Why Green Manure?

Why would you sow seeds and then dig in the resulting lush greenery?  Why bother?

Below is a photo of a healthy bed of broad beans. They will grow a bit higher still, then produce flowers, and before you know it, there will be a nice crop of juicy broad beans.

Alternatively, this would be the perfect time in the life of these plants to cut up all the lush greenery with a spade and dig it all in. If I did that, I would be using these broad beans as a green manure.

 

The term ‘green manure’ is confusing because there is no manure involved in the process. The term is used for the practice of using lush green plant material to improve drainage and soil fertility.

 

 

Green manure is great for food gardens for six reasons

• Nutrients: Some green manures (see below) develop long roots that bring nutrients from down below to the surface. Dig them in and you will be adding these nutrients to your soil. Some green manures (see below) form nitrogen in root nodules, and are a great way to prepare a soil for nitrogen-demanding crops such as Brassica.
• Soil structure: dug-in green manures add much needed humus to sandy soils and help them retain moisture. They break down heavy clay soils and improve drainage. 
• Weed suppression: Some green manures (see below) are a great way to avoid or suppress persistent weeds because they will smother those weeds. 
• Soil protection: bare soils dry out and are subject to leaching of nutrients and erosion. A green manure, while growing protects the soil from the elements. Once it is in the soil, the added organic matter helps retaining moisture and nutrients. 
• Disease management: crop rotation is the number one way to avoid pests and diseases returning each year and gradually taking a bigger and bigger toll. Some green manures (see below) produce compounds that actively work against certain pests. Farmers call disease management by way of green manure biofumigation because it is pest control without chemicals.
• Cost: most seed is inexpensive, so improving your soil with green manures will be cheap if you compare it with what you pay for mulches, manures and composts.

The benefits of green manure are really quite impressive, and this is why many farmers use it to  improve their soils.  Some people call it a cornerstone of ecologically sustainable gardening.

How to green-manure

 

Some food gardeners sow a green manure whenever they have an empty space in their garden, and dig it in whenever thereafter they need the space for another crop.

If you want to plan things a bit more than that, consider that, on average, a green manure crop will need four weeks minimum to make a difference, and six weeks ideally. It is best practice to give dug-in green manure four weeks to compost, before sowing or planting the next crop. In total, the area will be available for the next crop 2 – 2½ months from sowing the green manure. Make both growing and composting periods longer if soil improvement is your main priority.

It is generally recommended that you sow green manure seeds in Spring before it becomes too warm and dry, or in Autumn before it becomes too cold. That recommendation assumes you don’t have time to irrigate the area (at least) once a day, every day, until seeds comes up. If you can, any time from September to early May is fine.

Do not use green manure under fruit trees as green plant material encourages bacteria in soils, whereas fruit trees do best in fungi-dominated soil.

Many green manure seeds are small, so first break up clumps of soil and make the surface as smooth as possible before sowing.

If the seeds are small it is a good idea to put them in a small bucket, add some sand and mix it in. Broadcast this mix by hand when it is not windy. Cover the seeds with a thin layer of soil that is once or twice the diameter of the seed.

Minimise the chance of air-pockets in the soil. After sowing, compact the soil lightly with your hand, or a tool, and then moisten the surface, but do not over-water.

I have a resident blackbird or two in my garden.  Without fail they make a complete mess of my nicely tilled open soil when I forget to cover it with mesh.  Don’t forget to remove this when you see the first signs of germination.

Until most seeds have come up, make sure that the soil is moist but not wet  at all times. Once most seeds are up, revert to normal irrigation.

 

a lush mix of plants ready to be chopped up

 

Monitor your green manure crop for signs of flowering 

This is really important because ..... 

• You don’t want to add weeds to your garden by allowing green manure plants to go to seed.
• Green manure is most beneficial to your soil when it is soft, lush and green.
• Cutting up lush soft green plants is much easier than cutting up plants that have formed a tough stem to support their flowers.
  

Cutting it all up with a spade or a pair of scissors

 

When your crop is lush and green .....

 

If you don’t need this part of your food garden any time soon, give the greens a good haircut first. Leave the cut material in situ, and watch the greenery re-grow.

The most beneficial way to end the crop, is to actually not dig it in, but to cut all foliage off at ground level. You could use a mower. Leave all the foliage in place, make it wet and then cover it completely with a layer of soil or mulch.

Alternatively, cut the foliage into bits, and dig it in with a spade. If some of your green manure plants are legumes, make sure their roots (and their nodules that contain nitrogen) don’t end up out of the ground.

 

Let the green material compost for at least four weeks, then plant or sow your next crop. Composting can have a negative effect on seed germination, so it is best not to sow this area within the four-week period.

 

 

You could use your old seeds as green manure

 

Seed packets have a Sow By date. A great way to use seeds that are past this date is to go through your old seed packets and empty the ones that are past it in a big bowl.  Mix it all together when you are finished and you may have found a new purpose for those old seeds. It will be very interesting to see how many of them turn out to be still viable.

 

 

 

Plants that are recommended as green manure

 

Any plant can be chopped up and used as green manure, but some plants are more suited than others. The list below shows plants recommended for Tasmanian climate conditions.

Black Oats: Good weed suppressor. Non-invasive. Good provider of foliage.

Brassica Rapa: Helps to control nematodes and root rot fungal pathogens. Long roots loosen soil. Adds a lot of organic matter.

 

Broad beans: Nitrogen fixer. Fast growing. Good in our cool climate. Extensive roots loosen soil.

Buckwheat: Sow only in summer. Good weed suppressor. Shallow roots make it easy to pull up.

French Marigold: Summer crop. Helps to control Nematodes.

Lucerne: Nitrogen fixer. Summer crop. Good weed suppressor. Long roots break up soil and bring nutrients to the surface.

Lupin: Nitrogen fixer. Summer crop. Long roots break up soil and bring nutrients to the surface.

Mustard: Fast growing. Helps to control nematodes.

Oats: Good in our cool climate. Good weed suppressor. Good in a wide range of soils.

Pearl Millet: Summer crop. Good weed suppressor. Extensive root system breaks up soil and brings nutrients to the surface.

 

Serratas: Good soil loosener. Good nitrogen fixer. Good weed suppressor. Cold tolerant.

Sub clover: Nitrogen fixer. Good weed suppressor. Good soil loosener. Suppresses a number of pests.

Mustard happily growing away

Green & lush and ready for digging in

Where to buy green manure seeds

 

Many seed suppliers sell the individual green manure varieties mentioned above. Try a nitrogen fixing plant mixed with oats. The combination will work well, the legume providing nitrogen, and the oats the bulk of the organic matter.

Some seed suppliers sell ready-to-use mixes of green manure varieties. That is a good and easy way to get started with green manuring. I am looking here at a packet of green manure mix of a mainland seed company, and notice that some of the varieties in the mix are less than ideal for Tasmania’s cool climate.

There are two Tasmanian seed companies that sell green manure seeds that perform well in Tasmanian conditions:

• Inspirations – see https://vegetableseeds.net.au/product-category/green-manure-fertiliser/
• Southern Harvest – go to https://www.southernharvest.com.au/site-search/green%20manure

You can get larger quantities of green manure seeds and mixes at

• Roberts stores around the state
• Hollander Imports in Hobart.

Happy green manuring!