Soil Science

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Basalt soils tend to be more fertile than granite soils. This is in part due to nutrients inherited by the soil from the parent material. Three such nutrient elements that are typically more abundant in basalt than in granite are?

Correct! Ca, P and Mg are typically more abundant in basalt than in granite, and of these P usually has the most significant effect on soil fertility. K tends to be more abundant in granite than basalt. N occurs in very low concentrations in rocks, and soil N depends on nitrogen fixation from the atmosphere.

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A basalt landscape in eastern Australia comprises red soils and black cracking-clay soils. From your understanding of the factors of soil formation, which of the following is most likely to be true??

Correct! 2:1 clay minerals are formed in less leached soil chemical environments (higher concentrations of silica and cations in solution) than 1:1 clay minerals. In this soil landscape the upper slope soils are dominated by kaolinite with some hematite and the lower slope cracking clays by smectites.

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The ratio of annual precipitation to evaporation (P/E) is one of the main aspects of climate which affects soil properties. Which of the following is least likely to be affected by P/E??

Correct! Soil texture is determined mainly by soil parent material. The other three are all strongly influenced by the degree of leaching of the soil, which is in turn influenced by P/E.

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Imagine a sequence of soils of different ages (a chronosequence) formed from granite in a leaching environment. Let us say that the oldest has been forming for 500 000 years and the youngest for only 5000 years. Which of the following would tend to accumulate in the oldest soil??

Correct! Gibbsite and the other sesquioxide minerals tend to accumulate in very old soils because they are the most chemically stable in a leached soil environment. Under these conditions 1:1 and 2:1 clay minerals decompose and lose silica to form gibbsite. The primary mica mineral biotite is weatherable in soil and will tend to disappear from soil over time. Organic matter accumulation is relatively rapid and would not necessarily be greater in the oldest of these soils.

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You dig a soil pit in alluvium and observe a duplex profile has formed (A1, A2 and B2 horizons with indications of illuviation in the B2 horizon). What can you conclude as to the likely minimum age of the soil??

Correct! 10 000 years. Observations in eastern Australia have shown that Al horizons can become strongly developed in a few thousand years and clay B2 horizons can form in a few tens of thousands of years.

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Three duplex soils formed from granite in a sub-humid climate oceur in a toposequence. In random order: soil 1 has a dull grey-brown B horizon of clay that is very hard when dry and disperses when wet, soil 2 has a red coloured B horizon, and soil 3 has a yellow coloured B horizon. The most likely order of these soils is:?

Correct! Soil 2 above soil 3 above soil 1. Red soils (soil 2) containing hematite tend to occur above yellow soils (soil 3) containing goethite because hematite is preferentially formed under drier conditions than goethite. The B horizon of soil 1 has the characteristics of sodicity, and sodic soils preferentially form in lower slope positions. These soils might be classified as a red podzolic (Red Kurosol), a yellow podzolic (Yellow Kurosol) and a solodic (Sodosol).

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