Soil Science

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From your understanding of the behaviour of ions in the soil solution, which of the following would you expect to be the most likely to contaminate ground water?

Correct! Unlike the other ions, nitrate ions are only very weakly adsorbed in soil, so are easily leached. Because excessive nutrate concentrations are toxic, nitrate contamination of aquifers used for drinking water can be a problem in some agricultural areas.

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Which of the following is likely to increase the amount of P in the soil solution?

Correct! In nutrient cycling, mineralisation is a microbially mediated chemical reaction whereby a nutrient is transformed from an unavailable organic form to an available inorganic form. It is the opposite of immobilisation. P mineralisation releases phosphate from organic matter into the soil solution.

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In waterlogged soils denitrification reduces nitrate to dinitrogen (N2) and which oxide of nitrogen?

Correct! Denitrification produces nitrous oxide or N2O.

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Blooms of blue-green algae periodically pollute the inland waterways of Australia, and P, largely derived from soil, is essential for their growth. What is likely to be the main pathway by which phosphate is transported into waterways?

Correct! Erosion. The orthophosphate anions (HPO42- and H2PO4-) are very resistant to leaching because they are so strongly adsorbed. However, phosphate can be transported by erosion bound to soil particles. Under the anaerobic conditions of sediment in a waterway phosphate becomes more soluble, and therefore available for uptake by blue-green algae.

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