Put simply, compost is decomposed organic matter. So those leaves breaking down on the forest floor are compost, as too are the bodies at the cemetery. All organic matter lives, then dies and breaks down into different qualities of compost. That breakdown of organic matter is carried out by animals, plants, moulds, microbes, air and water, basically 'nature' or 'mother earth' depending on how whimsical you feel.
That was the easy answer, but the long answer is dependent on what kind of gardener you are. Do you make your own garden compost, and if so how? Or do you just buy it in bags from the store? Every gardener who is a fan of garden composting has a slightly different method. They will use slightly different ingredients in their garden composters. So everyone's garden compost is a bit different!
Garden soil tends to be a combination of crushed rock and mineral mixed up with hummus (the end result of your garden compost bin). Compost is the bulk within the soil but not necessarily the nutrient provider. Hummus improves the soil structure, allowing it to hold moisture and air.
The more hummus the soil has the better the structure. The soil will not be compacted, as some clay soils are apt to do. Hummus opens them out leaving air pockets which are so vital for the micro-organisms and insect life so vital to the health and vitality of the soil and eventually your plants.
Hummus is spongy and great at holding water so is vital for those with sandy soils. But, any soil will be improved by the addition of more hummus. Home compost is free and easy to create. There is no reason not to compost waste from the home and garden. Much easier to trapse to the bottom of the garden with garden waste or kitchen scraps to compost, than sort them out and place in bins for a destiny in municipal landfill.
Brilliantly, many local authorities across Europe and the US are recycling organic waste on a commercial scale. But if you have space for even the smallest beehive compost bin it makes sense to keep your garden waste for yourself and make your own garden compost. Your compost will be a very locally sourced product and free to boot!
Compost or hummus provides the soil with slow release nutrients. The variety of nutrients will depend entirely on what the compost originally was. For example composting a nitrogen rich poultry manure, will give a nitrogen rich compost.
The very best garden composts are made from a wide variety of ingredients so the resulting hummus is full of the widest variety of nutrients. The hummus which is the end product of the garden composter should be spongy in texture and full of all the trace elements needed in the garden.
So when you are making compost at home the very best approach is to put as many different things in your garden compost bin, as possible. That way you will get the widest variety of goodness to put on your garden.
It is worth telling friends and neighbours if you are starting garden composting. That way you can get more 'food' for the compost bin from them. Composting is the ultimate in garden recycling. And, the more you can recycle to the composter the better the compost will be.
The biggest problem most people face is not being able to fill the garden compost bin fast enough. The whole point of garden compositng, is to improve on what nature does all day every day. The earth tends to compost slowly. Moulds, bacteria, insects, scavengers all slowly turning what was once alive, into hummus to feed the next generation. If you leave a pile of dead leaves in the corner of your garden, eventually nature will break them down. But since they're dry and exposed to the elements, along with being one solitary type of matter, the process will be very slow. Indeed you may find they've all blown away before you get a decent leaf mulch!
Garden composting means helping nature out. Gathering all the dead matter you want to recycle into garden compost, and then protecting it from the elements will speed up the process no end. Also using as wide variety of ingredients for your compost bins will introduce lots of different organisms that work together to make your compost useful in no time.
When carried out successfully garden compost is a beautiful, nutty product that improves the fertility and productivity of your garden. To the successful gardener, composting is a way to save money, work with the environment, recycle and reduce waste. Perhaps most importantly it is a way of ensuring the garden remains productive over the long term. If we are forever harvesting crops we literally reduce the soil bulk and soil vitality of our garden. You can see this in practice as over the years the actual level of soil on a vegetable plot will sink. Maybe more importantly crop yields and performance will falter unless something is done.
Although garden compost is not particularly rich (say in comparison with an organic fertilizer) in nutrients immediately available to plants, it is a feeder as well as bulker for the soil. The nutrients locked up in the compost will be 'slow release' nutrients which means putting composted matter on the garden, means feeding in the long term. This will promote tough plants which are fully developed and strong, not the sappy growth susceptible to disease which quick release fertilizers can give.
The soil life; microbes, bacteria, worms and so on will pull down the compost into the soil where it can do even more good. Best of all you don't have to dig it in!