Everything about Soil Life totally explained
Soil life or
soil biota is a collective term for all the organisms living within the soil.
Overview
In a balanced soil, plants grow in an active and vibrant environment. The
mineral content of the soil and its physical structure are important for their well-being, but it's the life in the earth that powers its cycles and provides its fertility. Without the activities of soil organisms,
organic materials would accumulate and litter the soil surface, and there would be no food for plants.
The soil biota includes:
- Megafauna: size range 20 mm upwards, for example moles, rabbits, and rodents.
- Macrofauna: size range 2-20 mm, for example woodlice, earthworms, beetles, centipedes, slugs, snails, ants, and harvestmen.
- Mesofauna: size range 100 micrometre-2 mm, for example tardigrades, mites and springtails.
- Microfauna and Microflora: size range 1-100 micrometres, for example yeasts, bacteria, fungi, protozoa, roundworms, and rotifers.
Of these, bacteria and fungi play key roles in maintaining a healthy soil. They act as
decomposers that break down organic materials to produce
detritus and other breakdown products. Soil
detritivores, like earthworms, ingest detritus and decompose it.
Saprotrophs, well represented by fungi and bacteria, extract soiluble nutrients from delitro.
Bacteria
Bacteria are single-celled organisms, and are the most numerous denizens of the soil, with populations ranging from 100 million to 3 billion in a gram. They are
capable of very rapid reproduction by binary fission (dividing into two) in favourable conditions. One bacterium is capable of producing 16 million more in just 24 hours. Most soil bacteria live in close proximity to plant roots and are often referred to as rhizobacteria. Bacteria live in soil water, including the film of moisture surrounding soil particles, and some are able to swim by means of
flagella. The majority of the beneficial soil-dwelling bacteria need oxygen (and are thus termed
aerobic bacteria), whilst those that don't require air are referred to as
anaerobic, and tend to cause
putrefaction of dead organic matter. Aerobic bacteria are most active in a
soil that's moist (but
not saturated, as this will deprive aerobic bacteria of the air that they require), and neutral
soil pH, and where there's plenty of food (
carbohydrates and
micronutrients from organic matter) available. Hostile conditions won't completely kill bacteria; rather, the bacteria will stop growing and get into a dormant stage, and those individuals with pro-adaptive
mutations may compete better in the new conditions. Gram positive bacteria produce spores in order to wait for more favourable circumstances, and Gram negative bacteria gets into a "nonculturable" stage.
From the organic gardener's point of view, the important roles that bacteria play are:
Nitrification
Nitrification is a vital part of the
nitrogen cycle wherein certain bacteria (which manufacture their own
carbohydrate supply without using the process of photosynthesis) are able to transform
nitrogen in the form of ammonium, which is produced by the decomposition of proteins, into nitrates, which are available to growing plants, and once again converted to proteins.
Nitrogen fixation
In another part of the cycle, the process of
nitrogen fixation constantly puts additional nitrogen into biological circulation. This is carried out by free-living nitrogen-fixing bacteria in the soil or water such as
Azotobacter, or by those which live in close symbiosis with
leguminous plants, such as
rhizobia. These bacteria form colonies in nodules they create on the roots of
peas,
beans, and related species. These are able to convert nitrogen from
the atmosphere into nitrogen-containing organic substances.
Denitrification
While nitrogen fixation converts nitrogen from the
atmosphere into organic compounds, a series of processes called denitrification returns an approximately equal amount of nitrogen to the atmosphere. Denitrifying bacteria tend to be anaerobes, or facultatively anaerobes (can alter between the oxygen dependent and oxygen independent types of metabolisms), including
Achromobacter and
Pseudomonas. The putrefaction process caused by oxygen-free conditions converts nitrates and nitrites in soil into nitrogen gas or into gaseous compounds such as
nitrous oxide or
nitric oxide. In excess, denitrification can
lead to overall losses of available soil nitrogen and subsequent loss of soil fertility. However, fixed nitrogen may circulate many times between organisms and the soil
before denitrification returns it to the atmosphere. The diagram below illustrates the nitrogen cycle.
Actinobacteria
Actinobacteria are critical in the decomposition of
organic matter and in
humus formation, and their presence is responsible for the sweet "earthy" aroma which is associated with a good healthy soil. They require plenty of air and a pH between 6.0 and 7.5, but are more tolerant of dry conditions than most other bacteria and fungi.
Fungi
A gram of garden soil can contain around one million
fungi, such as
yeasts and
moulds. Fungi have no
chlorophyll, and are not able to
photosynthesise; besides, they can't use atmospheric carbon dioxide as a source of carbon, therefore they're
chemo-heterotrophic, meaning that, like
animals, they require a chemical source of energy rather than being able to use light as an energy source, as well as organic substrates to get carbon for growth and development.
Many fungi are parasitic, often causing disease to their living host plant, although some have beneficial relationships with living plants as we'll see below. In terms of soil and humus creation, the most important fungi tend to be
saprotrophic, that is, they live on dead or decaying organic matter, thus breaking it down and converting it to forms which are available to the higher plants. A succession of fungi species will colonise the dead matter, beginning with those that use sugars and starches, which are succeeded by those that are able to break down
cellulose and
lignins.
Fungi spread underground by sending long thin threads known as
mycelium throughout the soil; these threads can be
observed throughout many soils and
compost heaps. From the mycelia the fungi is able to throw up its fruiting bodies, the visible part above the soil (for example,
mushrooms,
toadstools and
puffballs) which may contain millions of
spores. When the
fruiting body bursts, these spores are dispersed through the air to settle in
fresh environments, and are able to lie dormant for up to years until the right conditions for their activation arise or the right food is made available.
Mycorrhizae
Those fungi that are able to live symbiotically with living plants, creating a relationship that's beneficial to both, are known as
Mycorrhizae (from
myco meaning fungal and
rhiza meaning root). Plant root hairs are invaded by the mycelia of the mycorrhiza, which lives partly in the soil and partly in the root, and may either cover the length of the root hair as a sheath or be concentrated around its tip. The mycorrhiza obtains the carbohydrates that it requires from the root, in return providing
the plant with nutrients including nitrogen and moisture. Later the plant roots will also absorb the mycelium into its own tissues.
Beneficial mycorrhizal associations are to be found in many of our edible and flowering crops.
Shewell Cooper suggests that these include at least 80% of the
brassica and
solanum families (including
tomatoes and
potatoes), as well as the majority of
tree species, especially in
forest and woodlands. Here the mycorrhizae create a fine underground mesh which extends greatly beyond the limits of the tree's roots, thus greatly increasing their feeding range and actually causing neighbouring trees to
become physically interconnected. The benefits of mycorrhizal relations to their plant partners are not limited to nutrients, but can be essential for plant reproduction: in situations where little light is able to reach the forest floor, such as the North American
pine forests, a young seedling can't obtain sufficient light to photosynthesise for itself and won't grow properly in a sterile soil. But if the ground is underlain by a mycorrhizal mat then the developing seedling will throw down roots that can link with the fungal threads and through them obtain the nutrients it needs, often indirectly obtained from its parents or neighbouring trees.
David Attenborough points out the plant/fungi/animal relationship that creates a "Three way harmonious trio" to be found in forest
ecosystems wherein the plant/fungi symbiosis is enhanced by animals such as the wild boar, deer, mice or flying squirrel which feed upon the fungi's fruiting bodies, including truffles, and cause their further spread (
Private Life Of Plants, 1995). A greater understanding of the complex relationships which pervade natural systems is one of the major justifications of the
organic gardener, in refraining from the use of artificial chemicals and the damage these might cause.
Further Information
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