Soil Science

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The main decomposers of lignin are members of which group of microorganisms??

Correct! Some fungi are able to produce enzymes which can attack lignin.

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The soil organic matter in Australian surface soils commonly has an average c:N ratio of between 14:1 and 19:1. Which of the following statements is true of such soil??

Correct! Net mineralisation will occur (since C:N < 20), which is the same as saying organic N is converted to inorganic forms (NH4+ and NO3-) which are available to plants. Such mineralisation is responsible for the increase in available N that occurs when a soil is fallowed between crops.

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Which of the following statements is false:?

Correct! Humus makes soil acid rather than alkaline, due to the presence of phenol and carboxylic acid groups. As these dissociate in soil the humus acquires a negative charge and a high cation exchange capacity.

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Which of the following residues is most likely to cause a temporary decrease in N availability when incorporated in soil??

Correct! Wheat straw as it is the only one with a high C:N ratio.

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Which of the following is the least resistant to decay in soil??

Correct! Cellulose. Waxes and lignin are the most resistant plant residues, and microbial debris is stabilised when associated with clay particles.

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For one of the following pairs, soil organic matter is the main source of both elements to plants. Which one??

Correct! P and S. Much of the N, P and S in soil is held in organic forms and becomes available to plants as the organic matter is decomposed. K and Na do not form organic compounds in the soil, and in any case Na is not generally required by plants. Plants obtain C from photosynthesis, not soil organic matter.

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