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.

Perma-thoughts, 2

Here's an example of how my Permaculture course has changed my perspective on elements of the Patch:

On the northern boundary we're establishing a windbreak against hot, drying winds. At the moment, this consists of 15 Flinders Ranges wattle saplings. All but one are doing well, and should start to be effective in the next 24-36 months. I was quite pleased with this idea when I first thought of it: 'Good idea to create a microclimate, Steve!' I thought.

Will eventually be a windbreak: 15 tiny wattle seedlings , Aug 2011

Now, applying some Permaculture principles, I can see ways to make it a whole lot better:

Principle: Build diverse, stable ecosystems.

OK, a 15 x 2 metre windbreak (eventual mature size) isn't exactly an ecosystem. But what if I planted sheokes on the leeward side of the wattles, and grew a small-leafed (because not rampant) hardenbergia up the sheokes when established, and dug a swale to catch water runoff, and sowed the bank of the swale with dichondra and native violets, and planted blueberries in the lee of the windbreak? And put logs in the swale to provide habitat for lizards and amphibians?

Would this be an ecosystem, or just a collection of plants and landscape features? Let's have a look:

The wattles and the sheokes together will make a more effective windbreak; both are nitrogen fixing. When they are cut back (mulch for the veg beds), some of their root mass will also die off, making nitrogen available to the soil. Hardenbergia is also nitrogen fixing, so we have three plant species improving the soil. Blueberries on the leeward fringe of the windbreak will benefit from the added nitrogen, and also the acidic environment provided by the mulch of sheoke needles.

All of these are flowering plants and will provide forage for bees and nectar-eating birds. In addition the swale should become an ephemeral wetland, attracting the numerous frogs from the surrounding area - we're lucky to have two established wetlands to the northeast and northwest of the site.

So this begins to look like a functioning ecosystem, with beneficial interactions between the various species. In time further species would be introduced (or colonise by themselves) and it would become more diverse.

Principle: Every element of a design should function in many ways.

This is also referred to as 'stacking' functions. How does our windbreak fit in with this?

As envisaged above, it now serves the following functions:

• reduces the impact of hot, drying winds
• provides mulch for veg beds
• provides forage for bees (pollinators, suppliers of honey)
• provides habitat for insectivorous birds, reptiles and amphibians (pest control)
• provides fruit
• improves soil nitrogen
• improves water infiltration
• removes the need for mowing (saves labour)

Now, not all aspects of the plan will turn out as envisaged. For example, it may be that blueberries just won't flourish where I want them, so I would have to use other berries. That would tie in with other Permaculture principles:

Apply self-regulation and accept feedback.
Use small and slow solutions. (Otherwise referred to as incremental design.)
Creatively use and respond to change.

Soil

For the past couple of years I've been paying more attention to the soil in my garden – 'soil' is a short word, but what is it exactly?

Soil is an amalgam of tiny particles of rock with humus (decayed vegetable matter), fungi and bacteria. This provides a substrate for plants to anchor themselves in and a sponge to capture water, making it available to the plants' roots.

Soil contains a wide variety of nutrients which are essential to healthy plant growth, including the big three – potassium, nitrogen and phosphorus. Then there are the secondary nutrients calcium, magnesium and sulphur. Thirdly, there's a range of micronutrients: boron, copper, iron, zinc, chloride, molybdenum and manganese. 'Micro' because they are only present (and needed) in tiny quantities. They're also called 'trace elements'.

So far, so complicated. But we're not done yet. Not only do plants need all these nutrients in varying quantities, it appears that they also need the help of a type of fungus to enable them to digest their food. This symbiotic relationship is called mycorrhizal (fungus and root).

So there's a lot more to 'soil' than just some brown crumbly stuff. But how can I apply this knowledge practically in the garden?

Well, I monitor soil pH (acidity/alkalinity), because I know that it affects a plant's ability to take up nutrients. A pH test kit is quite cheap and easy to use. An excessively acidic (under 5.5) or alkaline (over 7.5) soil can be corrected by the addition of lime or sulphur respectively. I check the soil pH about once a year.

Other than measuring pH, soil care is fairly unscientific in my garden. I adopt a variety of approaches, hoping that they will all help.

• I try to get as much organic matter into the soil as possible, through the use of green manure (a crop which is grown, slashed down and dug in to feed the soil) and home-made and bought-in compost.
• I boost micronutrients through the addition of rock dust.
• Now that my worm farm is flourishing I can also add worm castings and worm 'wee' to the soil, hopefully boosting its population of beneficial bacteria.
• The organic mulches I apply, such as lucerne and pea straw, protect soil microbes and plant roots from overheating and desiccation, and add some carbon and nitrogen as they break down.
• I try to avoid deep digging, which brings sterile subsoil to the surface.
• I'm experimenting with the use of comfrey and lucerne - deep-rooted perennials - to recover lost nutrients from the subsoil.
• I don't sieve soil unless absolutely necessary, because it disrupts the soil structure and can produce a hydrophobic surface.
• I'm very careful about what I put in and on my garden soil. No detergents, no poisons like glyphosate (Roundup or Zero), which may harm beneficial bacteria.

As well as these basic measures, I've recently started to experiment with two new products which have come on the market: biochar and Bactivate, a bacterial fertiliser.

Biochar is the 'black earth' that is supposed to have given the naturally poor Amazonian soils great fertility, supporting intensive agriculture in pre-Columbian times. Basically it's charcoal, in a form which binds water and nutrients, making them available to plants and improving soil structure. Bactivate is also carbon based: it's pellets of coal dust coated with beneficial soil bacteria.

By the use of all these methods, I'm hoping to transform the rather poor, sandy and hydrophobic soil I inherited for my vegie patch into a rich, fertile growing medium. Comparison with other parts of the garden suggests that I've already made quite a bit of progress.

What I'd really like is the rich, dark soil of my mum's and stepdad's garden in western France. Come spring, the vegies in the Limousin leap out of the ground at an astonishing rate. However, it's taken probably 3,000 years of organic cultivation to make that soil, so I may have a way to go yet!

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.