Root exudates have numerous functions beyond affecting microorganisms.  One is to suppress or encourage the growth of surrounding plants Gardeners experience this as plant companions and antagonists.  Walnut tree root exudates are very antagonistic to many other species.  And members of the onion family prevent beans from growing well if their root systems are intermixed.

Many crop rotational schemes exist because the effects of root exudates seem to persist for one or even two years after the original plant grew That’s why onions grow very well when they are planted where potatoes grew the year before.  And why farmers grow a three year rotation of hay, potatoes and onions.  That is also why onions don’t grow nearly as well following cabbage or squash.  Farmers have a much easier time managing successions.  They can grow 40 acres of one crop followed by 40 acres of another.  But squash from 100 square feet may overwhelm the kitchen while carrots from the same 100 square feet the next year may not be enough.  Unless you keep detailed records, it is hard to remember exactly where everything grew as long as two years ago in a vegetable garden and to correlate that data with this year’s results.  But when I see half a planting on a raised bed grow well and the adjacent half grow poorly, I assume the difficulty was caused by exudate remains from whatever grew there one, or even, two years ago.

In 1990, half of crop “F” grew well, half poorly. this was due to the presence of crop “D” in 1989.  The gardener might remember that “D” was there last year.  But in 1991, half of crop “G” grew well, half poorly.  This was also due to the presence of crop “D” two years ago.  Few can make this association.

These effects were one reason that Sir Albert Howard thought it was very foolish to grow a vegetable garden in one spot for too many years.  He recommended growing “healing grass” for about five years following several years of vegetable gardening to erase all the exudate effects and restore the soil ecology to normal.

Mycorrhizal association is another beneficial relationship that should exist between soil organisms and many higher plants.  This symbiotic relationship involves fungi and plant roots.  Fungi can be pathogenic, consuming living plants.  But most of them are harmless and eat only dead, decaying organic matter.  Most fungi are soil dwellers though some eat downed or even standing trees.

Most people do not realize that plant roots adsorb water and water-soluble nutrients only through the tiny hairs and actively growing tips near the very end of the root.  The ability for any new root to absorb nutrition only lasts a short time, then the hairs slough off and the root develops a sort of hard bark.  If root system growth slows or stops, the plant’s ability to obtain nourishment is greatly reduced.  Roots cannot make oxygen out of carbon dioxide as do the leaves.  That’s why it is so important to maintain a good supply of soil air and for the soil to remain loose enough to allow rapid root expansion.