Composting


Compost = home grown fertility

Organic gardeners avoid using modern synthetic fertilisers, and rely primarily on organic matter to feed their plants. Fresh organic matter has to be processed or 'composted' in some form to release the plant nutrients it contains.

Even though composting depicts decay of once living materials into constituent ingredients, it is useful to think of compost as a living organism, at least whilst it is being made. It will give you an insight as to how composting is best controlled and the processing time minimised.

Aims of good compost making

The idea of composting often brings up visions of ‘muck and magic’, but the aim of good composting is, in a few months, to produce a rich soil-like material full of plant nutrients.

Well-matured compost is not unpleasant to handle, and has  a soft, pleasant  earthy smell.

Needs

Think of 'working' compost as a living organism and its needs are the same as your own:

Does this list look familiar ?

Ingredients

The secret behind successful composting is to use the right mix of materials and provide a suitable environment for the composting microbes to live in. Warmth, moisture and air are all very important.

The correct mix of raw materials is one in which 'greens' are mixed with more fibrous ‘browns’ at the rate of four parts (by volume) of ‘greens’ to one of ‘browns’. Too many greens, especially grass clippings can lead to a smelly, slimy mess which will take a year to compost.

'Greens' 'Browns'
grass clippings
leafy hedge clippings
green weeds
vegetable trimmings
seaweed
fresh horse or farmyard manure
urine
green manure crops
straw
dry grass or hay
shredded prunings
dead leaves
sawdust or shavings
cardboard
cotton rags

Beware - do not put cooked food scraps or meat or cat and dog faeces into the compost heap.  They will compost but they may attract vermin (eg rats) or possibly lead to a risk of disease.

Other pet manures such as from rabbits, hamsters, guinea pigs, gerbils are excellent compost materials, but not cat and dog faeces.

If you intend to use large quantities of grass clippings for compost, then make sure they are well mixed with 'brown' materials such as straw, and that the heap is turned regularly.

Unless you can be sure of a hot composting process it is better not to put the roots of perennial weeds such as couch grass (scutch) into a compost bin.

Newspaper and cardboard can be composted in small quantities as long as they are well mixed with 'greens'.

Compost activators and additives

A small amount of soil mixed into the heap will add soil microbes and contribute to the composting. Mature compost or well-rotted manure also acts as an activator by 'seeding' the new heap with the right microbes.

A sprinkling of lime mixed in will help to keep the heap 'sweet' by stopping the heap getting too acidic. High acidity slows down the composting process.

Seaweed is an excellent compost ingredient, mainly contributing trace element and minerals.

Not for the squeamish - urine is a good compost activator and a lot cheaper than buying compost activators from the garden centre. It contains nitrogen.

Activators are added to compost bins to 'activate' the compost and get it going quickly.  They are normally materials rich in nitrogen such as comfrey leaves, grass clippings, poultry manure, urine, young weeds, nettle tops.

Composting process

Once composting starts, the temperature in the bin rises very rapidly to 60-70°C. Turn the mixture with a fork after about two weeks to get more air into the material and keep the heating going.

It is this heating process which kills weed seeds, disease organisms and perennial weeds.

It might need turning again a few weeks later. It will then slowly cool down, and as it does so the materials will become progressively unrecognisable for what they originally were.  As the compost finally matures, manure worms and other creatures will move into the compost.

Once the compost looks, feels and smells like soil it is ready to use.

Generally speaking, the longer the temperature stays high, the shorter time will be needed for composting to reach completion.

Making the compost

BIG IS BEAUTIFUL

The bigger the volume you can start composting at one go the easier and quicker the process will be. Collect ‘browns’ in advance and then mix with the ‘greens’ to fill the compost bin. Add water if required so that the mixture is like a damp sponge, and cover.

Alternatively, though the overall process will be slower, layer ‘greens’ and ‘browns’ as they become available.

Shredding is useful for woody materials (browns) as the microbes have a bigger surface area to work on.  It also allows you to compost tougher materials than you might normally do.

Compost containers

What about a compost bin. For small gardens you can buy nice plastic or wooden bins, but a cheaper alternative is a plastic dustbin. Drill holes around the bottom to let air in. For large gardens which produce a lot of compostable material, sheep wire around four fencing posts in a square, and lined inside with polythene or cardboard, makes a cheap, though not particularly attractive, bin. Whichever type you use put a six-inch layer of sticks at the bottom to help provide aeration.

