How to Interpret Soil Foodweb Assays

Example

How to Interpret Soil Foodweb Information
Be sure to take samples from that place that will answer the question you want answered. Typically, that is from the place that most affects the plant. Keep this in mind when looking at drip wells versus drive lanes, etc.
Where is the food web that most affects the plant roots?

There is a best soil food web for each combination of crop type, climate region, soil type, amount of organic matter and water supply. The ideal food web balance for row crops in Arizona is different than the ideal balance for fruit trees or grapes in California.

State the question you are trying to answer very clearly. The tests you need then should be come clear.

• For example, you want to know if your soil is healthy. That means a full foodweb analysis is needed, since you don’t know what part of the food web may or may not be "out of whack".
• If you have done a foodweb analysis in the past, and know your soil lacks fungal activity, for example, then all you need to assess is fungal activity, and probably total fungal biomass. Perhaps mycorrhizal colonization as well, since this assay includes disease encountered on the root system, as well as insect feeding damage.

What information is given by which test?

Active Bacteria/Active Fungi

• These tests measure the numbers and biomass of aerobic bacteria and fungi that are actively feeding and reproducing. Active bacteria and fungi rapidly enhance soil structure, nutrient retention, disease suppression and residue/pollutant decomposition.
• If your soil is deficient in disease suppression, you need to know whether it is because bacterial activity or fungal activity is lacking.
• If water puddles on the soil surface, perhaps the reason is that soil structure is not being maintained. If the roots of your plants only grow a short distance into the soil, it is a clear indication that the soil is compacted, and lacks oxygen. Bacteria and fungi need to grow into that soil, and build the hallways and passages ways to let water flow into the soil as well as allow oxygen to move into the soil.
• These tests are used to determine:
  1. Is nitrogen being retained at this time?
  2. Is this soil dominated by fungi or bacteria? Is it bacteria or fungi that are playing the greatest role in decomposition?
  3. Is there a decent set of bacteria to support protozoa and bacterial-feeding nematodes?
  4. Did addition of a product, compost, or compost tea, or some aspect of management cause a bloom of bacterial activity or fungal activity, or kill, harm or otherwise reduce activity of the bacteria and fungi?
  5. Did herbicides or other pesticides kill or stimulate significant numbers of organisms?

Total Bacteria/Total Fungi

• This test measures the total amount of bacteria and fungi in the sample. Total biomass includes the active populations determined in the previous tests, as well as all of the inactive (sleeping, moribund, semi-awake, just woken up, just about to go to sleep, not really wide awake yet, and dead but not yet decomposed) organisms.
• Total biomass assesses the amount of carbon or nitrogen held in these organisms, disease suppressiveness, potential benefit to soil aggregation, and relate to decomposition rates.
• There is a clear correlation between diversity and total bacteria or total fungal biomass. The higher the biomass present, the more diverse the bacterial or fungal populations. It’s not a perfect correlation, but in general it holds.
• These tests are used to determine:
  1. Are fungi or bacteria dominant or is there equal biomass of both? Are there minimum levels of fungi, or bacteria, or high levels of both?
  2. Is there a pool of retained nitrogen in the form of organisms that can be released to plants later?
  3. Is there enough fungal biomass to immobilize solution calcium so it doesn’t leach?
  4. Are fungal biomass and bacterial biomass great enough to support the organisms that graze on bacteria and fungi? These higher forms balance the population levels of bacteria and fungi and release nitrogen into the soil in the form of ammonium for plant growth.

SFI can perform morphological diversity testing. In general this is a significant improvement over plate counts, since so few species of bacteria and fungi actually grow on any plate count medium. However, it takes molecular methods to assess the full diversity of bacteria and fungi in soils. We work with other programs that are in the process of developing these methods for practical applications.

