Grow Your Own Fertiliser with Green Manures

Mung bean and french white millet green manure seedlings.

Mung bean and french white millet green manure seedlings.

Why buy fertiliser when you can grow it yourself at low cost? The best manure for your garden isn’t manure at all. You can feed your annual and perennial plants and improve the soil in your garden by growing green manures.

What are Green Manures?

Green manures, or cover crops, are leafy annual or perennial plants grown to add nutrients and organic matter to soil and to improve its’ structure and water holding capacity. They can also help suppress weeds, pests, and diseases.

What are the Benefits?

A proven technique used by organic gardeners for centuries, green manures are more sustainable than using conventional fertilisers that feed your plants directly at the expense of the life in your soil. Cultivating healthy soil life is like having a strong, healthy, and fit body; it is more naturally resilient to disease and stress than a body dependent on synthetic drugs. A healthy soil can better withstand changes and extremes like drought that can damage soil life.

Nutrients are often present in the soil but not in a form available to your plants. Green manures are food for the soil organisms that convert these nutrients into forms that your plants can use. So not only do they add nutrients to the soil directly when they decompose, they also make the nutrients that are already in the soil available to your plants.

Their root systems help open the soil and release substances that bind soil particles, creating a crumbly structure which allows more air and water to find its’ way into the soil. As the roots rot, they also become food for soil organisms. Some green manures have deep roots, which bring water and nutrients to the surface, while legume green manures draw nitrogen from the air and add it to the soil when they decompose.

How Does it Work?

Typically, you’ll sow green manures as a combination of beans and grasses or grains. The beans add nitrogen drawn from the air in a process known as nitrogen fixation, while the grass or grain adds bulky organic matter. Examples include oats, vetch, peas, clover, mung bean, and millet.

Traditionally, instead of harvesting green manures they’re slashed and tilled into the soil to decompose. Many organic farmers and gardeners now recognise that tilling can damage soil life and that it’s better to leave the green manure on the soil surface. Ideally, layering a brown mulch material on top of the green manures helps speed up the process and prevents loss of nutrients to the air. Essentially, it’s composting right where it’s needed, in the garden bed.

Garden beds being prepared with mung bean and french white millet green manure.

Garden beds being prepared with mung bean and french white millet green manure.

Sow and slash annual green manures before your next crop and after a previous crop. This usually means you won’t be able to grow a productive crop for one season, but if timed correctly, you can squeeze one in at the beginning or end of a season.

For example, prepare your soil before the next growing season by sowing a winter cover of oats in autumn. The snow will kill the oats so you’re ready to plant in spring. Winter rye is another option, but since it will live through winter and early spring it’s better suited to late season crops. You can even under sow some crops with green manures so they establish once you harvest your crop. Japanese natural farming pioneer, Masanobu Fukuoka, was a master of this technique.

For fruit trees or perennials, green manures that grow year round and can be continually slashed are also known as a living mulch.

To choose a green manure suitable for your particular climate, crop, or season, contact your favourite organic seed retailer.