In 2025, the global organic farming market exceeded $100 billion, and the green manure sector is growing rapidly—forecast to rise from $2.32 billion in 2024 to $2.49 billion in 2025, with a CAGR of about 7.6 percent.
Green manure is a cover crop grown specifically to be tilled into the soil for fertility and structure. Instead of synthetic inputs, organic farmers use these living crops as natural fertilizers. In practice, a green manure crop is planted, allowed to grow, and then killed (by mowing or tillage) so its biomass adds nutrients and organic matter to the soil.
As one extension notes, adding green plant material “benefits many soil characteristics, including drainage, nutrient content and microbial activity.” In other words, green manures form a cornerstone of organic farming’s closed-loop system: they cycle nutrients on the farm, build long-term soil health, and replace the need for external fertilizers.
Key Benefits: Why Farmers Use Green Manure
Cover crops globally (forage crops) reduce soil bulk density by around 3.2 percent, increase water-stable aggregates by 15.9 percent, and decrease soil penetration resistance by 11.8 percent.
A. Soil Fertility & Nutrient Management
Legume green manures (e.g. clover, vetch, peas) host nitrogen-fixing bacteria in their roots, pulling atmospheric N₂ into the soil as ammonia. This built-in fertilizer is released to the next crop when the legume is turned under. In addition, deep roots of green manure crops draw up nutrients like phosphorus and potassium from lower soil layers.
When the crop is incorporated, those nutrients become available, effectively recycling minerals that would otherwise leach away. Green manures also contribute carbon-rich biomass. This organic matter feeds microbes and builds Soil Organic Matter (SOM), enhancing the soil’s ability to hold nutrients and water.
In practice, farmers see much better nutrient retention with green manures – one survey reported “added fertility” and “better nutrient retention” as key results of green-manuring. Overall, a green manure crop boosts soil fertility by fixing nitrogen, conserving minerals and feeding the soil ecosystem.
B. Soil Structure & Physical Health
Cover crops improve the physical condition of soil. Growing roots and decayed plant material bind the soil and prevent erosion by wind and water. For example, organic growers often report “added erosion protection” from green manure cover crops.
After termination, the decayed roots leave channels that loosen the soil, improving aeration and water infiltration. The new organic matter acts like a sponge: soils rich in SOM from green manures retain moisture much better.
In fact, research shows cover-cropped fields often have higher infiltration and retention of water than bare fields. In short, green manure crops turn compacted or bare fields into friable, moisture-holding soils, reducing runoff and compaction in future crops.
C. Weed, Pest, and Disease Suppression
A dense green manure canopy shades out weeds and can release natural bio-herbicides. Certain cover crops (e.g. cereal rye, sorghum-sudangrass, mustards) contain allelopathic chemicals that inhibit weed seed germination. For example, rye residue on the soil surface suppresses pigweed, lambsquarters and other common weeds.
On many organic farms, adding green manures is reported to significantly reduce weed pressure. Meanwhile, rotating in a cover crop breaks pest and disease cycles. Mustard family green manures (rapeseed, radish, etc.) have biofumigation properties that can reduce nematodes and soil pathogens.
Farmers note drops in soil diseases and pests – for instance, fewer nematodes, wireworms or Rhizoctonia damage – after using green manures. Thus green manuring acts as a cultural control: it interrupts pests and smothers weeds without chemicals.
D. Boosting Soil Biology
Green manures feed and invigorate the soil food web. As the crop grows and later decomposes, it delivers a feast to soil bacteria, fungi, earthworms and more. This stimulates microbial activity and diversity.
According to one report, green manuring causes “a substantial increase in activity of soil fungi and bacteria.”
A vibrant microbial community in turn improves nutrient cycling, disease suppression and overall soil productivity. Organic growers frequently mention that green manures “stimulate soil organisms” and improve soil health. In effect, green manures act as a probiotic for the soil – they feed the living soil body that sustains crops.
Types of Green Manure Crops & Selection Guide
In recent trials, green manure treatments increased total soil carbon by up to 19 percent, with an average boost around 10 percent compared to no-cover controls.
A. Legumes (for Nitrogen Fixation)
Legumes are chosen when you need to add nitrogen. Common legume green manures include crimson clover, hairy vetch, winter peas, fava beans, and cowpeas. These plants form root nodules with Rhizobium bacteria that convert atmospheric N₂ into fertilizer.
When you mow or plow them under at bloom, their nitrogen-rich tissue breaks down and releases N into the soil. Use legume green manures before heavy nitrogen-feeders (like corn, brassicas or leafy greens) to provide a natural N boost.
B. Grasses & Cereals (for Biomass & Weed Suppression)
Grass-type covers build bulk and protect soil. Examples include annual ryegrass, oats, winter rye, and sorghum-sudangrass. These species grow quickly and form dense mats, which are excellent for erosion control and “taking up” leftover nutrients. Their thick stems and roots add plenty of organic carbon to the soil when incorporated.
Many farmers seed a cereal cover (like winter rye) after harvest simply to catch nutrients and hold soil – rye is especially noted for its year-round growth and nutrient-scavenging ability. In addition to biomass, grasses suppress weeds by creating a thick mulch; their residues reduce sunlight to emerging weed seeds.
C. Broadleaves & Non-Legumes (for Diversity & Biofumigation)
Other cover crop choices add special benefits. Buckwheat grows fast in summer and smothers weeds; its flowers attract pollinators. Brassicas (mustards, oilseed radish, etc.) can biofumigate: when incorporated, their residues release compounds that suppress nematodes and soil pathogens.
