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Soil and life

Green Manure

Terra Humana editorial teamLast updated: 2026-10-126 min read

A flowering green manure of faba beans, vetch and oats between young fruit trees

In brief

Plants grown on purpose to feed the soil: often a mix of legumes and grasses, cut at flowering and left on the ground as biomass and nitrogen.

What is green manure?

Green manure is a crop grown to feed the soil rather than the table. It is sown, allowed to grow until it has produced a large amount of fresh plant mass, and then cut. The material is returned to the ground, where soil life breaks it down. In the process it releases nutrients, feeds earthworms, fungi and bacteria, and over years contributes to humus.

The best known green manures are legumes, because they bring something into the system that other plants cannot: nitrogen from the air. Bacteria in their root nodules, the rhizobia, convert atmospheric nitrogen into a form plants can use. A legume green manure is therefore biomass and fertiliser at once, grown on the spot. In practice legumes are usually sown together with cereals such as oats or barley. The cereals produce bulk and carbon, give climbing legumes something to hold on to, and make the cut material break down more evenly.

A crop that feeds the soil,,not the tableLearning panel: green manure is a crop that feeds the soil, not the table. It is sown, grown, cut and returned to the ground. Usually it is a mix: the legume takes nitrogen from the air through its root nodules and so is a fertiliser grown on the spot. Cereals such as oats or barley produce bulk and carbon and give climbing legumes something to hold on to. In the Mediterranean the growing season is winter: it is sown with the first autumn rains and cut in spring at the start of flowering. In a dry year it is cut earlier, because every week of growth in spring draws on moisture that will be missing in summer. Working it into the soil releases nutrients fast, but destroys fungal networks and crumbs and leaves the ground bare. Left on top, the cut stand acts as mulch through the summer, nutrients are released more slowly, and the soil stays protected; this is the path Terra Humana follows. Green manure is especially valuable at the beginning, when there are not yet trees and shrubs to provide material. TERRAWIKI · LEARNING PANEL A crop that feeds the soil,not the table Sown, grown, cut, and returned to the ground. 1THE MIX nodules Legume takes nitrogen from theair: a fertiliser grownon the spot Cereal oats or barley producebulk and carbon and giveclimbers something tohold on to 2IN THE MEDITERRANEAN IT GROWS IN WINTER AutumnWinterSpringSummer sow with thefirst rains cut at the startof flowering In a dry year, cut earlier: every week of growthin spring draws on moisture that will be missingin summer. 3WORK IT IN OR LEAVE IT ON TOP? Work it inreleases nutrientsfastdestroys fungalnetworks and crumbsleaves the groundbare Leave it on topacts as mulchthrough the summerreleases nutrientsmore slowlythe soil staysprotected Terra Humana’s way Especially valuable at the beginning, when thereare not yet trees and shrubs to provide material. Basis: SARE, Managing Cover Crops Profitably (2007) · TerraWiki: Green Manure

Green manure in a Mediterranean climate

In central Europe green manures are often sown after the summer harvest. In the Mediterranean the rhythm is turned around: the growing season is winter. The green manure is sown with the first autumn rains, grows through the mild, moist months, and is cut in spring.

The moment of cutting is the critical decision. The common rule is the beginning of flowering, when the stand has built most of its mass and the legumes hold a lot of nitrogen, while the stems are still soft. On dry sites there is a second criterion that weighs at least as much: soil water. Every week the green manure keeps growing in spring, it draws on moisture that will be missing in summer. In a dry year the stand is therefore cut earlier, even at the price of less biomass.

Suitable species for Mediterranean conditions are the winter-growing legumes. Faba bean roots deeply and copes with heavy soils. Common vetch climbs on oats or barley and covers the ground densely. White lupin produces a lot of mass and loosens the soil with its taproot, but it does poorly on limy soils. Mixtures of several species are generally more reliable than a single one, because each year favours different plants.

Dig in or leave on top?

Classic green manuring ends with the plough: the cut stand is worked into the soil, where it decomposes quickly. This releases nutrients fast, but it comes at a cost. Turning the soil destroys fungal networks, breaks the crumb structure and leaves the ground bare, which on slopes and in hot climates invites erosion and drying.

The alternative is to leave the cut stand on the surface. There it acts as mulch through the summer: it shades the soil, slows evaporation and is drawn down gradually by soil life. Nutrients are released more slowly, but the soil stays protected. Terra Humana follows this path. On dry, sloping land the surface is the most valuable part of the soil, and anything that keeps it covered is worth more than a faster nutrient pulse.

