A soil that has been air dried and stored for months will, on rewetting, usually show
Answer: A
Drying kills part of the population and disrupts aggregates, and rewetting makes that dead material and newly exposed organic matter available at once, so respiration spikes before settling down. Survivors, especially spore formers, multiply quickly on the released nutrients.
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Antibiosis in soil describes
Answer: B
Antibiosis is specifically chemical inhibition, the phenomenon that made soil actinomycetes the source of so many antibiotics. Mutual benefit is symbiosis, engulfing prey is predation, and competing for a shared resource is competition.
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The suppression of a soilborne plant pathogen by resident soil organisms is called
Answer: C
Biological control uses living organisms, whether naturally present or deliberately introduced, to hold a pathogen below damaging levels. Solarisation and fumigation kill by heat and chemicals respectively, and liming corrects acidity.
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Trichoderma is used in agriculture chiefly as
Answer: D
Trichoderma species parasitise other fungi, compete for space on the root and induce plant defences, which is why they are formulated as seed and soil treatments. They fix no nitrogen and produce no streptomycin, which comes from Streptomyces.
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A suppressive soil is one in which
Answer: A
In a suppressive soil the pathogen may be fully present and the crop susceptible, yet disease stays light because the resident community holds it in check; the property is lost when the soil is sterilised, which is how its biological basis was demonstrated.
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Two microbial species competing for the same limiting nutrient in soil is an example of
Answer: B
Competition is the interaction in which both parties are worse off for the other's presence. Commensalism benefits one and leaves the other unaffected, mutualism benefits both, and parasitism benefits one at the direct expense of the other.
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A mycorrhizal fungus supplying phosphorus to a plant in exchange for sugars is an example of
Answer: C
Both partners gain: the plant obtains phosphorus it could not reach and the fungus obtains carbon it cannot fix, which is the definition of mutualism. Under some conditions the balance can tip and the fungus behave more like a parasite, but the association is normally mutual.
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Which of these is NOT a step in the nitrogen cycle?
Answer: D
Photolysis is the splitting of water in photosynthesis and has nothing to do with nitrogen. The other three are the successive microbial transformations that release ammonium from organic matter, oxidise it to nitrate, and return nitrate to the atmosphere as gas.
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Nitrate is lost from soil more readily than ammonium chiefly because
Answer: A
Clay and organic surfaces carry a net negative charge, so they retain the ammonium cation while the nitrate anion stays in solution and moves with drainage water. That is why nitrification effectively converts a held form of nitrogen into a leachable one.
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Applying a nitrification inhibitor to soil is intended to
Answer: B
Slowing the conversion of ammonium to nitrate keeps nitrogen in the form that clay retains, reducing losses to leaching and to denitrification. The inhibitors target the nitrifying organisms specifically and leave fixation and decomposition alone.
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