Soft waste

For the average gardener, the easiest way to cope with most soft waste is to make a “high-fibre” compost heap. The method described here is based on that developed by the Centre for Alternative Technology (CAT) in Wales, in the UK. Green kitchen and garden waste and paper and card are essential ingredients. Any paper or card you add to a high-fibre heap should be crumpled to make irregular, three-dimensional shapes. Egg boxes and toilet-roll tubes are perfect without any pre-treatment. Start with a layer of paper and card to improve the drainage at the base, then just add paper and green waste as it comes along – no need for any distinct “layering”.

The only rules are to make sure that you have approximately equal volumes of paper and green waste, and not to add pure green waste in a layer more than 15cm (6in) deep.

Raking them in

The traditional advice on composting tree leaves is clear and unambiguous: don’t bother. Leaves from trees are low in nitrogen and calcium and high in carbon, especially in tough, indigestible stuff like lignin (the main component of wood) and tannins, so they are best kept away from the compost heap. Yet not all tree leaves are the same, and the usual advice is founded on the assumption that gardeners can’t tell them apart. The real story, for gardeners who can tell them apart, is that leaves of some trees are tough and slow to break down. However, other leaves are much richer in nitrogen and calcium and make a useful addition to the compost heap.

Good leaves Ash, cherry, elm, lime (linden or basswood), maple, poplar (cottonwood), willow.

Bad leaves Beech, birch, hornbeam, oak, sweet chestnut.

Compost for realists

Woody waste can cause such trouble that I’m going to start out by assuming you don’t have any. Later, we will see what can be done with woody waste, but first let’s consider what you can do realistically with the soft garden and household waste and lawn mowings.

The trial heaps were

• a standard, slatted, wooden bin bought from a large DIY chain store, with a volume of 0.75 cubic metre (26 cu.ft)

• a typical, local authority plastic bin, of 0.3 cubic metre (11 cu.ft)

• an open heap, also sized about 0.3 cubic metre (11 cu.ft)

All contained the same mix of typical garden waste: large, woody material was chipped and smaller material was shredded. The trial was started in late autumn: although the weather at this time is too cold to be ideal for compost-making, it is one of the few times that many gardeners will have a large quantity of waste material available.

The trial heaps were compared with the main heap at the RHS Garden, Wisley, which is enormous – at least 30 cubic metres (1,060 cu.ft). Despite air temperatures below 10°C (50°F), the temperature of this heap quickly rose to around 70°C (158°F). After two weeks, the heap was turned and the cold air admitted briefly reduced its temperature to below 50°C (122°F). But it was soon back up to over 60°C (140°F) again, and remained above 50°C, with monthly turning, for several months.

No such luck for the trial heaps. None ever achieved more than a few degrees above air temperature, with or without turning. Probably because of its larger size and better insulation, the wooden bin was the best of a bad bunch, and the plastic bin was slightly warmer than the open heap. Crucially however, none got anywhere near a temperature that would kill disease pathogens or weed seeds. So much for the bad news.

Classic composting - part 2

Which raises the awkward question: what do you do with compostable material that accumulates meanwhile? Clearly, if traditional composting is to work at all, you must have at least two compost heaps. “Fill your compost bin with at least a cubic metre of waste.” Naturally, this assumes that you have a compost bin that will hold that amount. So how big are commercial compost bins?

Many retailers sell their own version of the classic, wooden “New Zealand” box; a quick check of catalogues and websites shows that the standard box bin varies from 0.3–0.75 cubic metre (11–26 cu.ft).

You can find larger bins, up to 1.3 cubic metres (46 cu.ft), but you have to look quite hard to find them.

Plastic compost bins, of the type often provided free or at a subsidized price by local government, are generally smaller. A leaflet from my local authority lists three bins, ranging from 0.23–0.6 cubic metres (8–21 cu.ft). Fancier options, such as bins disguised as beehives and tumblers, are smaller still. I’m forced to conclude that manufacturers of compost bins do not expect the average gardener to assemble a cubic metre of compost at one go.

