A. 7. Interpreting Soil Foodweb information

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.

Review the Organism Biomass Data table in a seperate window

Dry Weight: All three composts have moistu sre in good range. If the soil is too wet, aeration will be a problem and roots will be killed. Too dry and organisms cannot grow.

Active bacterial biomass: Control is in good range. HH is in good range, not significantly different from the control. In Biol, bacterial activity is low, resulting in poor decomposition, poor nutrient retention, a lack of soil structure building and limited disease suppression. Need to add bacterial inoculum, or bacterial foods to wake up the bacteria that are present (see total bacterial biomass).

Total Bacterial Biomass: Control has adequate total bacterial biomass but both treatments have low total bacterial biomass, for different reasons. Fungal growth is probably out-competing bacterial growth in Plus HH, while something in Bio1 is detrimental to bacteria. In all samples, sleeping, dormant organisms are present (active subtracted from total). Some unknown percentage of these dormant, sleeping organisms would grow on plate counts.

Active Fungal Biomass: Active fungi low in the control and in the Bio1 sample. Plus HH has in desired range activity. Disease suppression, nutrient retention, and soil building will be present in HH, not in the control or in the Bio1 samples. Need to add a fungal inoculum and fungal foods to these two samples.

Total Fungal Biomass: Both control and Bio1 too low, and therefore fungal diversity is lacking. Need to add fungal foods to get fungal decomposition going. Fungal foods are, for example, humic acids, many fulvic acids, dry, brown leaf materials, wood chips, sawdust, paper, cardboard. May need to add a fungal inoculum as well. The HH treatment has adequate fungal biomass, showing that just by adding humic acid that fungal biomass can be resuscitated.

Hyphal Diameter: As indicated by the footnote on the table, the diameter of the hyphae observed in these samples indicate typical soil fungi, a mix of beneficials and not-so-beneficial are present in the control and in the Bio1 treatment. In the HH treatment, this amendment has provided food for the beneficial fungi and they grew in preference to the less beneficial fungi. This is a very good result for any soil.

Protozoa: All too low, in all samples. Need to add an inoculum of protozoa, which is typically found in compost with higher moisture, compost tea, or in commercial products (see www.soilfoodweb.com for list). Note the variability in this assessment method. All three values of flagellates are probably not significantly different one for the other. How is this determined? From replicate sampling from a set of soil samples.

Nematodes: All too low numbers, low diversity. This is the hardest component of the foodweb to return to healthy conditions once the food web has been destroyed. An inoculum of beneficial nematodes is needed to re-establish this group of the soil foodweb.

Review the Fungal to Bacterial Biomass table in a seperate window

Total Fungal to Total Bacterial Biomass: Control and Bio1 both on the bacterial-dominated side, while HH is fungal-dominated. Need to add more fungal foods and possibly need fungal inoculum in control and Bio1 samples. Control and Bio1 samples reveal foodwebs appropriate for vegetables, annual plants. The HH sample has a ratio appropriate for bentgrass and other productive pasture or lawn grasses.

Active to Total Fungal Biomass: Only HH has decent levels of active fungi. Neither the control nor the Bio1 sample have enough active fungi to protect the plant against disease-causing organisms. An inoculum of beneficial fungi would be wise, but certainly fungal foods are needed.

Active to Total Bacterial Biomass: Both the control and HH have adequate active bacteria relative to total bacteria, but the Bio1 treatment has reduced the active bacterial component severely. This amendment is detrimental to both active and dormant bacteria,

Active Fungi to Active Bacteria: Which microbial group is winning? Bacteria are in the control, so the soil will become even more bacterial over the next few weeks. In the HH treatment, the beneficial fungi were enhanced and the ratio shows that the soul will become more and more fungal with time. In the Bio1 treatment, the bacteria were killed, so this ratio is very fungal. Because active bacteria were harmed more than the fungi, the ratio is quite skewed. Fungi will grow in this soil more than bacteria, until conditions change, but it may well be un-desirable fungi because the bacteria are not performing their jobs.

Plant Available N: Low in all three soils. Need a protozoan inoculum.

Nematodes: No root-feeders detected, but no beneficial nematodes found either. Need a beneficial nematode inoculum. Nematodes are the most difficult group to get re-established.

For additional questions, please e-mail Soil Foodweb Inc.

< previous next >