Green manure cover crop

Lesson 12

Lesson 12: Part A: Part B: Part C: Part D:

WHAT IS A GREEN MANURE? PREPARING SEEDS AND BED GROWING A GREEN MANURE (for senior students) HOW LEGUMES FIX NITROGEN

Lesson 12 – Part A

WHAT IS A GREEN MANURE?

First weigh all the seeds to find out how much you have altogether. Then divide this weight by the even number of sections that you will mark on your garden bed. You now know

A green manure is a crop of plants that are grown to recycle

the weight of how much seed to sow in each section.

organic plant foods, and replace organic matter in topsoil.

Place a small container on the scales and fill it with green

Organic gardeners grow green manure crops of grains

manure seeds to the correct weight for that section. Then

or legumes when they have a garden bed that they won’t be

measure seeds into the other containers in the same way.

using for growing food crops for at least 8 weeks. Bare soil or fallow (resting) periods allows plant food minerals and organic matter to wash away in heavy rain or be blown away

Don’t forget your hat and gloves!

in dry, windy weather. Green manures hold the soil together, protecting the soil, and are an important part of crop rotation.

They provide groundcover for the soil which is why they are often called ‘cover crops’.

Preparing the bed

You can grow a green manure crop whenever you have a

Choose an area of garden bed that you won’t be using for a

spare garden bed. However, close to the summer holidays is

while. For best results, legumes need a soil pH between 6.25

a good time for schools to grow a green manure crop because

and 7.5. If you want to grow a green manure legume, test the

green manures don’t need as much care as food crops.

soil pH as you learnt in Lesson 9, and adjust it if necessary.

Then dig some organic-allowed poultry-based fertiliser into

Lesson 12 – Part B

PREPARING SEEDS AND BEDS Preparing seeds

the top 10 cm of soil in the bed, and give the bed a drink of seaweed extract tea. This plant food will not be wasted. All the plant food soaked up by the green manure will be returned to the soil as microorganisms break down the green manure.

The next lot of vegetables that you grow in that bed can then

When sowing a green manure you will need a lot more seeds

use this plant food.

than you would use to grow a vegetable crop. To help you sow

Water the bed thoroughly, and then use a rake to make lots

a lot of seeds evenly over a large area, you can divide your green manure seeds into equal small amounts and sow each of these in a separate section that you will mark out on your garden bed.

of furrows close together ACROSS the bed surface. If dry patches appear, it means that you haven’t given the soil enough water, and you need to give the bed a gentle watering so that you won’t damage the furrows.

To do this you will need: • Green manure seeds • kitchen scales • 6 or more small containers

Lesson 12| Page 1

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Lesson 12

Divide your garden bed into equal sections, so that you will

soil half way across the bed at each end of the area where

know where to sow the small containers of seeds.

you want to grow the green manure. Lie some yarn along the

To do this you will need:

bed between these two sticks and tie the ends to the sticks, so

• A tape measure • Some knitting yarn or string

that you have a nice straight line.

Then measure the length of the area where you want to grow

• Some short sticks

the green manure. Divide this measurement into equal parts

Measure the width of the bed, and divide the measurement by

each section, and tie pieces of yarn across the bed as you can

2 to find the middle of the bed. Poke one of the sticks into the

see in the diagram below.

for sowing seed. Place sticks on each side of the bed to mark

Lesson 12 – Part C

GROWING A GREEN MANURE Sprinkle the seeds from one small container as evenly

If the weather is very dry, you can cover the bed surface with

as you can across one marked out section of the bed surface.

a very thin layer of organic mulch – about 1 cm thick.

Do this again in the next section until all the marked sections

The mulch will help to keep the seeds constantly damp until

have been sown. Take a rake and use a gentle chopping

they germinate. When your green manure crop has germinat-

motion to cover the seeds with soil. If you pull the rake

ed, all you have to do is to water it occasionally if the weather

across the bed, a lot of the seeds will end up in a pile.

is very dry. You can see in the photo on the left below, that the

You can now remove the yarn and sticks. Give the bed a gentle watering and keep grain seeds damp by watering the bed when necessary, but don’t water legume seeds

thin layer of mulch has not stopped the wheat seeds from germinating. In the photo on the right that was taken 7 days later, you can see how fast the plants are growing.

again until after they have germinated as too much water can cause legume seeds to rot.