Good insulation and some form of roof or cover will keep the heat in and the water out.

A range of commercial compost bins are available. Some of them may be difficult to use due to their small size - ie they don't hold enough material to heat up properly.  They can be quite expensive, though they can be more attractive than some home-made bins.

Compost tumblers may also suffer from small volume problems, though they do give good aeration of the composting material.  Very large tumblers will be heavy to turn.

Wooden bins can be home-made to a suitable size and can incorporate ventilation holes. These should not be so large that they let too much heat out as they let air in.  A slatted removable front makes removal of the finished compost easy.

A large compost bin can be made from old, but sound, wooden pallets. They may need to have the large gaps between planks filled with planks from other pallets to prevent heat loss. The resulting double wall gives good insulation. 

Compost bins made of mortared building bricks or blocks are obviously more permanent. They have good insulation properties.

Straw bales can be made into a temporary compost bin - temporary in that it will last one year before rotting.  It can then itself be mixed with fresh material in another bin - a compostable compost bin !

The bales are laid in a square or rectangle arranged like brickwork, and then covered with a polythene cover.

Siting

Depending on the construction, a compost bin may not be the most attractive of garden features.  Generally, site it well away from the house since at times it may give off its own distinctive smell !

Using compost

Garden compost can be used in the same way as well rotted farm yard manure as a basic nutrient and organic matter provider.

Compost also makes a top class feeding mulch for all plants.

If you have only small quantities then it is probably best used for making your own organic seed or potting mixture.

Nutrient content of compost

Well-made garden compost has a similar nutrient composition to well rotted farm yard manure.

Anaerobic composting

The normal idea of composting is a process which heats up, requiring air for the microbes. However, compost can be made in the relative absence of air, though it will take up to a year to complete, and the material will often be wet and heavy. It may often have a 'sour' smell at times.

In anaerobic composting a different set of microbes break the fresh material down, at a much slower rate.  This often happens naturally when a large mass of grass clippings put into the compost bin heat up for a short while but then compress down sealing the air out of the material.  You may also find layers of grass which are yellow-green and have not broken down at all.  When this happens you have preserved the grass by making a layer of silage. Breaking this up and letting the air in will start it composting.

It is good policy to turn an anaerobic heap in mid-late winter to get air in and complete the composting.  Beware - it is very heavy to turn when wet.  After turning it keep rain out to allow it to dry out before use.

Sheet composting

Fresh organic materials can be put into layers on the surface of the soil over the winter.  You might cover it with polythene.

The material will break down slowly, but nutrients may be washed out, and it may also attract vermin. Slugs may also breed underneath it and birds may spread it all over the place.

Trench composting

Trench composting is an old technique in which a trench is progressively filled with material for composting and covered with soil as the trench is filled.  It can be useful for preparing a high organic matter site for hungry crops such as runner beans.

Disease free semi-hard materials such as Brussels Sprout stumps can be put in the bottom where they will break down over a couple of years.

Worm composting

Worm composting uses brandling or 'manure' worms to break down normally smallish quantities of kitchen waste in a specially prepared 'worm bin'.

It can be done on a large scale, but requires large quantities of the correct mix of materials and a suitable covered site.

The compost (worm casts) from the process make excellent solid plant feed or a nutrient source for seed or potting mixtures, and the drainage liquid makes good liquid feed.

Leaf mould

Leaves break down slowly into a nutrient-poor leaf mould. Well broken down and sieved, it makes a good peat substitute in seed composts.

A suitable bin can be made from four posts and wire netting.  No insulation is needed, but a cover is needed to prevent the leaves blowing away.

Note: a lot of leaves make only a little leaf mould.

Hot beds

Victorian gardeners used hot beds for growing early crops, either outside or in a greenhouse.

In essence a hot bed is a large stack of fresh, strawy horse or farmyard manure which is layered with and covered with soil.  A cold frame is placed on top for growing the crops in.

As the manure composts it gives off heat which keeps the frame on top warm. It also provides nutrients for the growing crops.

The hot bed will slowly shrink and sink as it matures, and once cropping has finished the resulting 'compost' can be used like normal compost.


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