The soil food web

I'm reading a fascinating little book at the moment called Teaming with Microbes: The Organic Gardener's Guide to the Soil Food Web, by Jeff Lowenfels and Wayne Lewis. It's all about the complex interrelationships between organisms in the soil and the plants in our gardens.

I learned for example that plants form partnerships with particular bacteria and fungi. The bacteria and fungi feed on carbohydrates manufactured by the plant (through photosynthesis) and exuded by its roots. In return, their excreta provide nitrogen and other nutrients to feed the plant, and they coat its roots in a protective film. Individual plants actually control which organisms they attract, according to their needs!

Then the keen gardener comes along with crude chemical fertilisers and knocks the whole system out of balance. No longer reliant on the microbes, plants grow fat on the readily available nitrates poured into the ground. The microbes consequently die out. From then on, the gardener has to keep feeding the plants artificially or they won't thrive – remember, the microbes are gone.

When the microbes leave, the larger soil organisms such as earthworms leave, to the great detriment of soil structure and soil health. As the soil structure degrades, more and more of the synthetic fertiliser goes straight through into the groundwater, where it becomes an environmental pollutant.

(As an aside: ever wondered why we have toxic algal blooms in our rivers every summer? Yeah, right, it's 'natural'.)

With unbalanced nutrition, the plant is more susceptible to pest and disease attack. What to do? Spray it with various '-icides' to rectify the problem. Trouble is, they make the soil an even less hospitable environment for its natural inhabitants.

In other words, a quick fix solution has put the gardener on a chemical tread mill. From now on, gardening will be more labour intensive and expensive. It will rely on inputs which can't be manufactured within the garden, but are products of the petrochemical industry.

To my mind, it's just one more example of how organic methods are not just more effective than chemical methods, but easier and cheaper.

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.