Green Manure Crops for Free Nitrogen

By James Wilson · October 10, 2026

What Is Green Manure?

Green manure refers to crops grown specifically to be cut and incorporated into soil rather than harvested for food. Nitrogen-fixing legumes are the most valuable green manure crops—they capture atmospheric nitrogen through symbiotic bacteria in root nodules, adding 50-200 pounds of nitrogen per acre to the soil when decomposed.

Top Nitrogen-Fixing Cover Crops

Crimson Clover (Trifolium incarnatum)

A cool-season annual that fixes 70-150 pounds of nitrogen per acre. Sow in late summer for overwintering in zones 6 and warmer. It produces stunning red flowers that attract pollinators before termination. Dies naturally in hard freezes, making spring bed preparation easy.

Hairy Vetch (Vicia villosa)

The cold-hardiest legume cover crop, surviving to zone 4. Fixes 100-200 pounds of nitrogen per acre and produces abundant biomass. Pair with winter rye for a complementary mix—rye provides physical structure while vetch supplies nitrogen.

Field Peas (Pisum sativum)

Fast-growing cool-season annual ideal for short growing windows. Fixes 60-120 pounds of nitrogen per acre in just 60-70 days. Sow in early spring before main crop planting or in late summer after harvest. Produces edible shoots as a bonus.

Alfalfa (Medicago sativa)

A perennial legume with roots reaching 15+ feet deep, mining subsoil minerals and bringing them to the surface. Fixes 150-300 pounds of nitrogen annually. Best for long-term bed rotation where it can grow for two to three years before being incorporated.

Maximizing Nitrogen Fixation

Inoculate legume seeds with the correct rhizobium strain before planting. Each legume species has specific bacterial partners—using the wrong strain results in poor nodulation. Commercial inoculants cost pennies per pound of seed and dramatically improve nitrogen fixation.

Termination Timing

Cut green manure crops at early flowering for the best balance of nitrogen content and decomposition speed. At this stage, plant tissue is still relatively soft and breaks down within three to four weeks. Mature, woody stems take months to decompose and temporarily tie up soil nitrogen.

Incorporation Methods

Chop and drop: Cut at soil level and leave on surface as mulch (no-till approach)
Shallow incorporation: Cut and rake into top two inches of soil for faster breakdown
Crimping: Use a roller-crimper to flatten and kill standing cover crops

Integration With Crop Rotation

Follow nitrogen-fixing green manure with heavy-feeding crops like corn, tomatoes, or brassicas. The decomposing legume tissue releases nitrogen over four to eight weeks, perfectly timed for the peak nutrient demand period of these crops. This eliminates or reduces the need for purchased nitrogen fertilizers.