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Why make compost? Well, to start with, because it saves you money! It takes all those components of weeds, prunings, lawn mowings, kitchen waste that you throw away and converts them into plant nutrients that is easy for plants to use.

Making compost  is not difficult. If you're not sure of the value of compost then read on...

Trials carried out at Rodale, an American Horticultural/Agricultural College on three similar plots, comparing chemical fertilisers with compost/manure combinations found that nitrates applied in the conventional manner were initially at higher levels than the plants needed and that harmed the young seedling roots. Then some of the excess was washed away so that as the plants grew there was insufficient nitrate when they needed it. The nitrates which were washed away polluted water supplies too. By comparison the manure/compost trials showed that compost and manure, because they dissolve more slowly, provide the amounts that the plants needed as they needed them. That makes stronger and more disease resistant plants. 

Compost will improve the soil texture so that it retains more moisture coupled with better drainage so that you don't get water-logging that damages plant root systems. But more than that compost is a living substance that improves the balance in favour of the beneficial bacteria in the soil.

One final reason for making your own compost is an ecological one. In our consumer society there is an ever increasing amount of waste material that council collectors dump onto landfill sites. The compostible materials mixed with the other rubbish produce methane in quite large quantities during the rotting process, with the problems that implies. Some local authorities do tap this methane to produce useable energy but generally it goes into the atmosphere and increases global warming. And, it is wasting an important resource. 

Basics of the Process

Composting relies upon Oxygen - it is an aerobic process. (Without Oxygen it becomes anaerobic and smells - if your heap is smelly, rather than sweet smelling, then it needs to be aerated by stirring the contents to introduce more oxygen, add water to keep the heap moist at the same time.)

There are two basic ways of ensuring that your compost gets adequate oxygen: Building the pile correctly will ensure air can reach the centre of the pile. When a pile gets too little oxygen, it will go anaerobic. By turning the pile at intervals you introduce fresh air and oxygen to the centre of the heap. This has other beneficial effects: 

Composting relies upon a combination of creatures, including slugs(!), beginning the break down of the materials on the heap. It is taken to its final stages by worms and various micro-organisms, fungi and bacteria mainly. 

They all need water, but the amount of water is important, too dry, and the decomposition process will become very slow; too wet, and it will prevent sufficient oxygen reaching the materials and the process becomes anaerobic and smelly. A good nose is useful in keeping your compost heap on the right track! {:-} The ideal water content is about 50% - the easy way to tell is by grabbing a handful of leaves and squeezing them. If no water appears between your fingers then it is too dry, if water drips out easily then it is probably too wet.  You are looking for a small amount of moisture, rather like a wrung out dish-cloth (it helps if you are an experienced washer-up! {:-})

Micro-organisms in this context include bacteria and fungi. They need energy which they get from the carbon which they find in leaves, twiggy prunings and wood chippings. Their other major need is for nitrogen which they find in lawn-mowings but too much nitrogen can generate ammonia (another use for a good nose) and can pollute water leaching out of the heap.

It is important to get the balance between carbon and nitrogen right. If high nitrogen materials such as grass clippings are used, they must be thoroughly mixed with a source of carbon. The nature of the source of carbon is also important in this relationship, the carbon in leaves is much easier to digest than the carbon in wood chippings.

The micro-organisms generate heat as they convert the raw materials to compost. The critical temperature is 140ºF where the organisms start to die. Turning the pile before temperatures reach this point will prevent overheating. Eventually, the micro-organisms will have converted most of the waste and the process will slow and the temperature will gradually drop. The compost has become dark and has a soil-like texture, none of the original components, except the largest twiggy bits will be recognisable. If it is left it will continue to improve until it is ready for use.

The ideal ratio of Carbon to Nitrogen is about 30:1 with moisture hovering between 40-60 per cent. If you can arrange for the oxygen to be greater than 10 per cent and the temperature between 90 140ºF Then you've got it made! But how the ordinary gardener can tell is the tricky part.

Get the Balance Right

It's in the way that you make your compost heap. Most recommendations in a lot of books are for 'hot' composting and are for people who can make large quantities at a time. Let's assume for a moment that you are one of those. You begin by assembling enough materials to make a container full of raw materials. Keep the various components in separate piles:

Ideally you will have a large container made of a material which will insulate the contents and keep the rain off as we want to control the process and we don't want them to get too dry, too wet, too hot or too cold. It sounds complicated but in practice it isn't once you've made a good bin. Incidentally one good large bin is better than too small ones. A bin which is about a metre cube seems to work quite well for the average garden; too big and it won't get filled quickly enough and the compost won't be ready in time; nor will it be so good as that from a well-made smaller heap. 

