There are many wonderful resources for information on how compost is produced and what it does .  One of the best for understanding the role of microbes in the process is “Teaming with Microbes”, a well-researched, very enthusiastic look at the incredibly complex life of the soil. Read it, and you will never look at soil the same way again.

Organisms in and on the soil fall into two categories: those that can be seen with the naked eye (macro-organisms such as insects of all kinds, slugs, snails, spiders and daddy longlegs, centipedes, millipedes, worms, and larger creatures such as birds and small mammals) and those that cannot be seen without the help of serious magnification (micro-organisms such as bacteria, fungi, actinomycetes, algae, protozoa, and nematodes)

Most of these are beneficial to the soil; some are not.  Fortunately, the ratio of good to bad is about 9:1.  This means that if the soil (or any other environment for that matter) has a wide range of organisms in it, the good far outweigh the bad.  It is as if for every criminal there were nine policemen.  The soil “police” don’t actually need to subdue or kill the “criminals”.  Mere numbers of the good limit the growth of the bad by competing for food and other resources.

Bad organisms, often known as pathogens, are only a problem when their numbers increase to the level of causing disease.  Healthy bodies of all kinds contain enough good organisms to keep nearly all bad organisms in check.  A weakened body cannot always contain them, they multiply uncontrollably, and the body becomes sick.

Wouldn’t it be wonderful if we humans could work out a similar system?  With enough good people around, the bad guys would never establish a foothold and become a menace to society.

In other words, pathogens only become a problem in soil that is deficient in good microbes, which is, to my mind, a compelling reason to keep life in the soil varied and active.

 The compost process goes through many stages.  The macros begin and end the process, while a large array of micros deal with the material in the middle stages. Macros chomp, chew, shred and otherwise reduce the size of organic particles, giving more surface area for the microbes to work on. 

 In phase two the ever-present bacteria take over. Bacteria will work at near freezing temperatures although very slowly. Above 55^o F (13^o C), they really get going.The temperature of the pile is affected by the type of bacteria working in it.  Mesophiles flourish at temperatures between 55^o and 110^oF while thermophiles are active at temperatures between 110^o and 160 ^oF.  External temperature plays a role, too.  Higher temperatures keep the edges of the pile warmer, allowing for activity throughout most of the pile.

  Bacteria eat by oxidizing minerals in solution, and then ingesting them through their cell walls.  Therefore, they need four conditions to make them happy:

• carbon for energy
• nitrogen for growth and reproduction
• water
• oxygen

Activity happens where all four are present, and that happens when particles in the pile are big enough to leave air pockets, but small enough to trap water between them. 

 I love this illustration, because it explains perfectly the conditions necessary for the compost process to work.  We are talking here about aerobic bacteria which need oxygen to live.

There is a second class of bacteria known as anaerobic bacteria, meaning that they don’t necessarily need oxygen to go about their daily lives.  Like everything else in nature, they have a role to play, but they are not particularly welcome in the compost pile as they break material down through the process of putrefaction, rather than decomposition, and putrefaction means smells – nasty smells.  If your pile smells, it is because the anaerobes have taken over.  You need to turn the pile and incorporate as much air as possible.

When most of the nitrogen is used up, the bacteria take a break and the fungi take over.  They are the decomposers on the forest floor, and the primary  decomposers of the woodier, carbon rich materials, including the chitin shells of insects and the bones of animals.  Fungi also, very importantly, form symbiotic relationships with plants: the plants give off exudates which the fungi eat, and in return the fungi send filaments known as hyphae through the soil to gather minerals (phosphorus in particular) and bring them back to the plant.

Both bacteria and fungi immobilize minerals by absorbing them into their bodies, where they remain locked until the microbes are eaten by other organisms or die and are themselves decomposed.

Other soil organisms contribute, mainly by eating the bacteria and fungi, thus releasing the nutrients, which then become available for the plants. This activity continues after the compost is incorporated into the soil, slowly providing nutrients to the plants over several months.  Eventually, however, the organic matter is completely broken down, and it is time to add more. 

One application of compost imay not be enough to turn poor soil into good; rather it is an ongoing process in which the soil becomes richer and richer, and able to absorb more and more organic matter.