Green manure is a crop grown to be dug back into the soil while it is still green, so that its leaves, stems and roots rot down and feed the next crop. It is the dug-in form of a cover crop: the plant is the fertiliser, grown in place rather than bought in a bag. Legume green manures such as clover, vetch and field beans also fix nitrogen from the air, so they add a nutrient that was not in the soil before. Green manure is one of the cheapest and most effective ways to build soil fertility, and it has been used for thousands of years for exactly that reason. This guide explains what it is, which crops to use, when to dig them in, and how to get the full benefit.
The difference between green manure and compost is that green manure is grown in the bed it feeds, while compost is made elsewhere and carried in. Both add organic matter, but green manure also keeps the soil covered and alive while it grows, and legume green manures add nitrogen. The difference between green manure and a cover crop is mostly one of emphasis: a cover crop is grown mainly to protect the soil, and green manure is grown mainly to be dug in and feed the next crop. In practice the same plant often does both jobs, and our cover crops for soil guide covers the protective side in detail.
How green manure works
Green manure feeds the soil in three ways. First, the green material itself is rich in nitrogen and other nutrients, which are released as it rots down over a few weeks. A lush legume green manure can add the equivalent of a substantial fertiliser dressing — roughly 100–150 kg of nitrogen per hectare for a good clover or vetch crop, which works out at about 10–15 g of nitrogen per square metre, enough to feed a following crop of leafy vegetables well. Non-legume green manures such as rye and mustard add less nitrogen but still add organic matter and hold nutrients that would otherwise wash away in winter rain.
Second, the roots feed soil life while the crop grows, leaking sugars that support bacteria and fungi, and leaving channels for water and air when they rot. Third, the crop protects the surface from rain and wind, so the soil it feeds is not eroded while it grows. The net effect is a bed that is more fertile, better structured and more alive than a bed left bare — and the cost is a packet of seed and a few weeks of growth.
The best green manure crops
Legumes — the nitrogen fixers. Crimson clover, white clover, winter vetch, field beans and lupins all fix nitrogen through bacteria on their roots. Crimson clover is the best all-rounder for a vegetable garden: easy to grow, easy to dig in, and good for bees. Winter vetch fixes a lot of nitrogen and smothers weeds, but it can be vigorous and needs cutting before it seeds. Field beans suit heavy clay and cold winters, and they have deep roots that open the soil. White clover can be left as a living mulch between crops. Lupins suit acidic, sandy soils where other legumes struggle.
Grasses — the bulk builders. Winter rye, annual ryegrass and oats add a large amount of organic matter, hold nutrients over winter, and suppress weeds. They add little nitrogen themselves, but they are tough, reliable and cheap. Winter rye is the hardiest and the best for cold winters, but it must be cut early in spring before it becomes woody.
Brassicas — the deep-rooted cleaners. Mustard, daikon radish and oil radish grow fast, break compaction with deep roots, and are traditionally said to help suppress some soil pests when dug in, though the evidence is modest. Do not follow them with brassica crops such as cabbage or broccoli, or you risk carrying clubroot and other brassica diseases into the next crop.
Others. Phacelia grows fast, flowers for bees, and suits light soils. Buckwheat is the fastest summer green manure, ready to dig in within six to eight weeks, and it is traditionally said to help make phosphorus available, though that claim should be treated cautiously. Comfrey is sometimes grown as a green manure, but it is usually used as a liquid feed or mulch rather than dug in, because its deep roots make it hard to remove.
When to sow green manure
Sow green manure whenever a bed becomes empty and will stay empty for at least six weeks. The main sowing is late summer and autumn, after summer crops finish: sow crimson clover, vetch, field beans or winter rye from August to October, so they establish before winter. In spring and summer, sow fast green manures such as phacelia, buckwheat and mustard into gaps between crops, or on beds that will be empty for a few weeks. Scatter the seed evenly, rake it in lightly, and water if the soil is dry. Sow generously — a thin, patchy stand adds little. Check your planting dates on our zone pages, such as when to plant tomatoes in zone 7, and work backwards from your first frost to set sowing dates. If you are unsure which green manure suits your soil and climate, check your local extension service for advice.
When and how to dig green manure in
Timing is the most important part of green manure, and it is where most gardeners go wrong. Dig the crop in while it is still soft, green and leafy, before it flowers and sets seed, and before it becomes woody. For autumn-sown crops, that usually means cutting them down in late winter or early spring, two to four weeks before you want to plant the next crop. Cut the growth with shears or a strimmer, leave it on the surface for a few days to wilt, then fork it lightly into the top 15 cm of soil.