Begin by putting a layer of twiggy prunings about six inches deep, followed by a similar layer of green material, then another layer of twiggy material and so on till you reach the top. Check that nothing is compacted and that you've got the moisture level about right. Adding a little accelerator like HLA will probably help. In a few days the heap will have begun to get quite hot, some people use a thermometer. After a length of time which depends a little upon prevailing conditions, summer of course being quicker than winter, the temperature will have risen quite near that critical temperature. That is when you remove the front and lid of the container and take all the materials out of the bin, and then replace them giving them a good mixing ensuring that you incorporate plenty of air and again check the moisture. Put the front and lid back on. You may need to repeat the process several times but you will finally have some beautiful compost and probably within six to eight weeks.

The advantages of the 'Hot' method are that the heap generally gets hot enough to kill weed seeds and the compost is made more quickly. The disadvantage is that in today's sedentary workplace people aren't accustomed to the very heavy work which the turning of a compost heap can be and could do their back serious damage.

 So what is the alternative?

 'The Heap'

Most of us don't fall into the category of those who can fill a bin in one go. It is a case of 'add the materials as they arise'; bit of kitchen waste here, lawn mowings there and the odd prunings and weeds from a session of weeding.

For us the process is known as a 'Cold' compost heap. The disadvantages are that the process takes longer and doesn't always kill the weed seeds. In every other respect it is as good and because it is so much simpler it is easy to manage. Again a large bin is better than smaller ones and two is better than one as the process takes longer so one can be maturing as the other is being filled. 

The moisture in a cold heap is as important as in the hot process and can help to reduce those weed seeds. There will be some warmth generated by the process and coupled with the moisture the weeds seeds will germinate while still in the heap, but in the centre of the heap they will not find the conditions they prefer so will become stifled and die. That is one good reason for turning even a cold heap after a reasonable period.

Begin with a layer of greenery, leaves and grass mowings, about six inches deep and well mixed so that the grass doesn't compress into an anaerobic lump. Cover the layer with a thin sprinkling of soil - alternatively don't shake the soil of the roots of weeds before you put them on the layer. Continue with another layer, always topping off with a thin layer of soil. You can use a bacterial activator, especially if your heap is almost entirely composed of green material - that is because your heap will be high in nitrogen and to add too much more nitrogen in the form of HLA would be counterproductive. However, if you have included plenty of woody material, shredded prunings and such materials, which are higher in carbon, then HLA will be helpful. The composting process generates gases which the soil will absorb and turn into useful nutrients.

What can you put on your heap?

In theory anything that has grown, including you and me (at an appropriate time of course), can be composted but there are some arguments, some practical and some aesthetic, for not putting some things onto a compost heap, and especially a 'cold' one.

Most kitchen waste is alright, but some people fear that cooked material and pieces of uncooked meat and cheese attract rodents. Actually the evidence is not all that supportive of that claim but as very few allotments are actually free of rats and even foxes it will probably prevent upsetting other plot-holders if you put those kinds of contentious materials in a wormery back home. (There will be more about wormeries later.)

Don't put diseased plants on your heap, potato haulms with blight, cabbages stems from club root victims are examples. If in doubt then dispose of it, by burning if possible. Nor should you put the roots of pernicious weeds like convolvulus and couch grass on the heap. (Such pernicious weeds can still be composted but using a different method - so almost nothing need be thrown away.)

Building your bin.

Building your own compost bin is easy. The simplest can be simply some form of chicken wire framework to keep the materials in place and prevent them looking untidy. But for the reasons outlined above the heap will not produce such good compost nor as quickly.

The bin will be better if it is made of wood, or concrete blocks as they are more robust and will last longer, help to keep the heap warm and prevent excessive wetness with a lid to keep the rain from reaching the materials inside. The bins illustrated were made from old pallets. At the time of the photograph they had floating lids which were a combination of old carpet and some old plywood which sank into the bin as the compost matured.

But good bins have also been made from bales of straw which will also keep the heap warm and keep off the rain, with the added bonus that they gradually decompose adding to the compost.

With those points in mind the three most important elements are:

If you follow the rules then you won't have many problems, but just in case, here are a few tips:

Even a 'cold' heap will warm up a little - if it doesn't then possibly it is because you are only adding small amounts at each visit - it may be better to have a means of storing materials until you have a reasonable quantity. I use an old plastic dustbin, when it is full I put the contents on the heap in an even layer and begin to fill the dustbin again.

Or it may be that the heap is too dry. Give the heap a turn and add more water and/or some HLA. Sometimes just  mixing in a pile of lawn mowings may be enough. Experience will grow and you'll get the balance right increasingly often.

If the compost smells bad it may be that it has compacted or that it is too wet. The solution may be to add nitrogen-rich material like grass, leaves or manure

If the heap is soggy, this is usually accompanied by a smell, then either you added too much water, it suffered from too much rain, or perhaps you just put on too much wet material. It helps to leave materials for a couple of days to wilt before adding them to the heap. You can make matters better by adding some dry materials and tossing the materials to introduce more air.

Don't let anybody try to convince you otherwise, making compost isn't difficult and it will do your soil a power of good!

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