Sunday, August 8, 2010

Feed the soil with green manure crops

At yesterday's Harvest Basket I was talking with Linda and Dave about soil. They have been in their house about a year and are getting a veggie garden established in their front yard. Up until now it has been lawn and the soil sounds as if it is poor and sandy.

I suggested growing a green manure crop to remedy this, but didn't have time to elaborate.

Although a lot of experienced gardeners use green manure regularly, it sounds like there are plenty out there who don't know about this great, low-cost and low-effort way of building a rich, living soil in their veggie patch.

The basic idea is as follows: in a bed that you don't need for a few months (at least 10 weeks), you sow seed very thickly to produce a lush, green blanket of young plants. These will out-compete any weeds that might germinate in your soil.

It's a good idea to net the bed until the seedlings are up, otherwise the pigeons and sparrows will have most of your seeds. You'll end up with fat, healthy pigeons but little else to show. (I've often considered harvesting the pigeons, but Susan isn't keen.)

When the young crop is about 20 cm high you slash it down and dig it into the soil, where it rapidly breaks down, providing food for microbes, earthworms – and of course the veggie seedlings you're going to plant into the bed.

It's important to dig in the green manure crop before it starts to get tough and woody, otherwise it will take a lot longer to break down.

Green manure is a way of keeping the soil 'working' until you need the bed. Watering needs are lower than with a food crop; if you sow the green manure in the colder months you shouldn't need to water it at all once the seed has germinated, unless there's a very dry spell.

Typically a green manure mix will contain legumes such as beans, peas, lupins, with grasses like barley and rye. These nitrogen-fixing plants are ideal for a bed where you intend to grow hungry plants like cucurbits (pumpkins, zucchini etc) or brassicas. Van Loons sell a good mix in 500g bags, or you can use your own left-over seeds from last year.

You can also choose your green manure for specific purposes. For about 3 years now I've been using mustard as a bio-fumigant green manure. It is supposed to clean the soil of fusarium (causing wilt in tomatoes etc) and repel harmful nematodes. It seems to work: I've had great, healthy tomato plants ever since and haven't lost one to wilt, whereas I used to lose a couple of plants each year prior to this. (Typically they would grow well until first fruit set, then the leaves would curl and turn purple and the plant would die.)

Diggers do a bio-fumigant mustard mix, and Green Harvest do a whole range of different green manures for specific requirements.

Quick update on the wicking bed (see July post)

I've been adding salad and herb seedlings to the bed these last couple of weeks and they're all looking fairly perky. We've had 36mm of rain this week but the bed appears to be draining well. I suspect that the sandy soil mix has been important in this regard, even though it went against all my instincts - I'd normally have a rich mix of compost, lucern straw etc in a raised bed. It will be interesting to see how things go once we get into the dry, hot summer months.


  1. That is a sensational explanation Steve!
    I've often thought about planting green manure crop but really wasnt sure how to go about it. Your little blog really expains it well so thanks for the advice!

  2. Can I plant the green manure crop anytime or do you mostly plant in winter - I'm thinking about the mustard idea, sounds good.

  3. Hi Meredith

    You can plant a green manure crop any time, but in the warmer months you'll need to water it.

    If you're aiming to plant out tomatoes in December, there's just about time to sow a mustard bio-fumigant now.