slowly for a year or so.  Then, as spores of mycorrhizal fungi begin falling on the bed and their hyphae become established, scattered trees begin to develop the necessary symbiosis and their growth takes off.  On a bed of two-year-old seedlings, many individual trees are head and shoulders above the others.  This is not due to superior genetics or erratic soil fertility.  These are the individuals with a mycorrhizal association.

Like other beneficial microorganisms, micorrhizal fungi do not primarily eat plant vascular fluid, their food is decaying organic matter.  Here’s yet another reason to contend that soil productivity can be measured by humus content.

CHAPTER EIGHT

Maintaining Soil Humus

Organic matter benefits soil productivity not because it is present, but because all forms of organic matter in the soil, including its most stable form—­humus—­are disappearing.  Mycorrhizal fungi and beneficial bacterial colonies around plant roots can exist only by consuming soil organic matter.  The slimes and gums that cement soil particles into relatively stable aggregates are formed by microorganisms as they consume soil organic matter.  Scats and casts that are soil crumbs form only because organic matter is being consumed.  If humus declines, the entire soil ecology runs down and with it, soil tilth and the health and productivity of plants.

If you want to manage your garden soil wisely, keep foremost in mind that the rate of humus loss is far more important than the amount of humus present.  However, natural processes remove humus without our aid or attention while the gardener’s task is to add organic matter.  So there is a very understandable tendency to focus on addition, not subtraction.  But, can we add too much?  And if so, what happens when we do?

How Much Humus is Soil Supposed to Have?

If you measured the organic matter contents of various soils around the United States there would be wide differences.  Some variations on crop land are due to great losses that have been caused by mismanagement.  But even if you could measure virgin soils never used by humans there still would be great differences.  Hans Jenny, a soil scientist at the University of Missouri during the 1940s, noticed patterns in soil humus levels and explained how and why this occurs in a wonderfully readable book, Factors in Soil Formation. These days, academic agricultural scientists conceal the basic simplicity of their knowledge by unnecessarily expressing their data with exotic verbiage and higher mathematics.  In Jenny’s time it was not considered demeaning if an intelligent layman could read and understand the writings of a scientist or scholar.  Any serious gardener who wants to understand the wide differences in soil should become familiar with Factors in Soil Formation. About organic matter in virgin soils, Jenny said: