Soil Productivity

By Thembi Borras

In a workshop I recently attended on biointensive farming, I was challenged to harvest vegetables as a byproduct of growing soil. With the exception of the ocean and the atmosphere, soil is the medium in which everything we consume grows. Trees depend on the 25% or more of their biomass found in the soil, which physically functions to store water, circulate air and water and support tree roots.

Despite the importance of soil, soil loss, soil compaction and organic matter loss continue to diminish soil productivity in forests. Forest site productivity, which is the capacity of the land to grow trees, is, in part, a function of soil productivity. Forest site productivity can be measured in annual production in bf/acre/year and is an important standard used by foresters to plan, describe and compare forestlands. As forest site productivity declines so does annual production rates and the length of time between harvests may become longer.

In New Zealand, a 20 percent drop in site productivity was revealed after 1" of topsoil was removed. In my own experience I have walked areas in Central Mendocino County that obviously suffer from topsoil loss and compaction, the magnitude of which being frequently connected to the extent of the legacy skid trail network and drainage associated with it as evidenced by the light colored subsoil, stunted trees and gullies.

Soil loss reduces the supply of nutrients and water. Soil compaction retards root growth and the circulation of air and water. And, organic matter loss accelerates erosion and may decrease water retention, structure, porosity, and resistance to compaction. These conditions are not easily reversed given the amount of time it takes for easily crumbled, humus and nutrient enriched topsoil to develop. In the context of agriculture, it takes 500 years to build 1" of topsoil which is significantly longer than the time that it is taking to lose 1" of topsoil, only 28 years in some cases.

For the most part, we have inherited the soil productivity we use to prosper today. To insure that future generations are able to flourish we must reverse the depletion trend and begin to accumulate soil wealth. In the context of forestry, practices exist to minimize the loss of soil productivity, however there are no economically viable practices to restore soil, soil structure and organic matter once it has been lost. My challenge and that of future generations will not be growing trees, agricultural crops or raising cattle but will be growing the soil on which they depend.

A portion of this production was gleaned from Sustaining Site Productivity on Forestlands; A User's Guide to Good Soil Management published by the University of California and the Proceedings from the Soil, Food and People Conference held in 2000 at UC Davis.