Nematode Numbers and Community Structure

• We extract all the active nematodes from 50 to 100 grams of soil or compost. We count and identify those individuals and report numbers of individuals per gram dry soil.
• Nematodes are identified to genus and placed in one of four functional group classes according to what they eat. The report differentiates root-feeding nematodes to genus. Reports list the beneficial bacterial-feeders, fungal-feeders and predatory nematodes, if any.
• Beneficial nematodes are important in preventing root-feeding nematodes from finding the roots of plants. Beneficial nematodes are a very important part of root protection, one which most agricultural soils lack.
• Identification of insect-feeding nematodes can also be performed.
• This test is used to determine:
  1. Are any root-feeding nematodes present? Are they at economic damage thresholds?
  2. Are any beneficial nematodes present?
  3. Bacterial-feeding nematodes help balance total bacteria populations and release nitrogen back to the plant.
  4. Fungal feeders balance total fungal levels, including root rot fungi, and also help release the nitrogen locked up inside fungi back to the plant.
  5. Predatory nematodes are higher-order predators that help balance all other nematodes. It is desirable to have some of these around but they are especially delicate and easily hurt by tillage.

Protozoa

• Protozoa are single celled organisms that mostly eat bacteria, although some prefer to consume pathogenic, disease-causing fungi. Protozoa are very important in recycling the nitrogen and other nutrients locked up inside the bacteria.
• Some protozoa also attack nematodes and some will attack fungi. All in all, having good populations of the right kinds of protozoa makes for a balanced soil.
• Protozoa come in three major groups, the ciliates, flagellates, and the amoebae. The relative numbers of these groups assess whether the sample is aerobic or anaerobic.
• This test is used to determine:
  1. Are enough protozoa present to cycle adequate nutrients? Will enough nutrients become plant available?
  2. Are ciliates numbers too high, indicating anaerobic conditions in the soil?
  3. All three groups of protozoa help balance total bacteria populations and release nitrogen back to the plant.

Mycorrhizal fungi (VAM)

• The kind and amount of beneficial mycorrhizal colonization on the roots is determined in this test. Mycorrhizal fungi are extremely important fungi for plants that require colonization, such as most crop, vegetable, orchard and landscape trees and shrubs.
• If you have plants in the soil, you need know the percent of the root system colonized by mycorrhizal fungi. We not only assess VAM versus ectomycorrhizal colonization of the roots, track nodulation by N-fixing rhizobia, necrosis by disease-causing bacteria and fungi, but insect and soil pest feeding on the roots.
• Please remember that we need a representative sample of roots of the plant you want to know about included in the sample. It is best to send all the roots picked from the composite soil sample (see below on obtaining the soil sample).
• This test measures:
  1. Is enough of the root system protected by mycorrhizal fungi from disease-causing organisms?
  2. Is the root system colonized enough to supply nutrients at the rate the plant requires?
  3. Would the plant benefit from improved colonization?
  4. Percent of the root being attacked by disease-causing organisms.
  5. Percent of the root being attacked by root-feeding insects.

Microarthropods

This test provides information on the numbers and identification to major group of the visible soil critters. The important groups are the fungal-feeding, herbivorous, and predatory microarthropods.

Generally, soils disturbed by plowing, disking, chiseling, etc will have not significant microarthropod populations for a year or more unless mulch is placed on the soil surface. Still, many predators of pests are microarthropods, and you would want to know if you have these important bio-control organisms present in your soil.

Those microarthropods that are true soil-dwellers are usually small and inconvenient to see with the naked eye. The principal role of these creatures is to recycle nutrients and make them available for plants.

Foliage Assay™:
Allows determination of the area of leaf surface occupied by microorganisms such as bacteria and fungi.

• The work so far performed suggests that if 70% or more of the leaf surface is occupied by beneficial microorganisms, then foliar disease can be significantly reduced. Plants with 70% or more of the leaf surface occupied by beneficial microorganisms also appear to have higher leaf tissue concentrations of important nutrients.
• More work is needed to determine which species of bacteria or fungi will be most suppressive and whether different cultivars of plants will respond in different ways.

Let’s take a look at SFI results from some soil samples:

Vocabulary:

Hyphae (plural), Hypha (singular): The threadlike strands of fungi; see pictures in the SFI gallery.

Inoculum – means a set of individuals of a designated group to be added in order to re-establish these organisms in the material.

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