Most microorganisms cannot withstand temperatures much over 130 degree.  When the core of a pile heats beyond this point they either form spores while waiting for things to cool off, or die off.  Plenty of living organisms will still be waiting in the cooler outer layers of the heap to reoccupy the core once things cool down.  However, there are unique bacteria and fungi that only work effectively at temperatures exceeding 110 degree.  Soil scientists and other academics that sometimes seem to measure their stature on how well they can baffle the average person by using unfamiliar words for ordinary notions call these types of organisms thermophiles, a Latin word that simply means “heat lovers.”

Compost piles can get remarkably hot.  Since thermophilic microorganisms and fungi generate the very heat they require to accelerate their activities and as the ambient temperature increases generate even more heat, the ultimate temperature is reached when the pile gets so hot that even thermophilic organisms begin to die off.  Compost piles have exceeded 160 degree.  You should expect the heaps you build to exceed 140 degree and shouldn’t be surprised if they approach 150 degree

Other types of decomposing organic matter can get even hotter.  For example, haystacks commonly catch on fire because dry hay is such an excellent insulator.  If the bales in the center of a large hay stack are just moist enough to encourage rapid bacterial decomposition, the heat generated may increase until dryer bales on the outside begin to smoke and then burn.  Wise farmers make sure their hay is thoroughly dry before baling and stacking it.

How hot the pile can get depends on how well the composter controls a number of factors.  These are so important that they need to be considered in detail.

Particle size. Microorganisms are not capable of chewing or mechanically attacking food.  Their primary method of eating is to secrete digestive enzymes that break down and then dissolve organic matter.  Some larger single-cell creatures can surround or envelop and then “swallow” tiny food particles.  Once inside the cell this material is then attacked by similar digestive enzymes.

Since digestive enzymes attack only outside surfaces, the greater the surface area the composting materials present the more rapidly microorganisms multiply to consume the food supply.  And the more heat is created.  As particle size decreases, the amount of surface area goes up just about as rapidly as the number series used a few paragraphs back to illustrate the multiplication of microorganisms.

The surfaces presented in different types of soil similarly affect plant growth so scientists have carefully calculated the amount of surface areas of soil materials.  Although compost heaps are made of much larger particles than soil, the relationship between particle size and surface area is the same.  Clearly, when a small difference in particle size can change the amount of surface area by hundreds of times, reducing the size of the stuff in the compost pile will: