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The Rhizosphere
THE RHIZOSPHERE
The rhizosphere is the word used to describe the area of soil surrounding plant roots. It is the most biologically active layer of the soil; populated with micro organisms interacting and benefiting from chemicals released by plant roots (1,2,7). There are more micro organisms present in a teaspoon of soil than there are people on the earth; the rhizosphere can carry 1000-2000 times this amount making it highly populated with microbial life (2). The rhizosphere has three zones (2,9,10)-the endorhizosphere, the rhizoplane and the ectorhizosphere (figure 1).
1. The endorhizosphere: This is the inner section of the rhizosphere. It is the section of the plant root occupied by micro organisms (which benefit from organic compounds released by roots).
2. The rhizoplane: This is the middle section of the rhizosphere. It includes the plant root surface with soil particles adhering to it.
3. The ectorhizosphere: The outer area of soil surrounding the roots.
RHIZODEPOSITION
Rhizodeposition is what makes the rhizosphere an interesting place for biological study. Plant roots release organic compounds into their surrounding soils (1). These compounds are what scientists refer to as rhizodeposits and the process is called rhizodeposition. As plants release organic compounds from their roots they lose some of their carbon to the surrounding soil; this means that they contribute to the soil carbon content (3). So the soil benefits from the plants lost carbon. The release of organic compounds in the root zone also supports the soil microbial life (3). The amount and composition of rhizodeposits from plant roots vary depending on the type of plant, the climate, the nutrient deficiency, and the physical, chemical and biological properties of soil surrounding the root (2). Rhizodeposits are released through the following ways (3):
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What Are Effective Microorganisms?
WHAT ARE EFFECTIVE MICROORGANISMS?
Effective Microorganisms (EM) are mixed cultures of beneficial naturally-occurring organisms that can be applied as inoculants to increase the microbial diversity of soil ecosystem. They consist mainly of the photosynthesizing bacteria, lactic acid bacteria, yeasts, actinomycetes and fermenting fungi. These microorganisms are physiologically compatible with one another and can coexist in liquid culture. There is evidence that EM inoculation to the soil can improve the quality of soil, plant growth and yield (Kengo and Hui-lian, 2000).
BACKGROUND AND CONCEPT OF EFFECTIVE MICROORGANISMS
Healthy soil ecology has the capability of protecting plants against soil associated diseases caused by pathogenic microorganisms and parasites. The soil system offers this protection through a balanced relationship between pathogenic and billions of beneficial microorganisms working together in synergy. The presence of these beneficial microorganisms in any soil system is what precisely distinguishes a “living soil” from a “dead soil”. They decompose and ferment organic fraction of the soil system converting it into humus containing nutrients while releasing hormones that facilitate plant growth. They are responsible for providing hormones, nutrients and minerals in a useable form to the plants through the root system. In addition, they bring together soil particles in the soil structure enabling it to retain nutrients and moisture (Kengo and Hui-lian, 2000).
Soil ecosystem can therefore be regarded as a “living system” costing of diverse groups of microorganisms. For this reason, farmers had long before been using animal manures, composts and “compost tea” which is a liquid extract of compost that also contains plant growth compounds and beneficial microorganisms. These mixtures could then be applied to soil and crops to improve the soil quality and help protect crop plants against microbiological infections (Ghosh et al., 2004).
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