How To Improve Soil With Green Manure

Green manure refers to cover crops turned into soil. When growing in your garden, these crops act like living mulch, suppressing weeds, adding nutrients, preventing erosion, and providing a home for beneficial predatory insects. When turned into the soil, they act like compost, providing beneficial microorganisms and nutrients while improving soil structure and getting your spring garden off to a great start!

There are numerous cover crops you can grow, all of which help to improve your soil. These come in the form of legumes, grasses, brassicas (plants in the cabbage and mustard family), and non-legume broadleaf plants like flax. Many gardeners prefer a mixture of different varieties. Learn to improve your soil with green manure!

Brassicas may produce a rotten egg smell as they decompose, and deer love legumes.

Materials You'll Need

All you need is seeds and gardening tools

Gardening tools in soil next to a potted plant. — Image credits: Case Yuion via Vecteezy

You'll require a few tools and some seeds to get your green manure started.

Green manure seed mix
Garden rake
Spade shovel
Garden hose and water

Choose Your Cover Crop

Legumes, grass, and brassica provide great cover crops

Crimson Clovers — Image credits: Bethany Wallace via Pixabay

You can select a specific cover crop for the benefits it provides or choose a variety. Local garden centers often sell green manure seed packets with a mixture ideal for your region. PennState Extension recommends a mixture containing a legume, grass, and brassica, such as annual ryegrass, crimson clover, and radish. This approach ensures you receive the many benefits each provides!

Cover Crop	Benefits
Oats	Fast-growing annual Prevents erosion Loosens soil due to thin, branching roots
Alfalfa	Fixes nitrogen Adds organic matter Retains moisture Prevents soil erosion Attracts pollinators
Buckwheat	Fast-growing annual Loosens tight soil Attracts beneficial insects
Red Clover	Fixes nitrogen Suppresses weeds Protects soil from erosion Ideal in orchards Slow growing
Winter Rye	Deep roots prevent compaction Protects soil from erosion
Mustard	Fast-growing annual Good for clay soil
Hairy Vetch (legume)	Fixes large amounts of nitrogen Improves soil health Protects soil from erosion Attracts beneficial insects Annual
Winter Field Beans	Fast-growing annual Fixes nitrogen Protects soil from erosion Suppresses weeds
Winter Field Peas	Fast-growing annual Fixes nitrogen Protects soil from erosion Adds organic matter Attracts beneficial insects

Hairy vetch and winter rye produce significant growth, making them potentially challenging to turn into the soil. Annual plants offer easy integration as they die off in the winter.

Buckwheat may attract Japanese beetles. If you're planting roses, strawberries, raspberries, eggplant, or other plants susceptible to these beetles, it's best to go with a different cover crop.

Plant Your Cover Crop

Timing is everything

Planting seeds in a row — Image credits: Binyamin Mellish via Pexels

Most cover crops are planted right after your garden is done. In northern areas, this often translates to late summer or early fall. They also need to be planted about one month before the first frost. This gives the seeds time to germinate and lets the plants get established before it gets too cold. It also gives the plants' roots time to penetrate into the soil.

First, remove any crop residue, weeds, and large stones. Break up any large clumps of soil and rake the garden area evenly.
Scatter seeds evenly over the prepared ground according to the rate on the package. Keep in mind it needs to be thick enough to prevent weeds from competing for space.
Gently rake the seeds into the soil, protecting them from birds. Larger seeds need to be covered with about ¼ inch of soil.
Water gently and thoroughly with a fine mist sprayer.

The University of Minnesota recommends planting buckwheat or beans, heat-loving crops, if you're creating a new garden bed in the spring or early summer. They suggest ryegrass or oats, cover crops that grow quicker in cooler weather, if you're starting the garden in late summer.

Caring for Your Cover Crop

Add a little water and sunshine

Watering a garden with a watering can. — Image credits: Karolina Grabowska

Most cover crops are relatively low-maintenance, requiring full sun (six to eight hours daily) and water when they're dry.

Keep the soil moist until the seeds germinate. The time it takes to germinate will depend on the crop and the environmental conditions. For instance, small grain seeds like oats and rye may take just a few days to a few weeks, depending on the temperature and moisture levels.
Once established, most cover crops are low-maintenance and only require watering when it's dry. Cover crops may require about four weeks to establish themselves.
Many thrive in at least six hours of full sun a day, but make sure to check on the growing requirements for your particular area.

Dig in Your Cover Crop

Turn your cover crop into compost

A person holding grass in a muddy garden area. — Image credits: Dominika Mazur via Pexels

Now, you get to turn your beautiful cover crop into compost! Ideally, dig in your cover crop after a killing frost or about one month before your spring planting, giving it time to break down and enrich your soil. If you're running out of time, you can turn it into the garden two to three weeks before planting.

If your cover crop has lots of top growth, mowing or weed whacking it may help make it easier to dig into the soil.
Use a sharp spade to turn the cover crop into the ground, ensuring you unearth the roots. Dig the cover crop into the top 10 inches of the soil.
Depending on what you plan on planting in the garden, now's also the time to add fertilizer to the soil.

If your cover crop flowers, dig it in before it starts seeding. Otherwise, it can become weeds in your garden.

Taking advantage of cover crops and green manure creates healthy soil with more beneficial microorganisms, organic matter, and improved structure. And, as an added benefit, you'll have fewer weeds!