Ivydene Gardens Soil:
What are Soil Nutrients?
 

What types of organism are found in the soil?

Traditionally soil organisms have been divided into 3 size class ranges.

Some fungi groups play an important role in the decomposition of woody tissue, which is difficult to decompose because of its high lignin concentration. The majority of nematodes feed upon members of the microbiota, helping to recycle nutrients - see A more in depth description of how Soil works on the interaction between these soil organisms in the right hand table.

 

In terms of nutrient cycling and beneficial effects on soil structure, earthworms are probably the most important members of the macrobiota. Earthworms tend to be greatest in neutral soils (pH 7), that have high concentrations of organic matter. They play a crucial role in the physical breakdown of organic matter into smaller units, thereby increasing its surface area and speeding up its decomposition by soil bacteria and fungi.

 

The following shows the size differences of soil organisms:-

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sedumflohispanicum1a

Sedum hispanicum

Microbiota
(less than 0.2mm)

Mesobiota
(0.2-10mm)

Macrobiota
(greater than 10mm)

 

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Bacteria

Nematodes

Earthworms

Cyanobacteria

Rotifers

Large Insects

Slime Moulds

Springtails

Snails

Fungi

Mites

Centipedes

Protozoa

Small arthropods

 

Algae

 

 

 

and the following shows the approximate numbers of organisms (per gram) commonly found in the microbiota:-

Organism

Estimated number/gram

Bacteria (not including Actinomycetes)

3 000 000 - 500 000 000

Actinomycetes

1 000 000 - 20 000 000

Fungi

5 000 - 900 000

Algae

1 000 - 500 000

Protozoa

1 000 - 500 000

 

A more-in-depth explaination of how soil works:-

"Plants are in Control

Most gardeners think of plants as only taking up nutrients through root systems and feeding the leaves. Few realize that a great deal of energy that results from photosynthesis in the leaves is actually used by plants to produce chemicals they secrete through their roots. These secretions are known as exudates. A good analogy is perspiration, a human's exudate.

Root exudates are in the form of carbohydrates (including sugars) and proteins. Amazingly, their presence wakes up, attracts, and grows specific beneficial bacteria and fungi living in the soil that subsist on these exudates and the cellular material sloughed off as the plant's root tips grow. All this secretion of exudates and sloughing off of cells takes place in the rhizosphere, a zone immediately round the roots, extending out about a tenth of an inch, or a couple of millimetres. The rhizosphere, which can look like a jelly or jam under the electron microscope, contains a constantly changing mix of soil organisms, including bacteria, fungi, nematodes, protozoa, and even larger organisms. All this "life" competes for the exudates in the rhizosphere, or its water or mineral content.

At the bottom of the soil food web are bacteria and fungi, which are attracted to and consume plant root exudates. In turn, they attract and are eaten by bigger microbes, specifically nematodes and protozoa who eat bacteria and fungi (primarily for carbon) to fuel their metabolic functions. Anything they don't need is excreted as wastes, which plant roots are readily able to absorb as nutrients. How convenient that this production of plant nutrients takes place right in the rhizosphere, the site of root-nutrient absorption.

At the centre of any viable soil food web are plants. Plants control the food web for their own benefit, an amazing fact that is too little understood and surely not appreciated by gardeners who are constantly interfereing with Nature's system. Studies indicate that individual plants can control the numbers and the different kinds of fungi and bacteria attracted to the rhizosphere by the exudates they produce.

Soil bacteria and fungi are like small bags of fertilizer, retaining in their bodies nitrogen and other nutrients they gain from root exudates and other organic matter. Carrying on the analogy, soil protozoa and nematodes act as "fertilizer spreaders" by releasng the nutrients locked up in the bacteria and fungi "fertilizer bags". The nematodes and protozoa in the soil come along and eat the bacteria and fungi in the rhizosphere. They digest what they need to survive and excrete excess carbon and other nutrients as waste.

The protozoa and nematodes that feasted on the fungi and bacteria attracted by plant exudates are in turn eaten by arthropods such as insects and spiders. Soil arthropods eat each other and themselves are the food of snakes, birds, moles and other animals. Simply put, the soil is one big fast-food restaurant.