Green manure, cover crop, nitrogen fixers: the difference

The terms overlap and are often confused. A cover crop between trees is about protection: the ground stays covered, permanently or every year. A green manure is about feeding: a stand grown for its biomass and cut on purpose. Living mulch grows at the same time as the crop it protects. And nitrogen-fixing plants are a group of species defined by their partnership with bacteria, which can serve in all these roles, from a winter vetch to a shrub that is pruned for years.

For Terra Humana the distinction matters less than the common principle: soil is fed from above, with plant material grown on the site, and it is never left bare. A green manure is the fastest way to produce a lot of such material in a single season, which makes it especially valuable at the beginning, when there are not yet trees and shrubs that could provide it.

Frequently asked

What is green manure?

Green manure is a crop grown not to be harvested but to be returned to the soil. Typical green manures are mixtures of legumes such as faba bean, vetch or lupin with cereals such as oats or barley. The legumes bring in nitrogen from the air through bacteria in their root nodules, the cereals add bulk and carbon and give the climbers support. Cut at the right moment, the whole stand becomes food for soil life. Traditionally it was ploughed in; in no-dig systems it stays on the surface as mulch.

When is green manure sown in a Mediterranean climate?

With the first autumn rains. In a Mediterranean climate the growing season for a green manure is winter, when the soil is moist and temperatures mild. Sown in autumn, the stand grows through winter and early spring and is cut in spring, before the soil dries out and before it starts to compete with trees and crops for water. Sowing in spring rarely works on dry sites, because the plants run out of water just as they should be producing their mass.

Which plants are suitable for green manure?

In the Mediterranean, winter-growing legumes are the core: faba bean with its deep taproot, common vetch, which climbs on cereals, and white lupin on soils that are not too limy. They are usually sown with oats or barley, which give the mixture structure and balance the nitrogen-rich legume material with carbon. Mixtures generally perform more reliably than single species, because one partner compensates for the weakness of another in a given year.

When should green manure be cut?

As a rule of thumb, at the beginning of flowering. By then the plants have built most of their mass and the legumes hold a large amount of nitrogen, while the stems are still soft enough to break down quickly. Later, the stems become woody, the plants set seed, and above all they draw more water from the soil. In a dry spring it is better to cut a little early than to let the stand use up the moisture the following crop or the trees will need.

Should green manure be dug in or left on the surface?

Both are practised. Digging in releases nutrients faster, but it disturbs soil structure and fungal networks and exposes the soil, which on slopes and in hot climates costs more than it gains. Left on the surface, the cut stand works as mulch: it shades the soil through the dry season and is drawn into it gradually by soil life. Terra Humana follows the second path, because protecting the soil surface matters most on dry, sloping ground.

Hugelkultur mound cut open at the end, logs and branches under the soilNext in this themeHügelkulturHügelkultur is a permaculture method: a raised bed over buried decaying wood that stores rainwater and slowly releases nutrients.

Related terms

A lifted layer of straw and wood-chip mulch with moist dark soil underneath, next to a dry, cracked pathMulchA layer of organic or mineral material on the soil surface that cuts evaporation, keeps the ground cooler, stops erosion and feeds soil life.An old olive grove on a slope in spring, the ground between the trees fully covered with grasses, clover and wildflowersCover CropA permanent or seasonal plant cover between trees or rows instead of open soil; it slows runoff and erosion and feeds soil life and beneficial insects.Young fruit trees, each in a circle of straw mulch, with flowering white clover and medics in betweenLiving MulchA low, living plant cover that grows alongside the main crop and protects the soil; in the Mediterranean it greens in winter and dries to straw in summer.Flowering fava beans between old olive trees in springNitrogen-Fixing PlantsPlants that host bacteria or actinobacteria able to convert atmospheric nitrogen into a form plants can use, fertilising the soil around them.Fava bean roots with small root nodules, one cut open to show its pink interiorRhizobiaSoil bacteria that live inside root nodules and turn air nitrogen into plant food there: the smallest fertiliser factories in the world.Hands holding dark, crumbly soil with fine rootsHumusThe dark, stable fraction of organic matter in soil, the part that holds water and nutrients and gives a living soil its colour, smell and structure.

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Sources

  • Andy Clark (ed.), Managing Cover Crops Profitably, 3rd edition, SARE (2007)
  • FAO, Conservation Agriculture: principles and practices of soil cover and crop rotation
  • Gómez et al., Soil erosion control, plant diversity, and arthropod communities under heterogeneous cover crops in an olive orchard, Environmental Science and Pollution Research 25 (2018)
  • Vicente-Vicente et al., Soil carbon sequestration rates under Mediterranean woody crops using recommended management practices: a meta-analysis, Agriculture, Ecosystems & Environment 235 (2016)

Last updated: 2026-10-12