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 the wonderful nutrients
tied up in the bacteria. Nutrients within 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.
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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.
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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.
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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.
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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.
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When ciliates are high, but flagellates and amoebae are also high suggests
that one of three things may be happening:
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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.
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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.
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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.
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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.
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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.