Bacteria are so small they need to stick to things, or they will wash away; to attach themselves they produce a slime, the secondary result of which is that individual soil particles are bound together. Fungal hyphae, too, travel through soil particles, sticking to them and binding them together, thread-like, into aggregates.

Worms, together with insect larvae and moles move through the soil in search of food and protection, creating pathways that allow air and water to enter and leave the soil. The soil food web, then, in addition to providing nutrients to roots in the rhizosphere, also helps create soil structure: the activities of its members bind soil particles together even as they provide for the passage of air and water through the soil.

Without this system, most important nutrients would drain from soil. Instead, they are retained in the bodies of soil life. Here is the gardener's truth: when you apply a chemical fertilizer, a tiny bit hits the rhizosphere, where it is absorbed, but most of it continues to drain through soil until it hits the water table. Not so with the nutrients locked up inside soil organisms, a state known as immobilization; these nutrients are eventully released as wastes, or mineralized. And when the plants themselves die and are allowed to decay in situ, the nutrients they retained are again immobilized in the fungi and bacteria that consume them.

Just as important, every member of the soil food web has its place in the soil community. Each, be it on the surface or subsurface, plays a specific role. Elimination of just one group can drastically alter a soil community. Dung from mammals provides nutrients for beetles in the soil. Kill the mammals, or eliminate their habitat or food source, and you wont have so many beetles. It works in reverse as well. A healthy soil food web won't allow one set of members to get so strong as to destroy the web. If there are too many nematodes and protozoa, the bacteria and fungi on which they prey are in trouble and, ultimately, so are the plants in the area.

And there are other benefits. The nets or webs fungi form around roots act as physical barriers to invasion and protect plants from pathogenic fungi and bacteria. Bacteria coat surfaces so thoroughly, there is no room for others to attach themselves. If something impacts these fungi or bacteria and their numbers drop or disappear, the plant can easily be attacked.

 

Negative impacts on the soil food web

Chemical fertilizers, pesticides, insecticides, and fungicides affect the soil food web, toxic to some members, warding off others, and changing the environment. Important fungal and bacterial relationships don't form when a plant can get free nutrients. When chemically fed, plants bypass the microbial-assisted method of obtaining nutrients, and microbial populations adjust accordingly. Trouble is, you have to keep adding chemical fertilizers and using "-icides", because the right mix and diversity - the very foundation of the soil food web - has been altered.

It makes sense that once the bacteria, fungi, nematodes and protozoa are gone, other members of the soil food web disappear as well. Earthworms, for example, lacking food and irritated by the synthetic nitrates in soluble nitrogen fertilizers, move out. Since they are major shredders of organic material, their absence is a great loss. Soil structure deteriorates, watering can become problematic, pathogens and pests establish themselves and, worst of all, gardening becomes a lot more work than it needs to be.

If the salt-based chemical fertilizers don't kill portions of the soil food web, rototilling (rotovating) will. This gardening rite of spring breaks up fungal hyphae, decimates worms, and rips and crushes arthropods. It destroys soil structure and eventually saps soil of necessary air. Any chain is only as strong as its weakest link: if there is a gap in the soil food web, the system will break down and stop functioning properly.

Gardening with the soil food web is easy, but you must get the life back in your soils. First, however, you have to know something about the soil in which the soil food web operates; second, you need to know what each of the key members of the food web community does."

 

Both these concerns are taken up in the rest of Part 1 of Teaming with Microbes - The Organic Gardener's Guide to the Soil Food Web by Jeff Lowenfels and Wayne Lewis ISBN-13:978-1-60469-113-9 Published 2010. This book explains in non-technical language how soil works and how you can improve your garden soil to make it suitable for what you plant and hopefully stop you using chemicals to kill this or that, but use your grass cuttings and prunings to mulch your soil - the leaves fall off the trees, the branches fall on the ground, the animals shit and die on the land in old woodlands and that material is then recycled to provide the nutrients for those same trees, rather than being carefully removed and sent to the dump as most people do in their gardens leaving bare soil.

 

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