The two-to-four-week gap is essential. Freshly dug-in green material temporarily uses nitrogen as soil organisms break it down, and seedlings planted straight into it can go yellow and stall. After two to four weeks the breakdown passes its peak and the nutrients are released, so the next crop gets the benefit instead of the loss. If you are short of time, cut the green manure and leave it as a mulch on the surface rather than digging it in — it breaks down more slowly that way, and you can plant through it sooner. For a no-dig approach, cut at ground level, leave the roots in the soil to rot, and plant into the bed through the mulch. The roots still feed the soil and leave their channels.
How much nitrogen does green manure add?
The amount depends on the crop and how well it grows. A good legume green manure — crimson clover, vetch or field beans grown well for a full season — can add roughly 100–150 kg of nitrogen per hectare, which is about 10–15 g per square metre. That is enough to feed a following crop of leafy vegetables or brassicas well, though heavy feeders such as tomatoes and cucumbers will still need extra feeding through the season — see our fertilizing vegetables guide. Non-legume green manures add less nitrogen but still add organic matter and hold nutrients. A thin or patchy stand adds far less, so sow generously and give the crop time to grow. Do not expect a precise number: the benefit varies with soil, weather and management, and a soil test through your local extension service is the only way to know your soil's actual nitrogen level.
Green manure for problem soils
For heavy clay, field beans and daikon radish are best: beans fix nitrogen and their roots open the soil, while daikon's thick root breaks compaction. See our clay soil improvement guide. For sandy soil, crimson clover and winter rye are best: clover adds nitrogen, rye holds nutrients against winter rain, and both add the organic matter sandy soil consumes quickly. See our sandy soil improvement guide. For compacted soil, daikon radish and alfalfa have the deepest roots. For acidic, sandy soil, lupins are the legume that thrives where others struggle. For a bed that will be empty all winter, winter rye or field beans are the toughest and most reliable choices.
Common mistakes
The most common mistake is digging green manure in too late, when it is woody and has set seed — the seed becomes a weed, and the woody material robs nitrogen for weeks. The second is planting straight into freshly dug-in green manure without the two-to-four-week gap. The third is sowing too thinly, so the stand is patchy and adds little. The fourth is choosing a green manure from the same family as the next crop, which carries disease — never follow mustard or radish with brassicas. The fifth is sowing too late in autumn, so the crop does not establish before winter. And the sixth is expecting green manure to replace all fertiliser: it adds a useful amount of nitrogen and organic matter, but heavy feeders still need feeding, and a soil test is the only way to know what is actually missing.
Safety notes
Wear gloves when handling seed and when cutting down green manure, and wash your hands afterwards. Some green manures, such as mustard, can irritate sensitive skin, so wear long sleeves when handling large amounts. Do not eat green manure crops unless you know the variety is edible — most are grown for soil, not food. Fresh manure should never be confused with green manure: green manure is plants dug in green, while fresh animal manure must be composted for several months before use on food beds. If you are unsure which green manure suits your soil, check your local extension service for advice.
Frequently asked questions
What is the difference between green manure and cover crops?
They are closely related and often the same plant. A cover crop is grown mainly to protect the soil, and green manure is grown mainly to be dug in and feed the next crop. In practice, one planting usually does both jobs, and the management — sowing, cutting before seed, and digging in — is the same.
How long before planting should I dig in green manure?
Two to four weeks. Freshly dug-in green material temporarily uses nitrogen as it breaks down, so planting straight into it can starve seedlings. Cut it down, dig it in, wait two to four weeks, then plant. If you are short of time, leave it as a surface mulch and plant through it instead.
Which green manure adds the most nitrogen?
Legumes add the most: crimson clover, winter vetch, field beans and lupins all fix nitrogen from the air. A good legume crop can add roughly 10–15 g of nitrogen per square metre. Non-legumes such as rye and mustard add organic matter but little nitrogen.
Can I use green manure in a small garden?
Yes. Even a single bed benefits, and a packet of seed costs little. Use fast summer green manures in gaps between crops, and an autumn green manure on any bed that will be empty over winter. In a very small garden, white clover as a living mulch between crops is a good low-effort option.
Does green manure replace fertiliser?
No, not entirely. It adds nitrogen and organic matter, but heavy feeders still need feeding, and a soil test is the only way to know what your soil is actually missing. Think of green manure as part of a feeding routine — alongside compost and fertiliser — not a replacement for them.