In fact, although this is unlikely to happen to you, spontaneous fires at commercial composting sites are not that uncommon, and are usually caused by a large heap that is (at least

in part) too dry. The message is that moisture content is critical: it must be high enough to allow composting to occur, but not so high that heat is lost too quickly. On a domestic scale, the moisture content of a compost heap is difficult to measure or control: quite small variations in moisture content are probably responsible for the rather unpredictable behaviour of the typical heap. Finally, does it work? That is, if you have survived the assault course needed to assemble a compost heap in the required manner, will it do what it should do? The Royal Horticultural Society research described previously plainly shows that it probably won’t. The only simple way of ensuring a compost heap gets hot and stays hot is to make it larger than recommended by

the books, and certainly larger than a typical compost bin.

It seems clear that classical hot composting was designed by (and for) people with big gardens, plenty of space, and ready access to outside sources of compost materials, typically animal

manures. In such gardens, classical composting is still alive and well. At the RHS Garden, Wisley, in the UK, huge quantities of waste are chipped, shredded, and mixed by special machinery, then

blended into huge heaps, typically 2x4x12m (6x12x40ft) in size, or about 100 times the minimum recommended size of the domestic heap. This makes excellent compost in about four months, but it’s a process the ordinary gardener can only dream about.

Of course, I’m not saying it’s impossible to make a traditional, hot compost heap in an average bin. It’s just that you will have to take some elaborate precautions, probably including more than one of these steps:

• importing composting material from outside the garden

• trying to make compost only in summer

• installing effective insulation around your bin

• siting the compost bin in the sunniest, most sheltered spot

• moving to somewhere with warmer summers.

At which point, the average gardener could be forgiven for

concluding that the game is hardly worth the candle. Gardening

is supposed to be fun, after all.

Classic composting

The traditional advice for constructing a classic compost heap is quite simple. First, collect together a mixture of soft, nitrogenrich waste and tougher, carbon-rich material. Shred any tree and shrub prunings.

Second, start filling your compost bin with layers of different materials, adding a spadeful of compost or soil now and then – the right micro-organisms will colonize your heap eventually anyway, but there’s no harm in giving them a helping hand. Carry on until your heap measures at least one cubic metre (1 cu.yd), or more if possible. If the heap seems dry, water it. Cover it with the bin lid or a piece of old carpet.

Your heap should soon start to get hot. When it starts to cool, turn it and it will heat up again. When it starts to cool again, turn it for a third time, and leave until done. Sounds simple, doesn’t it? Unfortunately, it’s this stately home approach that caused generations of gardeners to abandon the whole idea of making compost. In fact, this

apparently simple advice is

riddled with weasel words

that gloss over real difficulties.

“Turn your heap when it starts to cool down.”

Modern gardeners are busy people, with their compost heap somewhere near the bottom of their list of priorities. Turning a large compost heap is hard work, and doing it twice sounds like adding insult to injury. There’s also the anxiety of whether you’ve got the timing right. Is it cooling down yet? Have I left it too long?

“Collect together at least a cubic metre of waste.”

Pardon?

How often does the owner of a modern, small garden have a cubic metre (cu. yd) of waste at one time? Pause at this point and try to visualize a cubic metre of garden waste (go and get a tape measure if necessary).

“Well then,” goes the advice, “gather the materials over a

period of time.”

OK, but where do you store this stuff while you are waiting to collect enough? An even more interesting question is how you stop the green waste from starting to compost before it goes on the compost heap. Keep it in the fridge maybe?

Something else the books never mention. As described, composting is clearly a batch process, like putting a load in the washing machine. A full load of compost material is assembled and, after a time, finished compost results.

Sorting your stuff

The lists opposite are a reminder of most of the sorts of things that you might want to compost, divided into (1) soft, nitrogen-rich, (2) moderately nitrogen-rich, and (3) tough, carbon-rich material. Bearing in mind the ideal C:N ratio of 30 for composting, a heap made entirely of rich stuff will have too much nitrogen and one entirely of tough stuff will have too little, so always try to mix the two.

Rich stuff (C:N ratio of 5–25) Annual weeds, coffee

grounds, comfrey, feathers, general kitchen waste, hair, lawn

mowings, nettles, old cut flowers, pigeon manure, pond weed,

poultry manure (raw or pelleted), seaweed, pure wool or silk

clothing (shredded), soft, green garden waste, urine.

