What is Green Manure, and Should I Use It?

Introduction

Green manure is a sustainable gardening practice that involves growing specific crops to improve soil fertility, structure, and health. By planting green manure crops, such as clover, rye, or mustard, and later incorporating them into the soil, gardeners can naturally enrich the soil, suppress weeds, and prevent erosion. Green manure can be particularly beneficial for allotments, where soil fertility and structure are essential for productive gardening. In this guide, we’ll explore what green manure is, how it works, the benefits it offers, and how to use it effectively on your allotment.

1. What is Green Manure?

Green manure refers to cover crops that are grown specifically to improve soil health and then turned back into the soil before they reach maturity. Unlike crops grown for harvest, green manure plants are left in place to cover bare soil, add organic matter, and provide nutrients.

How Green Manure Works

Growth and Coverage: Green manure crops cover the soil, protecting it from erosion, and their roots help aerate and improve soil structure.
Nutrient Fixation: Certain green manure crops, like legumes, “fix” nitrogen from the air into the soil, enriching it for future crops.
Incorporation into Soil: Once grown, green manure plants are cut down and dug into the soil, where they decompose and release nutrients, enhancing soil fertility.

Examples of Green Manure Crops

Legumes: Clover, vetch, and field peas, which add nitrogen to the soil.
Grasses: Rye and oats, which add organic matter and improve soil structure.
Brassicas: Mustard and radish, which help control soil-borne pests and diseases.

2. Benefits of Using Green Manure on an Allotment

Green manure offers a variety of benefits that contribute to long-term soil health and allotment productivity. Here are some of the main advantages:

1. Improves Soil Fertility

Nitrogen Fixation: Leguminous green manure plants, such as clover and vetch, add nitrogen to the soil, an essential nutrient for plant growth.
Organic Matter Addition: Green manure crops add organic matter to the soil, enhancing its structure and water-holding capacity.

2. Suppresses Weeds

Natural Weed Barrier: Green manure crops cover the soil, creating a living mulch that suppresses weed growth by blocking sunlight and reducing available space for weeds.

3. Prevents Soil Erosion

Soil Stabilization: The roots of green manure plants bind the soil together, reducing erosion, especially during winter or in windy conditions.

4. Improves Soil Structure

Aeration: The roots of green manure crops help break up compacted soil, improving drainage and aeration for future plants.
Improved Tilth: Decomposing green manure plants improve soil texture, making it easier to work with and more suitable for root growth.

5. Encourages Beneficial Organisms

Soil Microbes and Worms: Green manure adds organic matter that feeds soil organisms, increasing microbial activity and worm populations, both of which contribute to healthy soil.

3. When to Plant Green Manure

Timing is essential to get the most benefit from green manure. Generally, green manure is planted during fallow periods, when beds would otherwise be left bare.

Fall Planting

Ideal Crops: Rye, clover, and mustard are common green manure choices for fall planting.
Benefits: Fall-planted green manure protects the soil over winter, prevents erosion, and adds nutrients in spring when incorporated.

Spring and Summer Planting

Ideal Crops: Buckwheat, phacelia, and field peas are good choices for spring or summer.
Benefits: Spring and summer green manure crops are ideal for quickly covering beds that will be replanted in late summer or fall, providing nutrients for the next crop cycle.

4. How to Plant and Grow Green Manure

Growing green manure is simple and requires minimal effort. Here’s how to get started:

Step 1: Prepare the Bed

Clear the Area: Remove any debris or existing plants, and loosen the soil with a garden fork.
Add Compost (Optional): Although not necessary, adding compost before planting green manure can further enrich the soil.

Step 2: Sow Seeds

Broadcast Seeds Evenly: Scatter the green manure seeds evenly across the bed, aiming for uniform coverage.
Rake Lightly: Gently rake the seeds into the soil to ensure good contact and prevent them from blowing away.

Step 3: Water and Maintain

Watering: Water lightly after sowing, especially if the weather is dry, to help seeds germinate.
Minimal Maintenance: Green manure crops require little maintenance. Just allow them to grow and cover the soil until you’re ready to dig them in.

5. When and How to Incorporate Green Manure into the Soil

Timing is important when it comes to incorporating green manure. Ideally, you should dig it into the soil before it starts to flower, as this is when it contains the most nutrients and is easiest to break down.

Best Time to Dig In

Before Flowering: Dig in green manure crops before they flower for the highest nutrient value.
Allow Decomposition Time: After digging in, allow 2-4 weeks for the green manure to break down in the soil before planting your next crop. This prevents nitrogen lockup, where nitrogen is temporarily unavailable to plants.

How to Incorporate Green Manure

Cut and Chop: Use garden shears to cut down the green manure plants, chopping them into smaller pieces for quicker decomposition.
Dig Into Soil: Turn the chopped plants into the top 4-6 inches of soil using a spade or fork.
Water and Cover (Optional): Water the area to help decomposition and, if desired, cover the bed with mulch or compost to retain moisture.

6. Choosing the Right Green Manure for Your Needs

Different green manure crops offer specific benefits, so choosing the right type depends on your allotment’s needs, soil type, and planting schedule.

For Nitrogen Fixation

Best Crops: Clover, vetch, and field peas.
Ideal Timing: Plant these in fall or spring to enrich the soil with nitrogen for future crops, especially if you plan to grow leafy greens or brassicas.

For Weed Suppression

Best Crops: Mustard, rye, and buckwheat.
Ideal Timing: These crops are ideal for quick-growing weed suppression. Plant them in spring or summer on beds that will be fallow for a short period.

For Soil Structure Improvement

Best Crops: Rye and phacelia.
Ideal Timing: Use rye in fall to improve heavy or compacted soil over winter. Phacelia, with its strong roots, helps improve drainage and structure in compacted areas.

7. Common Mistakes to Avoid with Green Manure

Using green manure is generally straightforward, but here are some common pitfalls to avoid:

Letting Green Manure Flower: Once green manure plants flower, they become harder to incorporate and may go to seed, introducing unwanted plants into your beds.
Planting Too Close to Crop Time: Allow at least 2-4 weeks after incorporating green manure before planting, as fresh green manure can temporarily reduce available nitrogen.
Choosing the Wrong Crop for Your Needs: Be mindful of your goals (e.g., nitrogen-fixing or weed suppression) when choosing green manure, as different types offer distinct benefits.

Conclusion

Green manure is an excellent tool for improving soil fertility, structure, and health on your allotment. By planting and later incorporating specific cover crops, you can add valuable nutrients, enhance soil structure, prevent erosion, and suppress weeds—all without synthetic fertilizers. Whether you need to fix nitrogen, improve drainage, or prepare fallow beds for the next season, green manure offers a sustainable solution that benefits both your plants and the environment. With careful planning and timing, green manure can become an integral part of your allotment rotation, supporting healthier, more productive crops.