Compost and Plant Health

by Bill Campbell

Compost is the familiar and valuable garden byproduct that we have good intentions of producing in quantity and utilizing to assure great soil tilth, fertility and bountiful yield. Beyond this, there is improved plant health due to the heat-killing of weed seeds, pathogens and pesticides. A properly prepared compost also exhibits a release of nitrogen and a balanced concentration of ions to stimulate the plant into optimum disease resistance. Futher, the beneficial microorganisms colonizing after the defeat of pathogens have many salutary effects.

  • Some produce plant hormones and so directly affect plant growth
  • Others make natural chelators keeping iron in high and available concentrations
  • Form water soluble humic substance to keep zinc, manganese et al in solution
  • Secrete antibiotics effective against pathogens
  • Parasitize pathogens; microarthropods may even seek out to destroy
  • Induce systemic resistance in plants, much similar to our immunizations

The composting process of enzymatic decomposition within mixed piles of organic matter has been the subject of terribly interesting research projects at notable universities - equally amazing are our individual accomplishments in stacking garden waste, some soil and manure and maybe even turning the pile to accomplish in time this incredible product. Briefly, the scientific consensus is a Carbon : Nitrogen ratio of 30:1 (woody : fresh green materials), aeration for oxygen, and a slightly damp moisture content. The turning of the materials for oxygenation also allows the recolonization of the compost with beneficial bacterial survivors in cooler parts of the pile. The temp range of 40 - 60 degrees C (104 - 140 F) destroys thermosensitive pathogens, fly larvae and weed seeds. Overheating of the mix above 65 C leads to die off of beneficial microorganisms, as can prolonged leaching and storage, or sterilisation by some fertilizer chemicals. Interestingly, sphagnum peat of itself is in a level of decomposition that cannot support beneficial microorganisms and so is conductive to disease organisms.

Bagged "compost" is an increasingly huge seller in the garden center. This is usually produced industrially by screening city wastes free of plastic and other trash, grinding the good stuff with accumulated yard and Christmas tree collections, adding sludge, dairy and such miscellanea, laying out in long windrows and turning with ingenious traveling heavy machinery. It winds up with a nice texture and smelling pretty good. Analysis is usually limited to the NPK percent; this is quite low in compost so not the most important quality. The consumer will never know the incidental chemicals, nor more importantly the beneficial microorganism activity after the vagaries of production, then hot and cold travel, storage and age. Though I'm a garden center parter with conflict of interest, we semi-rurals can do better making our own.

A couple of other advertised entities deserve attention. Twirling barrels of large and small sizes do speed and contain the process; the caution is to distrust the "14-day compost" ad. Compost must pass from heating to ambient temperature and thence "cure" - a longer term chemical stabilization process. The other misleading products are those promoting bacterial or other additives to accomplish composting. Addition of soil alone, with billions of microorganisms in a teaspoosful, is what Nature intended to produce this wonderful garden nurturer.