What Is Green Manure? Which Plants to Sow and When

If a bed in the field or garden will stand empty over winter, sowing a crop that feeds the soil is an old and widespread practice. This post explains what green manure is, which plants are used, when to sow them in autumn and when to turn them in during spring, based on material from Turkish faculties of agriculture.

What is green manure?

In the definition used in Ankara University’s Faculty of Agriculture course notes, green manures are plants grown to supply the soil with organic matter, which are ploughed or cut and brought under the soil at a certain stage of growth while still green. In other words, the crop is grown not for harvest but to return to the soil.

Turkey’s Regulation on the Principles and Implementation of Organic Agriculture also puts crop rotation, legumes, deep rooted plants and green manuring first among the ways of maintaining soil fertility. That is why green manure is a common method both in small gardens and in organic production.

Which plants are used as green manure?

The sources divide green manure crops into two groups:

Legumes: vetch (common vetch, Hungarian vetch, hairy vetch), broad beans, peas, alfalfa and clover. Because their roots can fix nitrogen from the air, they are the most preferred group.
Non legumes: rye, barley, oats, wheat and species such as rapeseed or mustard. They produce plenty of green mass and keep the soil covered over winter.

The course notes recommend sowing the two groups together; oats with peas and rye with vetch are given as examples. In a mix, the fibrous roots of the cereal and the deeper roots of the legume reach different layers of the soil, and more mass is produced from less seed. When choosing, look for fast growth, the ability to establish on poor soil and a crop that will not delay the sowing of the main crop.

When to sow green manure in autumn

Winter green manure appears in the course notes as a “winter catch crop”: it is sown in August or September and brought under the soil in April or May. In practice, in the Marmara region this window runs from September to November. A crop sown early has more time to grow before the cold sets in. In a trial at Ege University, common vetch sown in September was also the treatment that gave the best result for the crop that followed it.

November counts as late, but in coastal and sheltered gardens broad beans or a vetch and rye mix can still be sown. Sow when the soil is neither muddy nor rock hard; scatter the seed and cover it lightly with a rake. We cover what else can be sown in November and December in a separate post.

What to Plant in November and December: Winter Vegetable Guide →

When and how to turn green manure into the soil

According to the sources, the best time is just before or at the start of flowering. After this stage the nitrogen content of the plant begins to fall and hard to break down compounds such as cellulose and lignin increase. When a mature crop is turned in, decomposition slows down and soil life may compete with plants for nitrogen for a while.

The plants are cut or crushed and, once chopped, mixed into the top layer of the soil.
The course notes give a depth of 5 to 15 cm in heavy clay soils and 10 to 20 cm in light sandy soils.
If the soil has enough moisture, turning the crop in 10 to 15 days before sowing the main crop is considered suitable; in a dry spring this may take longer.
In a small garden, the cut plants can also be left on the surface as a cover instead of being dug in.

What to watch out for

Water: the turned in crop needs moisture to break down. The course notes recommend green manuring for areas with ample rainfall or irrigation and advise caution in dryland farming.
Timing: if you are late and a crop that has set seed or turned woody is dug in, decomposition slows down.
Planting plan: green manure is planned around the planting date of the vegetable that will go into that bed in spring.
Soil reaction: on light soils worked for years with green manure alone, the soil can become more acidic; an occasional soil test makes this easier to see.
Seed: which seed suits your area can be learned from the seed packet or from local growers.

A green manure plan for your own garden

Green manure is not a solution on its own; it works together with crop rotation, compost and keeping the soil covered. Which mix suits which bed depends on the soil, on what will grow there in spring and on water. The permaculture post on moving from observation to design in a small space can guide you as you build this plan.

Permaculture in a Small Garden: From Observation to Design →

At Şifa Köyü in Şile, food is grown from heirloom seeds, without pesticides and with methods that do not exhaust the soil. If you would like to talk through a soil, seed and seasonal plan for your own garden, balcony or field, you can write to us from the Permaculture and Herbal Farming Consultancy page.

Permaculture and Herbal Farming Consultancy →

Frequently asked questions

What is green manure?

Plants grown to add organic matter to the soil, which are cut or ploughed in while still green. Vetch, broad beans, peas, rye, barley and oats are the most common.

When is green manure sown?

In the course notes, winter green manure is a catch crop sown in August or September. In the Marmara region the window usually runs from September to November; sowing early gives the crop more time to grow before winter.

When is green manure turned into the soil?

Just before or at the start of flowering. After that the nitrogen in the plant drops and hard to break down compounds increase. Winter green manure is usually turned in during April or May.

Which plants are sown together as green manure?

The sources recommend sowing a legume together with a cereal. Oats with peas and rye with vetch are common examples.

How long should you wait after turning in green manure?

If the soil has enough moisture, the course notes consider turning it in 10 to 15 days before sowing the main crop suitable. In dry weather the plants may take longer to break down.

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