- expose more material to digestive enzymes;

- greatly accelerate decomposition;

- build much higher temperatures.

Oxygen supply. All desirable organisms of decomposition are oxygen breathers or “aerobes.  There must be an adequate movement of air through the pile to supply their needs.  If air supply is choked off, aerobic microorganisms die off and are replaced by anaerobic organisms.  These do not run by burning carbohydrates, but derive energy from other kinds of chemical reactions not requiring oxygen.  Anaerobic chemistry is slow and does not generate much heat, so a pile that suddenly cools off is giving a strong indication that the core may lack air.  The primary waste products of aerobes are water and carbon dioxide gas—­inoffensive substances.  When most people think of putrefaction they are actually picturing decomposition by anaerobic bacteria.  With insufficient oxygen, foul-smelling materials are created.  Instead of humus being formed, black, tarlike substances develop that are much less useful in soil.  Under airless conditions much nitrate is permanently lost.  The odiferous wastes of anaerobes also includes hydrogen sulfide (smells like rotten eggs), as well as other toxic substances with very unpleasant qualities.

Heaps built with significant amounts of coarse, strong, irregular materials tend to retain large pore spaces, encourage airflow and remain aerobic.  Heat generated in the pile causes hot air in the pile’s center to rise and exit the pile by convection.  This automatically draws in a supply of fresh, cool air.  But heaps made exclusively of large particles not only present little surface area to microorganisms, they permit so much airflow that they are rapidly cooled.  This is one reason that a wet firewood rick or a pile of damp wood chips does not heat up.  At the opposite extreme, piles made of finely ground or soft, wet materials tend to compact, ending convective air exchanges and bringing aerobic decomposition to a halt.  In the center of an airless heap, anaerobic organisms immediately take over.

Surface Area of One Gram of Soil Particles

Particle Size Diameter of Number of Surface Area
                 Particles in mm Particles per gm per square cm

Very Coarse Sand 2.00-1.00 90 11
Coarse Sand 1.00-0.50 720 23
Medium Sand 0.50-0.25 5,700 45
Find Sand 0.25-0.10 46,000 91
Very Fine Sand 0.10-005 772,000 227
Silt 0.05-0.002 5,776,000 454

Composters use several strategies to maintain airflow.  The most basic one is to blend an assortment of components so that coarse, stiff materials maintain a loose texture while soft, flexible stuff tends to partially fill in the spaces.  However, even if the heap starts out fluffy enough to permit adequate airflow, as the materials decompose they soften and tend to slump together into an airless mass.