Management of Soil Biological Characteristics
A complex array of soil-dwelling plants and animals decomposes organic matter, mineralizes organic forms of nutrients, and fixes nitrogen. These beneficial biological activities enhance the soil's ability to release nutrients needed for plant growth and to break down plant residues. Examples of cases where increased biological activity also reduces the incidence of soil pathogens are discussed in Nematode Management. Tillage practices that aerate the soil enhance biological activity by providing oxygen and mixing organic matter throughout the tilled area. On the other hand, broad-spectrum fumigants such as methyl bromide kill beneficial soil organisms along with soil pests.
Bacteria and Other Microorganisms
Both ammonium and nitrate are readily available sources of nitrogen to plants and soil microorganisms. A variety of soil microorganisms convert organic nitrogen to ammonium, but only specific (nitrifying) bacteria convert ammonium to nitrate (nitrification). Nitrifying bacteria are most effective in well aerated soils at soil temperatures of 80 to 90 degrees F, when the C:N ratio is low to medium and when soil moisture is adequate for plant growth. High activity of these bacteria increases N availability , but the nitrates released by nitrifying bacteria are highly soluble and subject to leaching. Thus, if not absorbed by crop or covers, nitrate may be lost from the field.
In addition, other soil bacteria (denitrifiers) reduce nitrates to elemental nitrogen or nitrous oxide which are lost into the atmosphere as they are volatilized. Denitrification is most likely to occur in poorly drained soils where oxygen levels are low (anaerobic conditions).
Not only do naturally occurring microorganisms play an essential role in the nitrogen cycle, they have also been reported to decrease some populations of pathogenic bacteria and fungi. For example, the fungus Gliocladium virens, which tends to increase with increased levels of organic matter, controls damping-off pathogens. G. virens is also available commercially. Agrobacterium, a naturally occurring soil microorganism, reportedly restricts growth of Fusarium, a fungal pathogen causing a number of diseases in vegetable crops. Beneficial microorganisms in the root zone, particularly species of Pseudomonas, have been associated with a decrease in take-all decline of wheat. Over time, two species of soil-dwelling amoebae in Canada reduced populations of two root pathogens.
Earthworms
Earthworms break up organic matter, combine it and humus with soil particles, and enhance microbial activity. Soils with high earthworm populations or soils containing earthworm castings generally have a greater ability to hold water and plant nutrients. Cation-exchange capacity, exchangeable calcium, magnesium, and potassium, and available phosphorus are generally higher in soils with earthworms. Earthworm activity has also been associated with reduced severity of several diseases. In Australia, Rhizoctonia solani caused less damage to wheat seedlings when earthworms were present. Mechanical soil disturbance also reduces disease severity, and it is possible that the physical mixing of soil by the worms reduces pathogen severity. The presence of earthworms also enhanced the movement and colonization of wheat roots by the biological disease control microorganism Pseudomonas corrugata.
When earthworms are introduced to the soil, they spread rapidly from the inoculation point - up to 36 feet per year. In most soils, however, if organic matter increases, earthworm populations will increase rapidly. Continuous cropping without adding o rganic matter usually reduces earthworm populations to a very low level, as do copper fungicides, soil fumigation, and frequent tillage.
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