Science Forums 2

[Michaelangelica]

Alternative Soil Amendments

Microbial Inoculants

Inoculants, which are dry or liquid preparations of one or more species of microorganism, fall into three broad groups: 1) those that inoculate individual plants with symbiotic organisms (chiefly Rhizobia spp.), 2) those that inoculate the soil with desirable organisms, and 3) those that are used as "cover crops" (algae).
Rhizobia

The most clearly beneficial microbial preparations for agricultural use are the different strains of Rhizobia used to inoculate legumes.
Specific strains of these bacteria live in a mutually beneficial (symbiotic) relationship with specific species of legumes.
The bacteria penetrate the plant roots, causing the formation of root nodules containing both plant tissue and bacteria. In very simple terms, the plant supplies the physical environment and certain nutrients to the bacteria; the bacteria "fix" nitrogen from the air into compounds that then become available to the plant. Typical nitrogen fixation rates vary from 50 lbs/acre to over 300 lbs/acre, depending on climate, species, and soil conditions. On most farms these rates make it possible to harvest good crops without purchasing additional nitrogen.
Mycorrhizae

The mycorrhizae (my-cor-ry-'zee) group of fungi live either on or in plant roots and act to extend the reach of root hairs: into the soil. Mycorrhizae increase the plant's uptake of water and nutrients, especially in less fertile soils. The superfine, root-like structures of these fungi are more extensive and more effective than plant root hairs at absorbing phosphorus, and other nutrients as well.
Phosphorus moves slowly in soils but the fungi can absorb it much faster than the plant alone can. This enhanced root feeding makes it possible to reduce fertilizer rates for plants having a healthy colony of mychorrhizae. Some plants including citrus, grapes, avocados, and bananas, are dependent on mycorrhiza fungi. Others that benefit from having them are artichokes, melons, tomatoes, peppers, and squash.

Roots colonized by mycorrhizae are less likely to be penetrated by root-feeding nematodes since the pest cannot pierce the thick fungal network.

Mycorrhizae also produce hormones and antibiotics, which enhance root growth and provide disease suppression. The fungi benefit from plant association by taking nutrients and carbohydrates from the plant roots they live in.

In soils where mychorrhizae have been killed off, an inoculation may be beneficial.
In healthy soils where they already exist there will be little or no benefit to adding more.
There are dozens of mychorrizae species in nature. Additionally, the species found on plant roots may change as the plant matures.
If those that are available are of the correct species, and are handled properly at all stages, they offer interesting potential benefits to farmers in well-managed systems. Generally it is preferred to inoculate with several species rather than a single one. For information on rhizobial and mycorrhizal inoculation for disease suppression, request the ATTRA publication Sustainable Management of Soil-borne Plant Diseases.
Free-living soil organisms

A great many of the products in this category are designed to be sprayed on the soil surface or on crop residues in order to inoculate the topsoil with desirable microorganisms. Manufacturers of these products make numerous and varying claims about their beneficial effects, which fall into three broad categories:

* The microbes will fix enough nitrogen from the air to allow the farmer to eliminate much or all fertilizer.
* The product improves soil organic matter and "releases" soil nutrients to the crop.
* The product produces better yields, especially during times of drought.

Many microbial products do indeed contain free-living (as opposed to symbiotic) microbes that are known to fix nitrogen in certain circumstances. Those species, however, work best in wet, oxygen-poor conditions that most farmers and their crops would prefer to avoid.

Rice paddies are a notable exception. In the vast majority of cropping situations other than rice production, the amount of nitrogen fixed by such free-living microbes is not generally considered economically significant (3).
In other words, the value of any fixed nitrogen may be less than the cost of the product. Far greater nitrogen fixation, for example, can be obtained via symbiotic Rhizobia on a legume sod or cover crop, for much lower cost.

Soil microbes, like all living things, will thrive only in the presence of their preferred environmental conditions-moisture, oxygen, temperature, pH, food, and shelter.
When conditions are not within favorable ranges, the microbes cease reproduction or die.
Natural microbial populations will be abundant if soil conditions are right. Adding a microbial amendment in such circumstances may not be cost-efficient, because the naturally occurring individuals will typically outnumber the same species supplied in a product by 10,000 to 1, or more

If soil conditions are not right, inoculant organisms will reproduce just as slowly as their naturally occurring colleagues, which is to say, not at all.
The consensus among agronomists appears to be that these products perform best when the soil is at or near optimum conditions to begin with.

So does that say it is a good idea to buy some or not?