© 1999 Streamline Publications The rhizosphere. It's the environment in which a plant's roots grow and thrive or are forced to struggle for life. It's the soil—and everything in it—that may affect plants for better or for worse. Traditionally, growers concerned themselves with soil's main chemical constituents: nitrogen (N), phosphorous (P) and potassium (K)—familiar to anyone who purchases fertilizers.

The only problem with careful scrutiny of soil samples is the threat of nematodes suing for invasion of privacy.

Though still important constituents in the soil, less attention has been given to the secondary nutrients: calcium, magnesium and sulfur; and the trace nutrients: boron, chlorine, copper, iron, manganese, molybdenum and zinc.

Other Stuff

Over recent years recognition has developed for the effects of fungi, bacteria and other naturally-occurring organisms in soil. There have been recent revelations about beneficial fungi such as:

* Trichoderma—a natural plant growth stimulator and disease fighter

* Mycorrhizal—which aids in a plant's uptake of water and nutrients by extending the reach of its roots.

Certain species of bacteria, nematodes and protozoa are also beneficial to plants and are present in a healthy rhizosphere. But how do you know what, exactly, is in your soil and what ought to be?

Answers

Now, comprehensive soil testing services are available from Soil Foodweb Inc. (http://www.soilfoodweb.com). The company uses data and methods developed by soil scientists at Oregon State University to compare their customers' soil to a database of over 50,000 soil samples. SFI consultants can then offer suggestions on how to restore soil health through cultural practices and the addition of beneficial organisms.

Balance

Different plants require different soils to achieve vigorous growth. Generally, trees and shrubs do best in soil dominated by fungi; row crops and garden flowers like an equal balance of bacteria and fungi; and turf prefers bacteria-rich soil.

Testing

Comprehensive soil testing includes measuring:

* The number of individuals or biomass of each group

* Type of organisms present and which is dominant

* How active the organisms are

* Relation of soil organisms to plant available nitrogen.

A Biological Six-Pack

SFI addresses six biological functions in a healthy rhizosphere—which the company refers to as the “soil foodweb.” Full descriptions for the following summarized biological functions, along with information on soil sampling services and other relevant issues, are available on the SFI web site.

1. Decomposition of crop residues, manure and other organic material: These materials will only decompose if certain species of fungi and bacteria, the “decomposers,” decay them and allow recycling processes to occur—getting rid of crop residues and feeding soil organisms.

2. Retention of nutrients: The function of nutrient retention occurs when bacteria and fungi multiply and increase their populations in the soil. When bacteria and fungi multiply they gather up free nitrogen from the soil and convert it to protein in their bodies. Nitrogen in this form will not leach away or be lost as a gas, reducing fertilizer needs with no reduction in crop yield.

3. Nutrient Recycling: The organisms that perform this function are beneficial nematodes that only feed on bacteria or fungi, the protozoa that feed on bacteria, and beneficial soil mites that feed on fungi. As these species go about their work they cause nitrogen especially, but also phosphorus and other nutrients, to be released at a gradual rate that supplies crops with a steady diet all season long.

4. Biological control of root rot and parasitic nematodes: A healthy soil that contains a broad diversity of microbial types most often contains species that kill, inhibit or suppress the kinds of fungi that cause root rots and the kinds of nematodes that attack roots.

5. Production of plant growth regulators: All plants depend on the presence of certain species of soil microorganisms in the root zone to produce various hormones and other chemical “signals” that stimulate growth and development.

6. Soil structure and tilth: Aggregates will not form unless many sand, silt and clay particles are “glued” together by the gums and gels that only certain species of soil bacteria can produce. It is impossible for a soil to maintain the ideal crumb structure in the absence of the particular species of bacteria and fungi that perform the gluing and binding functions.

Added Benefit

Clean up of herbicide or pesticide carry over is a by-product of balanced, healthy soil. Most herbicide and pesticide molecules can be “eaten” or degraded by certain kinds of microbes in the soil, if those species are present. A healthy soil will tend to rid itself of agricultural chemical carry over and other forms of pollution.