Legumes vs Nitrogen Fixing Cover Crops for Soil

For improving garden soil, nitrogen-fixing cover crops—which include legumes as a subgroup—are generally the best choice because they add organic matter, suppress weeds, and protect soil while also contributing nitrogen. Legumes specifically are the most effective at adding nitrogen, but a mix of legumes and non-legume cover crops often gives the best overall soil improvement.

What Are Nitrogen-Fixing Cover Crops?

Cover crops are plants grown primarily to benefit the soil rather than for harvest. Nitrogen-fixing cover crops are those that form a symbiotic relationship with soil bacteria called Rhizobia. The bacteria live in nodules on the plant’s roots and convert atmospheric nitrogen (N₂) into a form the plant can use, a process called biological nitrogen fixation. When the cover crop is cut down or dies, the nitrogen in its tissues becomes available to subsequent crops. Legumes—plants in the pea and bean family (Fabaceae)—are the most common nitrogen fixers, but some non-legumes, like certain trees and shrubs, also fix nitrogen. In gardening, the term “nitrogen-fixing cover crops” usually refers to legumes, but it can also include other plants like alder or autumn olive, which are less common in vegetable gardens.

How Legumes and Cover Crops Improve Soil

Both legumes and non-legume cover crops improve soil in several ways beyond nitrogen:

• Add organic matter: As the cover crop grows and is later incorporated or left as mulch, it increases soil organic matter, which improves soil structure, water retention, and nutrient-holding capacity.
• Prevent erosion: The roots and foliage hold soil in place during heavy rain and wind.
• Suppress weeds: A dense cover crop shades out weeds and competes for water and nutrients.
• Break up compacted soil: Deep-rooted cover crops, like daikon radish or cereal rye, can penetrate hardpan and improve aeration.
• Support beneficial organisms: Cover crops provide habitat for beneficial insects and promote a diverse soil food web.

Legumes specifically add nitrogen through fixation, while non-legumes like grasses (e.g., oats, rye) and brassicas (e.g., mustard) do not fix nitrogen but are often better at building soil structure and biomass. A mix of both can provide a balance of nitrogen addition and organic matter.

Choosing Between Legumes and Other Cover Crops

The choice depends on your soil’s needs and your gardening goals. Consider the following:

• If your soil is low in nitrogen: Choose legumes like crimson clover, hairy vetch, or field peas. They can add 50–150 pounds of nitrogen per acre, though in a home garden the amount is smaller.
• If your soil is compacted or heavy: Use a non-legume with a deep taproot, such as daikon radish or forage turnip, or a fibrous-rooted grass like cereal rye.
• If you want maximum organic matter: Non-legumes like winter rye or oats produce more biomass than most legumes, especially in cooler weather.
• If you want a winter cover: Winter rye and hairy vetch are cold-hardy; crimson clover may winter-kill in colder zones.
• If you want a quick summer cover: Buckwheat (non-legume) grows fast and smothers weeds, while cowpeas (legume) add nitrogen in warm weather.

In most home gardens, a mix of a legume (for nitrogen) and a grass or grain (for biomass and weed suppression) works well. For example, a classic mix is oats and field peas, or rye and hairy vetch.

How to Use Legumes and Cover Crops in Your Garden

To get the best results, follow these general steps:

1. Choose the right time: Plant cover crops in late summer or early fall after your main crops are done, or in early spring before planting. Some can also be undersown between rows.
2. Inoculate legume seeds: For legumes to fix nitrogen, the specific Rhizobia bacteria must be present. Inoculate seeds with the appropriate strain before planting (available at garden centers). This is especially important if you haven’t grown that legume before.
3. Plant at the right rate: Follow seeding rates on the seed packet. Too thick and they may compete with each other; too thin and weeds will take over.
4. Terminate the cover crop: Cut it down before it sets seed (unless you want it to self-sow). For legumes, the nitrogen is released when the plant decomposes, so cut them at flowering stage for maximum nitrogen content. For non-legumes, cut them before they become woody.
5. Incorporate or leave as mulch: You can till the cover crop into the soil, or cut it and leave it on the surface as a mulch. No-till methods preserve soil structure and organic matter better.
6. Wait before planting: Allow 2–4 weeks after incorporation for decomposition, especially if you added fresh, green material, to avoid nitrogen tie-up.

