Green Manure
Plant material - usually a cover crop - that is cut and incorporated into the soil while still green to decompose and add organic matter and nutrients to the soil rather than being harvested or composted.
Green manure is plant material incorporated into soil while still fresh and green, where it decomposes and contributes organic matter and nutrients to the soil profile. The term distinguishes this direct soil incorporation from composting (decomposing material separately before application) and from harvested crops.
Most commonly, green manure refers to a cover crop grown specifically to be turned into the soil. The crop is allowed to grow, then killed and incorporated - either by tilling, shallow hoeing, or burial under mulch - before or at flowering, when nutrient content is highest and stems are still soft enough to decompose quickly.
Why Incorporate Fresh Rather Than Compost
Composting before application is generally more predictable because the composting process standardizes the product and kills weed seeds. However, direct incorporation as green manure is faster, requires no composting infrastructure, and captures nutrients that might be lost during composting (particularly nitrogen, which can volatilize from hot compost piles).
The trade-off is timing. Fresh green material incorporated into soil begins decomposing rapidly, but the decomposition process temporarily ties up nitrogen as soil microbes break down high-carbon stems and leaves. This “nitrogen drawdown” can reduce available nitrogen for the following crop for 2-4 weeks. Planning a 2-4 week gap between incorporation and planting avoids this.
Leguminous green manures (clover, vetch, field peas) have a lower carbon-to-nitrogen ratio than grass covers, so they decompose faster and cause less nitrogen drawdown. They also contribute fixed nitrogen from the root nodules.
Nutrient Contribution
The nutrient value of a green manure crop depends on species, growth stage, and yield. General estimates:
- Hairy vetch (full grown, pre-flower): 3-5 lbs nitrogen per 1,000 sq ft
- Crimson clover: 2-4 lbs nitrogen per 1,000 sq ft
- Winter rye (maximum growth): 1-2 lbs nitrogen equivalent per 1,000 sq ft after decomposition (mostly from organic matter)
These numbers are rough estimates; actual release depends on soil temperature, moisture, and microbial activity during decomposition.
Organic Matter and Soil Biology
Beyond nitrogen, green manure improves soil structure through organic matter addition. Decomposing plant material feeds the soil food web - bacteria, fungi, protozoa, nematodes, and earthworms that together create the biological activity responsible for plant nutrient availability.
Regular green manure additions, like regular compost additions, gradually increase soil organic matter content. Soil with 3-5% organic matter drains better, holds more moisture at field capacity, and supports significantly higher crop productivity than the 0.5-1.5% organic matter common in compacted suburban soils.
Shallow vs. Deep Incorporation
Deep tilling (6-8 inches) incorporates green manure throughout the tillage zone but disrupts soil structure. Shallow incorporation (2-3 inches) or surface-mulching (laying cut material on the soil surface) preserves soil structure and supports no-till soil biology while still providing the organic matter benefits as the material decomposes from the top down.
For no-till beds, cut the cover crop at soil level, leave roots in place, and use the cut material as a mulch. Transplants can be pushed through the mulch layer directly. This preserves the soil channels created by cover crop roots and avoids the disruption and nitrogen drawdown of tilling.