Middling stuff (C:N ratio of 25–50) Brassica and other

fibrous, green stems, citrus skins, cotton rags, eggshells

(rich in calcium), soiled bedding from hamsters, rabbits,

guinea pigs (not dog or cat litter), spent hop waste, tea bags,

used potting compost, vacuum cleanings (but not if you have

synthetic carpets), waste from distilling, well-rotted cow/

horse/pig/sheep manure, wool shoddy, young hedge clippings

and soft prunings.

Tough stuff (C:N ratio of 50–600) Bracken, cardboard,

corn cobs (crushed), hay or straw, leaves of broad-leaved trees

and shrubs, leaves of conifers, nutshells, paper, sawdust,

tough hedge clippings, waxed paper, wood ash (rich in

potash), wood shavings, woody prunings.

what else can you use?

The principal goal of composting is to recycle organic matter on the spot, without the awful waste

of energy involved in moving it around the country first. Therefore, imported materials should not be added routinely to the compost heap. On the other hand, if there are local sources of composting ingredients that are available cheaply, or perhaps even free, it would be foolish to ignore them.

If your organic waste goes to a local composting scheme, it will be composted at a high temperature under controlled conditions. The resulting compost will be a clean, safe, weed-free material that can be bought and used straightaway in the garden. However, in common with other commercial soil conditioners that are based on recycled garden or forestry waste, it tends to be slightly alkaline, so is not suitable for use around rhododendrons or other acid-loving plants.

Strawy stable manure

can go straight on the garden,

while chicken manure is high in nutrients and makes an excellent

compost activator. Pigeon manure, often available even in the

centre of big cities, is similar.

Many other sources of organic matter are useful if available locally, including bracken, sawdust, seaweed, spent hop waste and mushroom compost, and straw. All these can go straight on the garden as a mulch and will help to improve the soil structure. Bear in mind, however, that in terms of nutrient content and other properties, these materials are very different.

Points to ponder

• mushroom compost is alkaline

• dead bracken, sawdust, and straw contain virtually no nutrients

• material from farms may have had pesticides used on them: always check and, if it has, compost it well before putting it on the garden

• landowners are often happy for you to collect bracken, but always ask first

• collect only loose seaweed washed up by the tide – do not, whatever you do, remove live seaweed from rocks

Active ingredients

Compost activators are materials intended to make up some deficiency – usually of nitrogen – in the compost heap. Some proprietary compost activators claim to contain micro-organisms, but there should be plenty of these in the compost heap anyway.

Direct contact between your heap and the soil will speed up colonization of the heap by bacteria and soil animals. To be absolutely sure, add a spadeful of soil or compost from an existing heap. Cheapest and best of all activators is human urine, applied directly or diluted and added via a watering can.

Something all activators contain is nitrogen. However, if you follow the high-fibre method, your heap will already have the right carbon:nitrogen (C:N) ratio and no extra nitrogen will be required. If your compost heap does contain too much nitrogen, the excess will simply be broken down and lost as gaseous nitrogen or – worse still – as ammonia, causing a nasty smell.

However, if you are composting low-nitrogen material such as hedge prunings or autumn leaves, extra nitrogen will probably speed things up. We’ll consider the best way to add this when we look at the practicalities of making compost. As we’ve seen, many compost materials that lack nitrogen are also low in calcium, and some proprietary activators do contain garden lime. Again, the high-fibre heap doesn’t require extra calcium, but the breakdown of naturally acidic materials like woody waste, autumn leaves, and (especially) conifer prunings will be accelerated by the addition of lime.

Good activators Ammonium sulphate (cheapest option); comfrey leaves or liquid feed made from comfrey (organic); dried blood (organic); lime (ground limestone); nettle leaves; fresh or pelleted poultry manure (organic); proprietary activator; spadeful of soil or compost; urine.

Woody garden wa ste

Larger yards, especially those with long hedges, can generate huge quantities of woody waste.

We’ll consider what to do with it later, but why is it a particular problem?

