Rhizobia are not capable of being active in compost piles, but another class of microbes is.  Called azobacteria, these free-living soil dwellers also make nitrate nitrogen.  Their contribution is not potentially as great as rhizobia, but no special provision must be made to encourage azobacteria other than maintaining a decent level of humus for them to eat, a balanced mineral supply that includes adequate calcium, and a soil pH between 5.75 and 7.25.  A high-yielding crop of wheat needs 60-80 pounds of nitrates per acre.  Corn and most vegetables can use twice that amount.  Azobacteria can make enough for wheat, though an average nitrate contribution under good soil conditions might be more like 30-50 pounds per year.

Once a compost heap has cooled, azobacteria will proliferate and begin to manufacture significant amounts of nitrates, steadily lowering the C/N.  And carbon never stops being digested, further dropping the C/N.  The rapid phase of composting may be over in a few months, but ripening can be allowed to go on for many more months if necessary.

Feeding unripened compost to worms is perhaps the quickest way to lower C/N and make a potent soil amendment.  Once the high heat of decomposition has passed and the heap is cooling, it is commonly invaded by redworms, the same species used for vermicomposting kitchen garbage.  These worms would not be able to eat the high C/N material that went into a heap, but after heating, the average C/N has probably dropped enough to be suitable for them.

The municipal composting operation at Fallbrook, California makes clever use of this method to produce a smaller amount of high-grade product out of a larger quantity of low-grade ingredients.  Mixtures of sewage sludge and municipal solid waste are first composted and after cooling, the half-done high C/N compost is shallowly spread out over crude worm beds and kept moist.  More crude compost is added as the worms consume the waste, much like a household worm box.  The worm beds gradually rise.  The lower portion of these mounds is pure castings while the worm activity stays closer to the surface where food is available.  When the beds have grown to about three feet tall, the surface few inches containing worms and undigested food are scraped off and used to form new vermicomposting beds.  The castings below are considered finished compost.  By laboratory analysis, the castings contain three or four times as much nitrogen as the crude compost being fed to the worms.

The marketplace gives an excellent indicator of the difference between their crude compost and the worm casts.  Even though Fallbrook is surrounded by large acreages devoted to citrus orchards and row crop vegetables, the municipality has a difficult time disposing of the crude product.  But their vermicompost is in strong demand.

Sir Albert Howard’s Indore Method

Nineteenth-century farmers and market gardeners had much practical knowledge about using manures and making composts that worked like fertilizers, but little was known about the actual microbial process of composting until our century.  As information became available about compost ecology, one brilliant individual, Sir Albert Howard, incorporated the new science of soil microbiology into his composting and by patient experiment learned how to make superior compost