Growing crops takes nutrients from the soil. Rain (especially the torrential sort we have from time to time in Southern Europe) washes nutrients away. So we need to put it all back. Ideally we should put in more than we have lost so as to increase the nutrient level of our soil, year by year. This is achieved by adding organic matter, which for best results should be in the form of well rotted compost. 

So what sort of materials can we compost? Basically anything organic but there are a few things to bear in mind. Newspapers will rot down and can be very good but paper products designed to last longer than one day may have been treated with fungicides or other preservative chemicals. Old woolen clothes are fine, if you can get them, but these days they tend to be made of a wool and synthetic mixture that will not easily decompose. Hard wood prunings will take forever to rot away and may harbour disease.

Leaves take so long that they should be treated separately as leaf mould. 

Any old pile of organic matter will rot down - eventually, but making good quality compost quickly is more of an art. The quickest way to make good compost in less than a day is to feed your organic matter to an animal and wait for it to emerge at the other end. If you don’t have enough livestock you will have to resort to a much slower method. 

Nature has thought of everything. You don’t see lots of dead things lying about the countryside. This is because there are organisms of various sizes that get rid of it all. To make good compost we need to work with these organisms in a natural way. There is a whole team of micro organisms, bacteria and fungi, that will break down our vegetable and animal waste for us. All we need to do is to provide them with the conditions they need. 

The four things they need most are nitrogen, oxygen, heat and moisture. 

If you build a compost heap in such a way that air can get in, thus supplying oxygen and nitrogen, the decomposition process will proceed rapidly and heat will be generated. 

Temperatures in excess of 80^oC may be reached. This heat will speed up the process still more and, provided you can achieve at lease 70^oC, it will kill weed seeds and disease organisms too. This is why poorly made compost just spreads weeds and disease. 

Atmospheric nitrogen may not be enough to activate the heap but you can increase the amount of organic nitrogen by adding animal manure, fish meal, blood or urine. So piddling on your compost heap is good for it! Moisture is essential for the aerobic bacteria and fungi, but too much will prevent good aeration.