How to Interpret Soil Foodweb Assays

Steps 6 - 11

Steps Six, Seven, Eight:
Flagellates (Six), Amoebae (Seven), Ciliates (Eight). These are the three groups of protozoa and they are critical in a bacterial-dominated soil, because the plants need a way to access all that wonderful nutrient tied up in the bacteria. Nutrients in the bacteria cannot be obtained by plant roots, so something has to eat the bacteria to release those nutrients. That’s what protozoa do. Protozoa also help build the larger soil pores by pushing aggregates around as the protozoa search for and try to reach the bacteria tucked away around soil particles.

1. 1. If the protozoa are too low in number, the nutrients remain tied up in bacterial and fungal bodies. Even if the bacteria and fungi die, they may not release the nutrients in their bodies until the protozoa come along. In many early microbial studies, microbiologists doing plate counts did not recognize that the protozoa were still in their "pure cultures", and it was the protozoa "mineralizing" nutrients, not the bacteria themselves. When protozoa are too low, and nematodes are too low as well, then inorganic fertilizer will have to be added in order to supply N, P, S etc to the plant. This is expensive and a large proportion of these nutrients will likely be lost from the soil, either by leaching or by volatilization. Until the protozoa are inoculated and brought to desired numbers, nutrient loss will continue to be a problem. Protozoa inocula are available in the form of good compost, good compost tea, or from a commercial source, Holmes Environmental, holmesenviro@attbi.com
2. If the protozoa are within the desired range, nutrients will be made available for the plants are minimal amounts over time. How much will be made available? That will be discussed in the section on Plant Available N made available to plants (see below). But reductions in fertilizer applications should be possible if protozoa are in good range.
3. If protozoa numbers are extremely high, or the different groups are very un-balanced, then nutrient cycling will be variable, and there may be periods when pulses of ammonium or nitrate may accumulate. These forms are subject to leaching and loss through gas production, and may result in weeds having the nitrate they need to germinate, grow and outcompete the crop or desired plant species.
4. If ciliates are too high, then the soil is either compacted or water-logged, and lacking oxygen. Ciliates are aerobic organisms, but prefer to consume anaerobic bacteria. They tolerate reduced oxygen conditions better than the other protozoa, so high numbers of ciliates indicate problems with the movement of oxygen into the soil which needs to be fixed. Of course, it the soil gets too anaerobic, all three groups of protozoa will be low.
5. When ciliates are high, but flagellates and amoebae are also high suggests that one of three things may be happening:
   1. The sample has just become compacted, or flooded, and the anaerobic conditions have just been initiated. Generally the number of ciliates is not extremely high.
   2. The sample has aggregates, which are anaerobic inside the aggregates. The high ciliate signal comes from the internal parts of those aggregates where anaerobic conditions exist, but outside those aggregates, aerobic conditions exist, and thus flagellate and amoebae numbers are typically high as well. Both anaerobes and aerobes co-exist, but in very different places within the spatial structure of this sample. This is very typical of good worm compost, particularly worm compost high in castings.
   3. The sample has been anaerobic in the past, but is just becoming aerobic. Flagellates and amoebae are growing because aerobic bacteria have begun to grow. Generally, ciliate numbers will be fairly high, while flagellate and amoebae are just barely in good range. Quite often this will result in nitrate pulses and germination of weed seeds.
6. When flagellates are high and amoebae low, or flagellates low and amoebae high indicates an imbalance in nutrient cycling, with pulses of nitrate being produced, resulting in weeds being able to out-compete the desired plants.
7. What do you feed protozoa? Bacteria. So, if you have taken care of step one and two, the bacteria should be there for the protozoa to eat.

Steps Nine, Ten, Eleven:
Bacterial-feeding nematodes (9), Fungal-feeding nematodes (10) and Predatory nematodes (11). The beneficial nematodes consume their prey groups, and in the case of bacterial- and fungal-feeders, release N, P, S, and micronutrients that would now be available to plants, if the majority of the cycling occurs in the root system. These nematodes also interfere with the ability of the root-feeding nematodes finding the root. The higher number of these organisms, the more nutrient cycling is occurring.

< Steps 5 - Step 12 >