The Biology Of Soil Health 3

Bacteria come in a wide variety of shapes and sizes, but the most common are rods, spheres, and spirals.

Parasites feed off of other living organisms, weakening them, sometimes to the point of death. Saprophytes only consume dead organic matter; symbionts partner with living organisms, to the benefit of both; and endophytes are microbes that live inside plants in a form of mutualism.

Arthropods are defined as invertebrate of the phylum Arthropoda. These creatures have a segmented body, jointed limbs, and usually a chitinous shell that undergoes moltings. It includes insects, spiders and other arachnids, crustaceans, and myriapods (e.g., millipedes and centipedes). It does not include snails, which are molluscs (invertebrates of a large phylum that includes snails, slugs, mussels, and octopuses).

The relative content of sand, silt, and clay does not change as soils get healthier. What does change is the level of soil aggregation and the amount of soil organic matter. These changes allow soils to infiltrate water and hold more of that water. As a result, the creatures of the soil food web increase in both number and diversity and it is the soil food web that increases natural fertiliy and disease suppression.

It is important to note the term "and the relationships between them". The "soil food web" is an ecological concept and ecology stresses relationships between organisms, rather than just the organisms themselves and their functions.

Plants can only take in nutrients in ionic form (e.g., NO3-), dissolved in soil water. Although soil microbes play several important roles in making nutrients available to plants, neither bacteria or fungi are necessary for a plant to be able to absorb nutrients through its roots -- that process can happen simply as a result of the root hairs intercepting nutrients in soil water, upon which the plant root absorbs the nutrients vis diffusion. The nutrients do not have to be in high concentrations -- although most absorbtion of nutrients by roots is a result of diffusion, plants have mechanisms for pulling nutrients into their roots even against diffusion gradients, if required.

Ciliates are not bad in and of themselves. They are good nutreint cyclers, just like other protozoa and nematodes. However, because they are better survivors of low-oxygen conditions than the other protozoa, seeing large numbers of ciliates relative to other predator organisms can indicate that oxygen levels in the soil or compost are lower than optimal.

Bacteria and protozoa may be more numerous, but they are single-celled organisms. Nematodes are found everywhere, from oceans to mountain tops, and are extremely numerous.

Collembola are tiny arthopods -- otherewise known as "springtails" because they use their tails to hop from place to place.

The rhizosphere is the area of soil directly around the roots of a plant. Because of the exudates, microbial populations in root zones tend to be much greater than in bulk soil -- this is known as the rhizosphere effect.

Assuming the average bacterium to be about 1 micron (one millionth of a meter), then one metre wouldn hold one million microbes and one centimetre (one hundredth of a metre) would hold 1/100 of 1,000,000, or 10,000 microbes.

As ecological succession proceeds, both above and below ground, diversity increases. This means that more of the innumerable "niches" in the environment are filled. This generally results in higher productivity. However, because there are more organisms requiring nutrients, competition for these nutrients is greater and nutrient availability for individual organisms decreases. Accordingly, the plants growing in later sucessional communities tend to be more efficient users of those nutrients. An analogy would be that early successional communities favour sprinters (faster-growing but inefficient nutrient users), while later successional communities favour long-distance runners (slower-growing but efficient nutrient users).

Compaction is the result of soil aggregate degradation, not the cause. The principal cause is the loss of the biological glues, which can happen for a variety of reasons. Some scientists think that OVERuse of synthetic fertilizers may be a factor; for example, feeding excess nitrogen and other nutrients to decomposer microbes can grow their populations, which then look for carbon and find it in the biological glues. Most certainly, tillage is hard on aggregates because it introduces higher levels of oxygen into the soil, which again causes a "bloom" of decomposer bacteria, which need carbon to grow and reproduce and will take it from wherever thay can find it (including the glues that hold the aggregates together).

Most fungi grow as long, thread-like hyphae (singular - hypha), which individually are microscopic and therefore not visible to the naked eye. The term mycelia (singular - mycelium) refers to the mass of hyphae, which can be so numerous as to be visible.

From the Greek words "myco " for "fungus" and "rhyza" for "root".