I have used a number of Green Manure strategies this Winter as part of my soil fertility building program. In fact I think that I have shifted from growing food to growing soil as my primary activity. Using a Green Manure provides a number of benefits for me, which I’ll describe below. Please note that this is an explanation of some of the things that I have done this Winter, and not an attempt to teach people how a green manure should be used.
Green Manure Benefits
There are some really good sources of information about the benefits of growing a green manure crop. This covercrop pdf lists an amazing number of crops, combinations, and advice. However I had some particular benefits in mind.
When looking at the factors that help maintain a healthy mycorrhizal population, keeping actively growing roots in the soil is possibly the most important factor. If I simply harvest a crop, spread compost, and then leave that bed alone, I have no roots for the fungi to colonise.
Microbial activity is stimulated by the secretion of sugars by plant roots. This is one of the primary drivers of the soilfoodweb. Although this is likely to be reduced in the Winter, any time that the plant is actively growing, I hope that my soil is benefiting from this.
Bare soil can be harmed by heavy rain, creating capping. A dense green manure crop can reduce this.
Growing a green manure crop creates more biomass, rather than just shifting materials in from outside.
A green manure crop sown in late Summer or Autumn can help to reduce the loss of Nitrogen through ‘leaching’. it does that by using the Nitrogen to build itself, effectively acting as a storage vessel. When the green manure is killed off, microbes make that Nitrogen available for a following crop.
My Green Manure Strategies
What I wanted to achieve was to use a variety of different Green Manure strategies to see how they performed. Whilst using green manure has a number of benefits, I also wanted to try some hardy plants that would give me a yield of food, as well as build soil fertility. The picture above is an example of that.
Edible Lupin and Garlic
Barely visible at the bottom right of the picture above is the top of some Edible Lupins, planted out in October, inter-planted with garlic. I was given some edible lupins to grow by a friend in Portugal. I harvested some from a Spring sowing last year, but the packet suggested that they were normally sown in Autumn in Portugal. I’m not sure how hardy they are so I used them as an over wintering crop to test that out. Unfortunately the Winter has been so mild, that I still don’t know how hardy they are.
The picture above shows the garlic and edible lupin intercrop. The lupins have grown so well that they are preparing to flower in FEBRUARY The picture blow shows one of those flowers forming. (Look at the colour of the soil at the bottom of the picture. I love soil)
Some might say that this isn’t a green manure, but the combination is performing all of the functions that I wanted from a green manure. Both plants will produce plenty of biomass to compost, or return to the soil as mulch. They both provide food, and the lupin also provides bee forage. The plants have been growing all winter whenever the temperature has allowed.
Multiple Species Green Manure Mixtures
Beyond the bed of Autumn sown garlic there are two beds that I have sown with multiple species green manure plants. These beds grew Onions and Garlic last year. Both of these are harvested relatively early, which allows a wider range of species to be grown. The furthest of these beds was sown with a mixture of broad beans, phacelia, crimson clover, and persian clover. All of these are winter hardy, and will need to be killed before the next crop, which are brassicas. This family should make good use of the nitrogen released by the decomposing green manure. The closer bed was sown with a similar mix of green manure plants, but with naked oats replacing the broad beans. The idea behind that was for the oats to be winter killed by the cold. That hasn’t happened, and I cannot be sure if that’s due to the mild winter, or if this species/strain of oat is fully winter hardy. Only a really cold winter will tell, and I cannot control that.
The picture above shows a close up of some of the oat plants growing amongst the phacelia. The picture below shows some of the Broad beans in the other mixture.
The broad beans are some of my Bunyard’s Exhibition seed. They have consistently struggled in hard winters, so I decided to use them here, not concerned about their survival. Again, the mild winter has left them relatively unscathed, and already flowering (late February). As I am also growing a Field bean (Wizard), I will need to cut the green manure beans back to avoid crossing. Whilst these mixtures have a range of functions, I have only used seeds that I had available to put them together. They are not customised for optimum benefits. For example, a mix of oats and beans may have been better at creating biomass, and Nitrogen lifting. a mix of oats and radish may have been better at opening up the soil.
The picture above shows some of these beds looking South. The bed in the center has field beans in the foreground. These have overwintered, and have recently been inter planted with Onion sets, which are yet to emerge. At the far end of the bed are Autumn sown onions. These have been planted in clumps of four, spaced at 12 inch intervals. This makes weed control easier than a uniform 6 inch spacing. The picture below shows this. Although not all of the sets have emerged, most have, and some have only emerged recently.
The bed to the left is the broad bean green manure mix, although there is some overwintered salad and celery in the foreground.
The bed to the right has Autumn planted garlic and edible lupins at the far end. The near end is being planted up with more field beans. These are the same age as the plants in the central bed, but have grown better in modules with protection. using ordinary soil in my potting mix has meant that the bean roots have been colonised by Nitrogen fixing bacteria, as can be seen in the picture below.
Hopefully this will give the plants a good start in life.
I still have plenty of work to do to design an effective green manure strategy. My recent reading has made the suggestion that soil fertility could be built just using rock dusts and green manure. I think that would be an interesting experiment to try.