Lesson 12 – Part A: WHAT IS A GREEN MANURE?
Lesson 12 – Part B: PREPARING SEEDS AND BED
Lesson 12 – Part C: GROWING A GREEN MANURE
Lesson 12 – Part D: (for senior students) HOW LEGUMES FIX NITROGEN
WHAT IS A GREEN MANURE?
A green manure is a crop of plants that are grown to recycle organic plant foods, and replace organic matter in topsoil.
Organic gardeners grow green manure crops of grains or legumes when they have a garden bed that they won't be 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 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’.
You can grow a green manure crop whenever you have a spare garden bed. However, close to the summer holidays is a good time for schools to grow a green manure crop because green manures don't need as much care as food crops.
PREPARING SEEDS AND BEDS
When sowing a green manure you will need a lot more seeds than you would use to grow a vegetable crop. To help you sow 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. To do this you will need:
Green manure seeds • kitchen scales • 6 or more small containers
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 the weight of how much seed to sow in each section.
Place a small container on the scales and fill it with green manure seeds to the correct weight for that section. Then measure seeds into the other containers in the same way.
Don't forget your hat and gloves!
Preparing the bed
Choose an area of garden bed that you won't be using for a while. For best results, legumes need a soil pH between 6.25 and 7.5. If you want to grow a green manure legume, test the soil pH as you learnt in Lesson 9, and adjust it if necessary.
Then dig some organic-allowed poultry-based fertiliser into 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 use this plant food.
Water the bed thoroughly, and then use a rake to make lots 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.
Divide your garden bed into equal sections, so that you will know where to sow the small containers of seeds. To do this you will need:
A tape measure • some knitting yarn or string • some short sticks
Measure the width of the bed, and divide the measurement by 2 to find the middle of the bed. Poke one of the sticks into the soil half way across the bed at each end of the area where you want to grow the green manure. Lie some yarn along the bed between these two sticks and tie the ends to the sticks, so that you have a nice straight line.
Then measure the length of the area where you want to grow the green manure. Divide this measurement into equal parts for sowing seed. Place sticks on each side of the bed to mark each section, and tie pieces of yarn across the bed as you can see in the diagram below.
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GROWING A GREEN MANURE
Sprinkle the seeds from one small container as evenly as you can across one marked out section of the bed surface. Do this again in the next section until all the marked sections have been sown. Take a rake and use a gentle chopping motion to cover the seeds with soil. If you pull the rake across the bed, a lot of the seeds will end up in a pile.
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 again until after they have germinated as too much water can cause legume seeds to rot.
If the weather is very dry, you can cover the bed surface with a very thin layer of organic mulch – about 1 cm thick. The mulch will help to keep the seeds constantly damp until they germinate. When your green manure crop has germinated, all you have to do is to water it occasionally if the weather is very dry. You can see in the photo on the left below, that the 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.
Green manure grains are grown until 45 cm high before they are cut down. Legumes are cut down as they start to flower. 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.
If you leave your green manure on the soil surface as mulch over the summer holidays, you will be able to plant seedlings at the start of the school year by pulling back small pieces of the mulch to make a hole for your seedlings. Mix some compost in each planting hole before you plant the seedling.
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 help them break down quickly. Then, if you keep the soil just damp, the bed will be ready to use about four to six weeks later, in warm weather, or a bit longer in cool weather.
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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.
Nitrogen fixation by rhizobia is helpful to good growth because 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.
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 hemisphere, and the rhizobia they need are not in our soils.
If you want vegetable and green manure legumes to fix nitrogen, you will have to sow seeds that have been coated with the correct rhizobium. These are called inoculated seeds, and 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 usable form is released into soil for the use of other plants.
Once you have grown an inoculated crop, the type of rhizobium 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 nodules can be clearly seen on the white clover roots in the photo below. Use your thumbnail (or a box cutter blade) to carefully split one of the nodules in half.
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. (See the close up of the split nodule, below.)
If the inside of the nodule is deep pink to dark red in colour, 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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