Common Problems and Misconceptions

One common misconception is that all legumes fix nitrogen automatically. In reality, the bacteria must be present, and the soil must have adequate phosphorus, potassium, and other nutrients. If the soil is too acidic or too wet, fixation is reduced. Also, if the legume is allowed to flower and set seed, it will use the fixed nitrogen for seed production, leaving less for the next crop. Cutting at flowering stage maximizes nitrogen return.

Another issue is “nitrogen tie-up.” When you incorporate a cover crop with a high carbon-to-nitrogen ratio (like mature grasses), soil microbes consume nitrogen to break it down, temporarily making it unavailable to plants. To avoid this, mix legumes (lower C:N) with grasses, or wait longer before planting.

Some gardeners worry that cover crops will become weeds. Most annual cover crops are easy to control if you cut them before they go to seed. Perennial cover crops like white clover can be more persistent and may require more management.

Costs and Practical Considerations

Cover crop seeds are relatively inexpensive, especially compared to synthetic fertilizers. A pound of crimson clover or winter rye costs a few dollars and can cover a small garden. However, you may need to buy inoculant for legumes, which adds a small cost. The main investment is time and effort: planting, watering, and terminating the crop. In a small garden, you can do this by hand with a scythe or shears. In larger areas, you might need a mower or tiller.

Another consideration is that cover crops take up space that could otherwise grow food. However, they can be planted in the off-season, so they don’t necessarily reduce your harvest. Some gardeners use “living mulches”—low-growing cover crops like white clover—that grow between rows of vegetables and are mowed occasionally.

Alternatives to Cover Crops for Soil Improvement

If cover crops aren’t practical for you, there are other ways to improve soil:

• Compost: Adding finished compost is the most direct way to add organic matter and nutrients.
• Manure: Well-aged animal manure adds nitrogen and organic matter, but must be composted to avoid pathogens and weed seeds.
• Mulch: A layer of straw, leaves, or wood chips suppresses weeds and slowly adds organic matter as it decomposes.
• Green manures: These are essentially cover crops that are grown specifically to be tilled in. They are a type of cover crop, so this is not a true alternative.
• Fertilizers: Synthetic or organic fertilizers can supply nutrients, but they don’t improve soil structure or organic matter.

For most gardeners, a combination of compost and cover crops yields the best long-term soil health.

Frequently Asked Questions

Can I plant legumes and non-legumes together? Yes, mixing legumes with grasses or grains is a common practice. The grass provides biomass and weed suppression, while the legume adds nitrogen. The mix also balances the carbon-to-nitrogen ratio, reducing the risk of nitrogen tie-up.

How much nitrogen do legumes actually add to my garden? In a home garden, legumes can add roughly 1–3 pounds of nitrogen per 100 square feet per season, depending on the species, soil conditions, and how much biomass is produced. This is equivalent to about 1–3 ounces of urea per 100 square feet, but it’s released slowly as the plant decomposes.

Do I need to inoculate legume seeds every time? If you’ve grown the same legume in the same area before and the soil has the bacteria, you may not need to inoculate again. However, it’s cheap insurance, and different legumes require different bacteria strains. Inoculating each time ensures good nodulation.

When is the best time to cut down a cover crop? For legumes, cut at early flowering to maximize nitrogen content. For grasses, cut before they produce seed to avoid self-seeding. The best time also depends on your planting schedule—cut at least 2–4 weeks before you want to plant your next crop.

Can I use cover crops in a no-till garden? Yes, you can cut the cover crop and leave it as a mulch on the surface. This is called “no-till cover cropping” and it preserves soil structure, reduces erosion, and builds organic matter. You can plant directly into the mulch using a dibber or a no-till planter.

Are there any cover crops that don’t fix nitrogen but are still called “nitrogen-fixing”? Not really. The term “nitrogen-fixing” specifically refers to plants that convert atmospheric nitrogen into a usable form. Non-legumes like grasses don’t do this. However, some non-legumes, such as alder trees, do fix nitrogen, but they’re not typical garden cover crops.

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