Lesson 12| Page 2

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Lesson 12

Green manure grains are grown until 45 cm high before they

in each planting hole before you plant the

are cut down. Legumes are cut down as they start to flower.

seedling.

The leaves and stems can be left to break down on top of the bed to protect the soil during the rest of the holidays. Helpful soil bacteria, fungi and other creatures will start breaking down the leaves, stems and roots of your green manure crop and turning it into humus. The roots of green manure crops add lots of organic matter to your soil as you can see from the photo at right.

At other times of the year your green manure crop can be slashed into short pieces and dug into the top 8-10 cm of soil after the foliage has wilted. Green manure crops that have been dug into the soil can be watered and covered with a layer of organic mulch to

If you leave your green manure on the soil surface as mulch

help them break down quickly. Then, if you

over the summer holidays, you will be able to plant seedlings at

keep the soil just damp, the bed will be ready

the start of the school year by pulling back small pieces of the

to use about four to six weeks later, in warm

mulch to make a hole for your seedlings. Mix some compost

weather, or a bit longer in cool weather.

Lesson 12 – Part D – for senior students

HOW LEGUMES FIX NITROGEN Legumes are plants that produce pods and are able to fix nitrogen in small lumps called nodules that they form on their roots. However, they can only fix nitrogen in these nodules if a special family of bacteria called Rhizobia (rise-o-bee-uh) is in the surrounding soil. Some types of rhizobium can help several different legumes to fix nitrogen, but other plants need a particular variety of rhizobium to fix nitrogen.

although 80% of our atmosphere is nitrogen, it is in the form of a gas that plants are unable to use. A suitable rhizobium is able to enter plant root hairs and stimulate the root into forming round or finger-shaped nodules where the rhizobium can change the gas into the ammonia form of nitrogen that plants can use for many processes. This process is called nitrogen fixation.

Nitrogen fixation by rhizobia is helpful to good growth because

hemisphere, and the rhizobia they need are not in our soils.

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Rhizobia that help wattles and other native legume plants to fix nitrogen live in Australian soils, but most of the vegetable legumes we grow came to Australia from the northern

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Lesson 12

If you want vegetable and green manure legumes to fix nitro-

nodules can be clearly seen on the white clover roots in the

gen, you will have to sow seeds that have been coated with

photo below. Use your thumbnail (or a box cutter blade) to

the correct rhizobium. These are called inoculated seeds, and

carefully split one of the nodules in half.

the inoculant (suitable bacterium) has to be ordered when you purchase seeds, as ordinary packets of legume seeds are not inoculated. Inoculated green manure legumes are cut down when they have fixed the most nitrogen, which is when they begin to flower. As the green manure breaks down, nitrogen in

Although, in gardening, we tend to associate nitrogen with the colour green, signs of nitrogen fixation are red due to the presence of a pigment that is similar to haemoglobin, which causes the red colour of blood.

usable form is released into soil for the use of other plants.

See the close up of the split nodule, below

Once you have grown an inoculated crop, the type of rhizo-

If the inside of the nodule is deep pink to dark red in colour,

bium that you have added to your soil will help other legumes that need the same rhizobium to fix nitrogen properly, too.

However, some crops, such as French beans, require a very special rhizobium that is not available in Australia, and these will not be able to fix nitrogen.

Nitrogen-fixing nodules form about 2 to 3 weeks after legume seeds have germinated. You can tell if your legumes are fixing nitrogen by gently digging down beside a legume plant to expose some of the roots with nodules. The nitrogen-fixing

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a suitable rhizobium is present in that garden bed and your legumes are doing an excellent job of fixing nitrogen. If the inside is pale pink, your plants are trying hard, but there is little of the suitable rhizobium in that bed. If the inside of the nodule is white, grey or green, your legumes are not fixing nitrogen.

This may be due to a suitable rhizobium missing from that area of soil, plant stress, or an excess of soluble nitrogen in surrounding soil. When excess fertiliser is applied, legumes shut down nitrogen fixation.

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