Why Compost?

Why do it?  and why use it?

Compost is plant (and often animal) remains which have been eaten and digested by a range of small creatures,  such as slugs,  snails,  woodlice,  worms,  moulds and bacteria,  and then passed out as "droppings".

"Decayed",  or "rotted" means that bacteria and fungi have "eaten" the remains of the plants and left behind the undigested bits,  their droppings and their own bodies.

While these beasts are eating,  digesting and leaving their wastes,  very little is lost from your compost heap.   A little Nitrogen,  perhaps,  becomes a gas or two and escapes.   Some of the Carbon becomes Carbon Dioxide and escapes.   But most of the elements in the plant (and animal) bodies simply get formed into another compound,  and another,  and another . .

Plants need Nitrogen,  Phosphorus and Potassium (NPK) (K is the chemical symbol for Potassium),  along with many other elements,  in order to grow.   They absorb these elements,  dissolved in water,  through their roots.   Most of these elements are present in compost in forms which dissolve slowly in water.

So compost is a slow-release fertilizer.

Some of your compost is humus,  that dark jelly which glues particles of sand and clay together to make friable granules of soil;  that mysterious colloid which absorbs water like a sponge and releases it when your plants need it;  that active group of cation exchange complexes which loosely bind and release the elements which your plants need for growth;  that magical mix of macromolecules which seems to protect your plants against diseases;  that indigestible synthesis of lignins and quinoles which is stable for decades and centuries.

That very basis of a fertile soil.

But why compost?

Why not just mix the wilted weeds and waste plants into the soil of your veg patch and let them get on with it?

Why?   Because they'd suck the life (actually the Nitrogen) out of your soil,  that's why.

The dead vegetables have a Carbon to Nitrogen ratio of between 11:1 and 30:1.   The bacteria which digest (rot) them have a Carbon to Nitrogen ratio of 5:1,  so they need more Nitrogen than their food can provide.   They draw the extra from the soil around them and so,  for a while,  deplete the soil of Nitrogen.   If this happens in your veg plot,  your plants will starve.   If it happens in your compost heap,  then the rate of decay will slow down for a while.

Eventually,  of course,  the Nitrobacter and Azotobacter will get together and use Nitrogen from the air to make up the deficiency.

A compost heap is a temporal buffer to get the decay process past the short-term shortage of Nitrogen.