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Micro Organisms in Soils - Horticultural Products - Viresco UK - Aquatic Weed Control and Horticultural Supplies

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Viresco (UK) Ltd Tel: 44 (0)1845 525585 Fax: 44 (0)1845 523133 Email:sales@viresco-uk.com

Viresco (UK) Limited is a company registered in England and Wales with company number 5079726 VAT No: 481 9223 35 © 2003-2012

Suppliers of speciality products for the amateur pondkeeper and gardener worldwide


A Good Soil is a Living Soil

In recent years, there has been increasing concern about the use of pesticides, particularly in food crops. As consumers, we never know what chemicals have been used in the cultivation of the fruit and vegetables that we buy and eat.


As a part response to this concern, chemicals are being increasingly taken out of use because of the possible harmful effects that they may cause when treated produce is eaten. Our products are ideal for those who do not wish to use pesticides and who practice organic gardening.

The use of chemicals causes imbalances in soil and other growing media. Chemicals are used to help increase yield, to reduce disease and to cut down on pests. Whilst these aims may to a greater or lesser extent be achieved, another consequence is that growing media are put out of balance regarding microbial populations and other problems then arise.


A good soil or other growing medium is a living soil. It is the home of numerous micro-organisms and other life forms representing many genera and species. The numbers, kinds and activities of these organisms are influenced by the food available or organic content of the soil, soil texture, pH, moisture, aeration, temperature and other factors.

They live according to the rule of the survival of the fittest. In some soils, a few of the micro-organisms may parasitise or injure plant roots. The vast majority, however, perform beneficial functions that are important for both soil and plant

Some of micro-organisms, eg yeasts and yeast-like enzymes, live on leaf and fruit surfaces. Others, especially bacteria, are present in very large numbers on living root surfaces where they feed on excreted organic food materials such as organic acids, sugars and discarded cell debris. Some are best adapted to decompose dead roots and other plant residues and others slowly utilise the more resistant soil humic substances formed through microbial activity.

Types and Numbers of Micro-organisms in Soil

To give an indication of the numbers of micro-organisms present in good soils, it is estimated that the weight of live micro-organisms would vary from about 500kg to 2000kg per hectare!! The primitive forms of plant micro-organisms include bacteria, actinomycetes, fungi and algae. Protozoa and nematodes are soil animals.

The listing below gives an indication of the numbers of organisms present in good fertile soils.

Bacteria: 3,000,000 to 500,000,000 in 1 gram
Actinomycetes: 1,000,000 to 20,000,000 in 1 gram
Fungi: 5,000 to 1,000,000 in 1 gram
Yeasts: 1,000 to 100,000 in 1 gram
Protozoa: 1,000 to 500,000 in 1 gram
Algae: 1,000 to 500,000 in 1 gram
Nematodes: 10 to 5,000 in 10 grams

In addition, there are large numbers of microbial viruses, slime moulds, insects and earthworms.

Functions of Micro-organisms in Soils

Soil micro-organisms have several important functions. These are the:-

1. Decomposition of organic residues with release of nutrient element constituents such as nitrogen, phosphorus and sulphur.
2. Formation of beneficial soil humus through their activities in decomposing organic residues and through the synthesis of new compounds and polymers.
3. Improvement of soil physical properties such as better aggregation or tilth, better infiltration of water and improved aeration.
4. Release of plant nutrient elements such as phosphorus, potassium, iron etc from insoluble inorganic minerals.
5. Fixing of nitrogen by utilising gaseous nitrogen from the air and synthesising it into protein and other cellular organic compounds containing nitrogen.
6. Improvement of plant nutrition through mycorrhizal relationships between fungi and plant roots.
7. Antagonistic action against plant pathogens which destroy plant roots.

Effect of Pesticides on Numbers of Soil Micro-organisms

Pesticides are applied to soils or plants to kill or control plant pests including disease micro-organisms. Much of the material that is applied to foliage may also reach the soil. Most of these insecticide chemicals, if used according to instructions, will not exert a permanent effect on soil micro-organisms. Whilst they may kill a large number of soil and leaf insects, they have little effect on soil bacteria and fungi.

However, soil fumigants and some fungicides kill large numbers of soil micro-organisms. After the initial kill, organisms not killed, or certain species which quickly re-establish, become dominant and may reach numbers far in excess of those in the untreated soil.

Effect of Fumigation on the
Numbers of Bacteria in a Loam Soil (pH 7.4)
Millions of Bacteria per gram after
1 day
10 days
50 days












This is illustrated in the above table and is a very good example of how the use of pesticide chemicals can put a growing medium out of balance. The table shows the effect of fumigation on the numbers of bacteria in a particular loam soil with a pH of 7.4. Two fumigant treatments, D-D and chloropicrin, were made and compared with control. The numbers of bacteria were checked 1 day, 10 days and 50 days after treatment.

The figures in the table, particularly for the treatment with chloropicrin, illustrate well what can happen with the numbers of bacteria in soils when fumigants are used. With chloropicrin, only one day after treatment 37 million bacteria per gram have been reduced to just 4 million. However, those 4 million, after a further 9 days, had increased to 96 million, which is over three times the number found in the untreated soil after the same time interval! Thus, quite drastic imbalances can occur that can cause subsequent growing problems.