First, prunings, hedge trimmings, and woody material generally have a high C:N ratio, so they are high in carbon and low in nitrogen. This in itself slows down decomposition, but that’s

only half the problem. The other half is that cellulose is only one of the carbon-rich materials in plants. Another, which may make up 20–30 percent of wood, is lignin. Lignin’s chief disadvantage is that composting bacteria are not good at breaking it down. Second, woody waste has much bigger stems than green waste, so it has a low surface area compared to its volume. This doesn’t give bacteria and other organisms much to work on.

The third problem is the exact opposite of that afflicting a pile of grass mowings, which collapses and runs out of air. A pile of raw prunings, by contrast, contains far too much air, and dries out too quickly. For some gardeners, these problems seem so intractable that composting doesn’t look like a serious option. But

don’t worry,

there are plenty of environmentally friendly ways to deal with even a mountain of woody prunings

Tricky stuff

Some types of garden waste require caution. One of the undoubted virtues of “hot” composting is that weed seeds and pathogens are killed. But traditional, hot composting is hard to achieve in the average garden, so what do we do with such problematic ingredients? Fungal and bacterial pathogens are the worst, but we do need to keep a sense of perspective.

Pests and diseases that live on leaves and stems don’t enjoy life in the compost pile, so you don’t need to lose much sleep over them. Mildew and black spot are unlikely to survive a long spell

in the average compost pile. On the other hand, most soil pests are quite at home in the pile, and are all too likely to survive and be spread around the garden, as are overwintering diseases. There really is no alternative to keeping material with these problems out of the compost pile.

Not very satisfactory solutions are to burn them or bury them in an unused corner of the garden. It’s better to add them to a habitat pile, a heap of miscellaneous woody waste intended

primarily to provide a wildlife habitat.

When looking at the examples listed opposite, bear in mind that they are for guidance only: no list could possibly be complete, and most available advice on what can be safely composted

is anecdotal and contradictory. Also, the longer a pile is left, the smaller the chance that anything horrible will survive. And last but not least, beneficial microbes in mature garden compost are quite effective at controlling many common diseases. But,

if in doubt, leave it out.

What goes in Material damaged by aphids, black spot, brown rot on fruits, canker, gray mold, leaf miners, leaf mites, mildews, potato blight, sawfly larvae.

What stays out Material that might harbor carrot or cabbage root fly pupae, clubroot of brassicas, eelworms and their eggs, honey fungus, pests that overwinter in soil such as pear midge grubs, root rots of beans and peas, rusts, and viral diseases.

From your garden

All normal green garden waste can be composted—after all, disposing of such material is one of the main reasons for composting in the first place. Nearly all of it can go on the compost pile without any treatment, but tough stuff like brassica stalks should first be chopped up, or smashed, with a club hammer or spade. The same goes for tough kitchen waste such as corn cobs.

What about weed seeds? Well, they are not such a serious problem. Even a classic, hot compost pile never achieved a 100 percent kill, so gardeners have always had to live with weed seeds in compost. Of course, the best defense is vigilance in the first place, so that weeds end up on the compost pile before they get around to setting seed.

Also, keep a lookout for weeds growing near the pile itself, or even on it. Trials at the Royal Horticultural Society in the UK suggest that most of the weed seeds found in an open compost pile were not there at the start, but blew in while the pile was rotting down.

From your house

Most types of paper and cardboard make excellent compost

ingredients. Large quantities of flat paper should be avoided, and

in any case are more suitable for conventional recycling. However,

such materials may be used in moderation if you are short of other

types of waste paper—printing inks no longer contain toxic

heavy metals.

Better for compost are those types of paper that are harder

to recycle, such as used tissues and cereal boxes. Birthdays are

bonanza times for the serious composter, with all that wrapping

paper. Cardboard with laminated plastic should be avoided: the

cardboard will compost okay, but you will have the annoying job

of fishing the plastic out of the finished compost.

And if you’re alarmed by media stories of identity theft and

are worried about what to do with all those old bank statements

and credit-card bills, here’s the perfect solution—compost them.

What goes in Cardboard packaging, egg cartons, old

greeting cards, shredded documents, toilet-paper rolls, used

tissues and kitchen towels, waxed paper, wrapping paper.

