We tried growing garlic last year, with some success, but I had the feeling that if we’d prepared the ground a bit better the bulbs might have been larger, so this year we decided to try adding green manure to the soil a month or so before planting the garlic. With this in mind we planted the bed up with lemongrass about three months ago.
Before reading further you need to understand that on our place “soil” is mostly a concept. We live in rugged sandstone country and what passes for natural soil is mostly rocky gravel with sand and (perhaps) some humic material. If we want good soil we have to make it.
Another source of complexity is that because of the depredations of possums, wallabies and bandicoots we need to grow most of our vegetables behind fences topped off with a strand of electric tape. It wasn’t always like this. For the first six or seven years we seemed to have a rock-solid agreement with the wildlife (apart from the parrots) that if we didn’t eat their food, they wouldn’t eat ours. However over the course of a year, and strangely it was a very wet year when there was plenty of natural food for the wildlife, they suddenly decided that our vegetable were infinitely preferable to bush tucker.
Anyway, this is a long-winded way of getting to the point that there are few things we can grow unprotected, but these include all the onion-garlic group, lemongrass, turmeric, ginger, and citrus. So, in order to be economical with the fenced area, the garlic gets grown ‘in the open’. This pretty much means that it goes into a terraced garden bed behind a rock wall, which tends to make the whole thing “well drained”, particularly if there isn’t much humic material in the soil to start with. Hence one of the attractions of green manuring, and the choice of lemongrass as one of the components of the green manure – nothing was going to graze it off.
The functions of green manure are said to include
- increasing the percentage of organic matter in the soil, and thereby improving water infiltration and retention, aeration and enhancing other desirable soil characteristics, including soil structure;
- promoting a more varied and healthy soil biota; and
- adding nutrient resources that deep-rooted green manure crops bring up from deep in the soil (but lemongrass is shallow-rooted).
Here’s the harvest of lemongrass from what will be the garlic bed. It was densely planted three months before we cut it in the middle of March. About half of the crop we gave away to friends in the Indonesian community here – it’s a staple of many Indonesian dishes – which is the material in the buckets and pots on the bank. The heap in the middle is for the green manure, and the lemongrass still growing on the left of the photo gives an idea of the size of the plants we harvested. The bed in the foreground is where the green manure, and later the garlic, will go.
I also wondered whether the aromatic oils in the lemongrass might have a useful effect in inhibiting soil pests like nematodes, but I couldn’t find anything on this. In fact I couldn’t find anything at all on lemongrass as a green manure.
We also decided to add the fresh green growth from the pigeon peas. Pigeon peas (Cajanus cajan) are an evergreen perennial legume shrub that grows to more than two metres in good conditions. Cath Manuel has an informative short article on pigeon peas over at Soil to Supper, and there’s even more detail on the Tropical Permaculture site. Both recommend them as a mulch or soil improver, and NSW Agriculture says they are an excellent rotation crop for building up soil nitrogen and breaking weed and disease cycles. Pigeon pea hosts the VAM fungus which allows the plant to access phosphorous and zinc in the soil, and presumably this is incorporated into the plant, adding to its usefulness as a green manure.
The other ingredient in our green manure mix was mulberry stems and leaves. We coppice mulberries rather than growing them as trees, for two reasons. First, the variety we have tends to grow straight up. No amount of pruning or shaping seems to be able to persuade it to grow as a relatively low spreading tree that one could easily harvest fruit from. Second, ultimately we will mainly want the leaves as a food supplement for poultry. We learned from our good friend Thanongsi, who runs a successful demonstration sustainable agriculture farm in central Laos, that he finds he does not have to provide any supplements to goats or poultry that are given mulberry leaves as a component of their fodder. So presumably these nutritional supplements are going to be incorporated into the green manure.
Here are our green manure ingredients ready for mulching.
In the background is the lemongrass, the heap in the foreground is the pigeon pea, and on the right is the mulberry. It is important to use only the green and sappy ends of the branches for green manure, to make sure that it breaks down quickly and easily in the soil. Of course the blades of the lemongrass are typically coarse and hard, but when finely mulched and combined with the other ingredients should break down readily. One advantage of the mix of ingredients is that when combined it isn’t so “wet” that it will choke up the chipper/mulcher (seen in the background of the photo above). I’d highly recommend this machine from Greenfields. It has a very sturdy direct drive from the motor to the chipper blades (rather than a belt-drive which would need regular adjustment) and can chip green branches up to 50mm diameter without even dropping the revs. We’ve chipped everything less than 50mm in diameter from about ten full-sized trees with this machine, as well as a lot of prunings, and it is still on it’s first set of blades (which can be re-sharpened, but haven’t needed it yet).
Here’s the mulched ingredients on the bed, ready for spreading and digging in, after which it will be watered and covered with a thick barley straw mulch for the next month. You can see the “shorn” pigeon pea bushes in the background – they hardly look like they’ve been touched.