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  • Sue

Did you smell the petrichor?

It is raining heavily today - at last. The garden is grateful and, as the first rain fell, the soil gave up a distinctive smell - called petrichor. This is given off by a soil bacterium (called streptomycene) when soil conditions are good. Tiny creatures (called springtails) respond to this, ingest the bacterium and carry its spores off to other part of the soil, spreading the benefits around - a little like bees pollenating other plants as they fly from flower to flower extracting nectar.

The purpose of that little diversion was to demonstrate that life in garden soil is complex and interwoven and it is vital that, as gardeners and farmers, we do our best to maintain and improve the ground we work. Healthy soil gives good crops, beautiful flowers and strong trees. It also emits tiny microbes which make us feel good as we work it. We neglect our soil at our peril and cannot work it year after year without putting back the goodness we take out. Neglect or mistreatment has resulted in soil erosion, where it is washed away, sterility, where all goodness has been extracted with no return, pollution where we flood it with chemicals such as nitrogen to maximise our crops short term at the expense of the overall biosphere and destruction of the balance of insects, microbes, mycelia and bacteria because we don't like them - without realising that they play an essential part in life underground.

One of the best (and cheapest) ways of enriching garden soil is though creating compost from garden waste and then spreading it onto the soil as a mulch.


Compost is easy and cheap to make, but time consuming, unless you buy a hotbin or rotating composter, and there are a few rules you must follow:

You need space for several bins and each one should be at least a metre cubed. If they are smaller than that then the composting material will not be able to generate enough heat. I would suggest that you have three, but two will do. Ideally make them from something like wooden pallets or corrugated metal sheets and position them side by side, with shared side walls (a little like a row of terraced houses) so that they insulate one another and plant material can easily be transferred from one to another. The bins should be open to the earth at the bottom - don't put them on a concrete base, for example, because you want to encourage worms, insects, microbes etc to move in. They should also be open fronted, but with a shutter to retain the contents while they are composting.

Start with bin no.1 and gradually fill it with suitable waste material. I find it is quicker and more manageable if the material you put into the no. 1 bin is cut up into smallish bits (say, not more than 20cm long).

The following things are good for composting: Grass cuttings, annual weeds, plant prunings, paper and cardboard (shredded and which has not been chemically treated), vegetable peelings except for potato peel (if you grow vegetables), chicken manure (which is fantastic because it is rich in urea), straw.

Do not compost: Perennial weeds such a couch grass or dandelions, cooked food of any kind, raw meat, fish and dairy products, thick branches, potato peel (if you grow vegetables). Anything inorganic or containing plastic.

Try to layer and vary the nature of your compost bin materials - the ideal is to achieve a roughly equal balance of dry, brown compost materials (dead plant prunings, dead leaves, paper and cardboard) and green composting materials - grass cuttings, green plant prunings and vegetable peelings. It is a little bit like making a cake mix or muesli which then, of its own accord, incorporates microbes, worms and insects etc. and takes on a life of its own. You are aiming to get a good balance of wet and dry matter. Too wet and green and it will rot down into a sludge, too brown and dry and it will have difficulty turning into compost at all and will stay coarse and twiggy. Turn it over with a garden fork every now and again to mix it all up and keep it covered with a tarpaulin or bit of old carpet/big piece of cardboard. This is to keep the heat in and stop it drying out. if it gets too dry then you will need to water it to keep things moving forward.

Once it has started to rot down - it will shrink in volume and start to develop a uniform colour and texture, but will still be too coarse to call compost - shovel it all into the No. 2 bin, which should be the one immediately next door. I find this takes about 4-6 months. You then keep mixing that, keeping it covered and moist but not wet, while you begin to fill up No. 1 bin with new base compost ingredients. 4-6 months later tip the contents of No. 2 bin into No. 3 bin and No.1 bin into No 2 bin. Start putting new raw ingredients into No. 1 bin and managing the other 2 bins until you are happy that the contents of No.3 bin are ready to use as mulch - i.e. they are sufficiently broken down, brown and crumbly enough to put onto the garden. You should see (in No 3 bin) little red worms which are a good sign of healthy compost as well as other insects and creepy crawlies. Learn to love them, they are part of your work force.

It will take you about a year to get from start to finish, but it really is not an exact science as so much depends on what you have put into the bins, how often you turn them and the weather. But what you end up with is mulch, which you add to the garden at this time of year.


Mulch is something which covers the soil surface. It comes in all sorts of shapes and sizes, depending on what you want it to achieve. There are ornamental mulches such as gravel, pebbles or clay granules. Bark, textile and coconut shell mulches which suppress weeds and, finally, compost mulches, such as the one described above. They all have some common purposes - weed suppression and moisture retention for example - and you choose your mulch according to the results you are after, but the one which feeds your soil, suppresses weeds, helps retain moisture and texture both as a surface covering and because it adds humus to the soil plus manages to look good is your own, cost free garden compost. And the best time of year to add it to your garden in now.

As you work around your beds and borders in late winter/early spring, moving plants, removing weeds and generally tiding up in anticipation of the growing season add a mulch of garden compost to your soil. Spread it thickly (say at least 5 cm deep) across bare soil, ensuring that you do not put it tight up against the collar of any plants. Make sure the ground is wet before you apply it because mulch will reduce evaporation of moisture from the soil, but could also stop water penetrating. And ensure that you have removed all perennial weeds from the soil because they will grown back with vigour if left undisturbed.

It is better to mulch a small area of your garden well than try to cover all of it too thinly. Next year do some, or all, of the soil that you missed this year. There is no need to dig it in, but it will disappear over the course of the year as earthworms drag the tiny particles down into the soil where they will enrich the soil structure, adding nutrients and making it more water retentive.

I never have enough garden compost mulch, so depending on what various areas of the garden are crying out for I supplement it with leaf mould (great under trees), spent compost from last year's pot plants, chipped branches, well rotted manure I have begged or bought from farmers and even shop bought bags of compost. But nothing beats the stuff you have made yourself, so, this year, build some compost bins, create some mulch and get down and dirty. Your garden will thank you for it.

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