What stays out Laminated cardboard such as juice and

milk cartons, large quantities of newspaper, telephone

directories, very shiny magazines.

Compost ingredients

Making compost is like cooking: the secret is in the correct ingredients, plus knowing what to leave out. But don’t worry—unlike baking a cake, getting the proportions right is not critical, baking time is up to you, and garbage ingredients are a positive advantage.

from your house

The house, and particularly the kitchen, is a major source of compost ingredients. Any green (or orange or yellow or purple) vegetable waste may go on the compost pile. Do not be put off by the tough appearance of things like grapefruit skins—they will compost just as quickly as any other vegetable matter. There’s also no problem with leftover cooked vegetables.

Theoretically, waste meat or fish (cooked or uncooked) can also be composted, but your compost bin must be animal-proof. In practice, it’s quite hard to construct a compost bin that cannot be penetrated by a determined rat.

What goes in Coffee grounds; old cotton, silk, or wool

clothing (torn up); eggshells; floor sweepings; hair and nail

clippings; paper and cardboard; rabbit or hamster bedding;

tea bags; vacuum-cleaner dust; vegetable waste; wood ash.

What stays out Cat or dog litter, coal ash, dairy

products, disposable diapers, fish and meat waste,

oil or fats.

The Calcium factor

The basic story of compost emphasizes the carbon: nitrogen ratio, and rightly so—this has to be at least nearly right, or nothing will work very well. However, the C:N ratio is far from the whole story, and there is one element that has not been given quite the attention it deserves: calcium. Why might calcium be important in the compost pile? One product of the breakdown of organic matter by bacteria is organic acids. In a well-aerated compost pile, these organic acids are themselves broken down in the later stages of the composting process, but if there is any shortage of oxygen, they can begin to accumulate, making the compost acidic. This is a problem because bacteria do not like acidic conditions at all.

The pH scale measures acidity or alkalinity, and goes from 1 (very acidic) to 14 (highly alkaline). Pure water has a pH of 7 and is described as neutral. Soil pH depends on geology and climate: acidic rocks, like granite or sandstone, and high rainfall result in acidic soil; calcium-rich limestone and low rainfall lead to alkaline soil. A soil pH of 6 to 7 is ideal for most plants, but there are some specialized species that grow best on acidic or alkaline soils. Few plants thrive on soils with a pH of less than 4 or more than 8. A roughly neutral pH is perfect for composting bacteria, but they are happy in alkaline conditions, up to pH 8.5. Which is why materials ontaining calcium are great for the pile. Acidic soil can easily be corrected by adding garden lime (ground limestone), but there is no simple way of making an alkaline soil more acidic. If your compost pile is acidic because of poor aeration, it’s far better to treat the cause rather than attempt to treat the symptoms by adding lime. However, although most green waste contains plenty of calcium, some compost materials are naturally low in calcium.

For example, fall leaves of oak and hornbeam are relatively rich—for tree leaves—in nitrogen, yet do not make good compost, which makes sense when you discover that both are low in calcium. Woody waste such as hedge prunings, especially of conifers, is also short of calcium, so here there is a case for adding lime to help it break down.

The correlation isn’t perfect, but materials that are high in calcium also tend to be high in nitrogen, and no low-calcium materials contain more than moderate levels of nitrogen.

High in calcium Annual and perennial weeds; brassica leaves and stalks; kitchen vegetable waste; leaves of ash, cherry, elm, hawthorn, linden, maple, and rose; seaweed.

Low in calcium Bark, bracken, and ferns generally;

conifer needles; hay and straw; leaves of beech, birch, hornbeam, and oak; moss; wood.

Waste paper is the answer to a composter’s prayer. just enough water

Soil animals and microorganisms need water. The bacteria and other microbes live in the film of water that covers compost particles, and most soil animals especially worms, the most important ones—are very intolerant of drying out. So obviously the ideal compost pile needs to be at least moist. But not too moist—too much water conflicts with the need for air, and there is no faster route to a smelly, anaerobic compost pile than waterlogging. The ideal water content for rapid composting is about 50–60 percent by weight, but knowing that isn’t very helpful, is it? In practice, this is the water content of a well-wrung sponge. How does this prescription square with the water content of typical compost materials? “Not very well” is the answer.

