GREEN MANURES

© Frances Michaels 

Green manures are a cornerstone of ecologically sensible agriculture. They can provide outstanding benefits for the soil, crop and you, the gardener by:

• Increasing organic matter, earthworms & beneficial micro-organisms

• Increasing the soil's available nitrogen and moisture retention

• Stabilising the soil to prevent erosion

• Bringing deep minerals to the surface and breaking up hardpans

• Providing habitat, nectar & pollen for beneficial insects and reducing populations of pests

• Improving water, root & air penetration in the soil

• Smothering weeds

GETTING STARTED

Growing a green manure crop is as easy as throwing out a handful of seed onto freshly cultivated ground, followed by raking to cover the seed.

"Digging the crop in" at the end isn't necessary, as by cutting the plants at the base while still green and lush, usually just as flowers form and leaving the green manure crop on the surface you have 'instant' mulch. This is cheaper than constantly buying in mulch and doesn't introduce new weeds. A combination of a legume and a grass works well, the legume providing nitrogen & the grass, such as oats in winter or Japanese millet in summer, the bulk of the organic matter in the form of large quantity of roots.

WHAT IS A LEGUME?

An important advantage of legumes is their unusual ability to obtain nitrogen, a major element needed for plant growth, from the soil air, as most plants are unable to do this. They do this by forming a symbiotic relationship with a group of bacteria called Rhizobium, which live within a specialised structure, called a nodule, on the plant's roots. The Rhizobia can take nitrogen (N₂) from the air and convert it to the form plants normally obtain from the soil. This process is called nitrogen fixation. Legumes are plants such as clover, lucerne, peas, beans, medics and chickpeas.

WHY INOCULATE?

Rhizobia occur naturally, but they are very host-specific, that is, any one species of Rhizobia will only live with a few different types of legume. Consequently, when sowing legumes, the seed should be coated with a culture of the correct Rhizobium before sowing. This is referred to as inoculation. The bacteria are stored in peat, and as this is a living culture, it must be treated with care. It should be stored in the fridge and used within 3 months. To use, moisten the seed with a small amount of milk and stir in the inoculant until seeds are coated. Then to allow even spreading of the seed over the seedbed, mix in enough agricultural lime or superfine lime to dry the mixture out (use approx. 6 g of lime to 100 g of seed). This is called pelleting and makes fine seed easier to spread. Do not inoculate the seed until you are ready to sow it and do not leave the inoculated seed in the sun. You can also use sand, vermiculite or perlite to help spread the seed evenly.

PLANTING TIPS

• Check the pH, legumes dislike acid soil

• Correct any nutrient deficiencies (except nitrogen) before sowing

• Cover the seed to a depth of 2-3 times the width of the seed & firm the soil

• Irrigate in all but rainy weather for best results

ADDITIONAL BENEFITS OF GREEN MANURE CROPS

• Green manures can be used to smother persistent weeds; they may also make conditions unsuitable for certain weeds by improving the structure and nutrient status of the soil. Good choices for weed suppression include lablab, cowpea, lucerne, sorghum and buckwheat. Green manures can also be used to undersow crops such as corn, reducing weed growth and protecting the soil from erosion. Useful species for undersowing include subclover, cowpea, lucerne and woolly pod vetch.

• Green manures can be used to interrupt pest and disease cycles in much the same way as crop rotation. Particular green manures can be used to control root knot nematodes and root rot fungal pathogens, reducing the need to use toxic chemicals for soil fumigation. When plants such as rapeseed are dug into the soil, exudates released from the decomposing plants suppress these diseases.

The soil should never be left bare, vulnerable to erosion and weed invasion, always put in a green manure crop.

DISCLAIMER:

No liability will be accepted by Green Harvest, its owners or employees as to the accuracy of any information. No responsibility will be taken for damage to property or persons due to information given about a product or technique. No responsibility will be taken for the loss of a crop or income due to information given about a product or technique.