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10.3: Soil Microorganisms

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  • Soil organisms usually comprise much less than 0.5 % of the dry soil mass. Practically nothing can be done to permanently alter the microbial population of a soil. Normally, the soil contains a vast array and abundance of organisms. Consequently, most direct additions of organisms (inoculation) to this community will be ineffective. Usually, microbial populations will be altered more by changing the crop growing in the soil than by adding other microorganisms. However, in soils that have been steam sterilized or fumigated to control plant pathogens, the microbial numbers have been reduced greatly (but not eliminated). Adding fresh soil to a steam-sterilized soil will reintroduce many microbes but may add undesirable plant pathogenic microbes with the fresh soil. The average biomass distribution in an acre-furrow-slice of soil is approximately as presented in Table 1.

    Table 1. Distribution of living organisms and organic matter in an acre-furrow-slice (about 2,000,000 lbs. soil) of a highly productive soil.
    Soil Organisms Estimated biomass

    Number of organisms

    lbs./AFS or kg/ha Number/AFS Number/g
    Earthworms 200 to 500 1.5 x 105 1.6 x 10-4
    Myriapods 5 to 40 2.2 x 106 2.4 x 10-3
    Insects 100 to 200 5.05 x 103 2.3 x 10-6
    Rodents 50 to 100 1.3 x 102 1.4 x 10-7
    Nematodes 1 to 2 9.1 x 107 1.0 x 10-1

    Plant roots + root hairs

    2,500 to 4,000 1.13 x 1013 1.35 x 104
    Bacteria 3,000 to 4,500 2.4 x 1018 2.6 x 109
    Actinomycetes 1,000 to 2,000 1.3 x 1017 1.4 x 108
    Fungi 2,500 to 4,500 3.3 x 1015 3.7 x 106
    Protozoa 10 to 75 1.2 x 1013 1.3 x 104
    Algae 5 to 10 2.5 x 1014 2.8 x 105
    Dead organisms + humus 40,000 to 60,000 --- ---

    Source: Brady, N.C. and R.R. Weil. 2002. The nature and properties of soils. 13th ed. Prentice Hall, Upper Saddle River, NJ.  AFS = acre-furrow-slice = 2,000,000 lbs. of dry soil which is assumed to contain about 2.5 % humus. 1 hectare-15 cm = 2,000,000 kg of dry soil which is assumed to contain about 2.5 % humus.

    A number of conditions affect microbial populations in soils. The optimum temperature range for decay organisms is between 70-100 °F (about 20-40 °C). Soil temperatures outside this range will retard the activity of most soil organisms. Excessive water in soil reduces the numbers and kinds of living organisms due to poor aeration. However, at low moisture levels soil organisms thrive better than do higher plants. Numbers of fungi, bacteria and actinomycetes vary with soil pH. If pH of the soil is <6.0, the fungi become the dominant soil microorganisms. The supply of nutrients, organic material for energy, and free oxygen gas affect microbial numbers. Fortunately, optimum soil conditions for most plants and for most soil microorganisms are similar.

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