Kitchen waste and grass cuttings are at least 80 percent water, so a pile made entirely from such materials would be too wet, which would contribute to a lack of oxygen. You could solve the problem by adding drier, woody waste, shredded or otherwise, but this would slow things down too much. The solution once more is paper and cardboard, which not only lowers the average water content of the pile, but also soaks up the liquid that is released as the softer materials start to decompose.

Why compost?

There are plenty of very good reasons for gardeners to make compost, but above all, it spares the environment the damage caused by
burying or burning waste, reduces the need to destroy natural habitats by excavating peat, and saves you money. don’t dump it, Recycle it US household garbage adds up to more than 200 million tons a year, or roughly 5 pounds (2 kg) per person per day, of which about a quarter is recycled. Only about ten percent of British household waste is recycled, which puts the UK near the bottom of the European league. Some countries, like Germany and Denmark, do much better, although a few are even worse. Most waste is incinerated or ends up in landfill, yet about half of all household waste could be composted. At the same time, every year gardeners around the world buy millions of tons of growing media, soil conditioners, and mulching materials, much of it based on peat. A high proportion of this could be replaced, free of charge, if gardeners started recycling what they now simply throw away. Incinerated waste is returned immediately to the air as carbon dioxide (CO2) and other gases, causing air pollution and contributing to global warming. Organic matter in landfill also slowly decays, but the main product is methane, an even worse greenhouse gas than CO2, and 25 percent of US methane emissions come from biodegradable waste in landfill sites. The soil is also the safest place for waste organic matter. Once returned to temperate soils as compost, some organic matter stays there, often for a long time. More widespread use of compost by farmers and gardeners could make a small but significant contribution to the commitments made in the Kyoto Protocol of 1997 to reduce CO2 emissions.

Treat your Soil

It’s hard to believe just how many benefits compost provides for soil. Even when spread on the surface as a mulch, and long before it actually enters the soil, compost elps to suppress weeds, retains soil moisture in dry conditions, and protects soil from the damaging effects of wind and heavy rain. But it’s when organic matter gets into the soil that it really starts to work its magic. Soil structure depends almost entirely on organic matter. In healthy soil, the organic humus and inert mineral particles are stuck together in tiny crumbs a fraction of an inch across. These crumbs are held together by fine fungal strands, or hyphae, and by organic glues produced by trillions of bacteria.

Leaves that are low in nitrogen and calcium, like those of larch and this pine, make truly awful compost.

materials containing calcium are great for the pile. Acidic soil can easily be corrected by adding garden lime (ground limestone), but there is no simple way of making an alkaline soil more acidic. If your compost pile is acidic because of poor aeration, it’s far better to treat the cause rather than attempt to treat the symptoms by adding lime. However, although most green waste contains plenty of calcium, some compost materials are naturally low in calcium. For example, fall leaves of oak and hornbeam are relatively rich—for tree leaves—in nitrogen, yet do not make good compost, which makes sense when you discover that both are low in calcium. Woody waste such as hedge prunings, especially of conifers, is also short of calcium, so here there is a case for adding lime to help it break down. The correlation isn’t perfect, but materials that are high in calcium also tend to be high in nitrogen, and no low-calcium materials contain more than moderate levels of nitrogen.

High in calcium Annual and perennial weeds; brassica leaves and stalks; kitchen vegetable waste; leaves of ash, cherry, elm, hawthorn, linden, maple, and rose; seaweed.

Low in calcium Bark, bracken, and ferns generally; conifer needles; hay and straw; leaves of beech, birch,

hornbeam, and oak; moss; wood.

What every gardener wants: compost.

Somewhere in this process—between the large amount of original material and the tiny fraction that is left as humus— lies The entire composting process can be seen on any woodland floor. On the surface are freshly fallen, unaltered leaves. Dig down a little way and the leaves become more fragmented and much less distinct. Eventually, you will come to a dark, crumbly material in which the original leaves are quite unrecognizable. This is usually called leafmold, but it’s just a special variety of compost, made only from tree leaves rather than the usual mixture of materials.