as fresh material is inoculated.

Hot composting kills most weed seeds (at 131°F for 3 days). Cold composting does not reliably kill seeds - avoid adding seed-bearing weeds to cold piles.

Passive (Cold) Composting

Pile materials, wait. Decomposition happens without management but takes 6-18 months. No turning required; pile doesn’t heat significantly. Acceptable for yard waste and most kitchen scraps. Does not kill weed seeds. Adequate for most home garden fertility needs without the effort of hot composting.

Vermicomposting

Vermicomposting uses red wigglers (Eisenia fetida) rather than heat to process organic material. The resulting vermicompost (worm castings) has higher available nutrient content and stronger biological activity than thermophilic compost. Best suited for kitchen scraps; worms can’t handle large volumes of yard waste. See also: Vermicompost.

Application Rates

For establishing new beds: work 3-4 inches of compost into the top 6-8 inches of soil. This is a substantial amount - a 4x8 bed requires roughly 10-12 cubic feet (about 0.4 cubic yards) for a 4-inch application.

For established beds: topdress with 1-2 inches annually. For raised beds with good existing organic matter, 1 inch is adequate to maintain fertility if combined with crop rotation and minimal tillage.

What Compost Doesn’t Do

Compost is not a substitute for macronutrients when soils are severely deficient. A vegetable garden that needs substantial nitrogen or phosphorus for an immediate season can’t rely on compost alone - the nutrient release from compost is slow and modest. Use compost for long-term soil building; use targeted fertilizers for acute deficiencies.