Phacelia, sunflowers, and other broadleaf flowers boost biodiversity, supporting beneficial insects and breaking pest cycles. By rotating in non-legume covers, farmers address niche goals (pest control, pollinators, soil structure) not covered by legumes or grasses. Selection Criteria:
Climate and Season: Choose winter-hardy covers (rye, vetch) for fall sowing in cold regions, or warm-season crops (buckwheat, cowpeas) for summer sowing in warm climates.
Soil Type and Needs: Test your soil. If soil is low in nitrogen, include legumes. If it’s compacted, use deep-rooted radish or grasses. If you need more phosphorus or potassium, pick species known to scavenge those nutrients.
Crop Rotation: Consider the next crop. Plant a high-N cover if the following crop is a heavy feeder. Leave a legume before a nitrogen-demanding crop, or a fast-decomposing mustard before a seed crop.
Primary Goal: Be clear on your goal. If you need biomass for organic matter, use a heavy grass cover. If weed suppression is key, use quick-canopy covers like buckwheat. Match the species to the problem you want to solve.
How to Use Green Manure in Your Rotation
Globally, cover crops increase soil carbon sequestration by 0.11–0.15 Pg C per year and can reduce nitrogen leaching by 34–41 percent.
A. Timing & Seeding
Green manures are typically seeded right after a cash crop is removed. In practice, many growers sow winter cover crops in late summer or fall, as soon as harvest is complete and the soil is prepared. For example, after pulling out vegetables, a farmer might broadcast rye or vetch seeds in September.
The cover then grows through fall and early winter. In mild climates (or between spring–summer crops), a spring or summer cover may be planted instead (e.g. buckwheat after a spring vegetable harvest).
Some farmers even interseed a cover into a standing crop (for example, drilling clover under a cereal before its final harvest) to get an earlier start. In all cases, sow green manure seed promptly after the cash crop so the cover has time to establish before adverse weather.
B. Termination (The “Manuring” Process)
The green manure is typically terminated at bloom, before seeds develop. This is when biomass and nutrients are high. For instance, field peas are usually mowed or plowed in at first flowering (about 5–8 weeks after planting). To kill the crop, farmers can mow or flail it; in no-till systems, a roller-crimper may be used to flatten and kill the plants.
Otherwise, the cover is tilled into the soil with a plow or hoe. If livestock are used, grazing the cover until it’s flat also works (the animals leave manure as they go). Once killed, the dead plant material is mixed into the topsoil or left as a surface mulch.
The goal is to uniformly spread the biomass so it can decompose. Proper timing is crucial: mowing or cutting at flowering not only captures maximum nutrients, but also prevents reseeding and weed issues.
C. The Waiting Period (Nitrogen Flush)
After incorporation, the soil needs a short rest before the next crop is planted. This 2–4 week waiting period allows microbes to decompose the green manure and release nutrients.
If planted too early, the new crop may suffer from nitrogen immobilization – a short-term N shortage as soil microbes “tie up” nitrogen to break down high-carbon residues.
Research defines nitrogen immobilization as “the temporary loss of available soil nitrogen resulting from decomposition of cover crop residue.”
In practice, this means waiting until the bulk of decomposition is done. During this time, soil microorganisms will mineralize the green manure’s nitrogen and other nutrients.
For example, crops like field peas should be incorporated and then a few weeks later the next crop seeded, to avoid the “nitrogen penalty.” Warm, moist soil speeds this process, while cool or dry conditions slow it, so adjust the timing to local conditions.
Challenges and Considerations
Management Intensity: Green manuring requires extra planning and labor. You must schedule time for sowing, termination and waiting. Equipment may be needed (seeders, mowers or rollers). Good planning of your crop rotation is essential.
Water Use: In dry climates, cover crops consume water, so they must be managed carefully. However, studies show well-timed cover crops often improve overall water efficiency. For instance, UC research found that winter cover-cropped fields had higher soil water retention and did not reduce yields compared to bare fallow; cover crops actually held more moisture for the cash crop.
The key is to match covers to rainfall and irrigation plans: don’t sow a water-thirsty cover just before a drought, and terminate it in time so it doesn’t compete with the cash crop.
Nitrogen Tie-Up: High-carbon covers (e.g. mature rye, wheat) can immobilize soil N if not managed. This is why timing and waiting are important. As mentioned, using legumes (low C:N) or terminating at flowering minimizes this risk.
Volunteer Weeds: If you let a green manure go to seed, those seeds can become weeds. Always kill or mow the cover before flowering to prevent a seed bank. In practice, most farmers mow green manures two times (if growing tall) to ensure no seeding.
Conclusion
Regenerative and organic farming methods like green manuring contribute to carbon sequestration and build soil resilience. Green manures are a multi-functional, sustainable tool at the heart of organic farming. By cycling nutrients, improving soil structure, suppressing weeds and feeding the soil life, they build resilient, fertile soils without synthetic chemicals.
As one survey of organic farmers reported, benefits include better weed control, erosion protection and “improved soil health” thanks to added organic matter and microbial activity. In short, green manure is an investment in long-term soil health.
It may require extra management up front, but it pays dividends through higher yields, lower inputs and a healthier farm ecosystem over time. In the words of many organic growers, green manuring is not a quick fix but “an investment in the future” of the land.
