Soil formation processes

The governing soil-formation concept is the movement and deposition of materials between soil horizons. Illuviation is the process in which materials washed or transported downward from an upper horizon accumulate in a lower horizon, commonly the B horizon. Clay particles and compounds of iron and aluminium may be carried downward by percolating water and deposited there, making the B horizon a zone of accumulation. Eluviation is the contrasting removal process that usually affects an upper horizon such as the E horizon. Evaporation, seismic waves and marine erosion do not describe this soil-profile movement. Therefore option A is correct.

Correct answer: A, a long-term water-saturated and low-oxygen condition. Gleying is a soil-forming process associated with prolonged waterlogging and poor drainage. When soil pores remain filled with water, air cannot circulate easily and oxygen becomes limited. This creates reducing conditions, which affect the form and colour of minerals such as iron compounds and may produce greyish or bluish features in the soil. A is correct because persistent saturation and oxygen deficiency are the essential conditions. B is wrong because extreme dryness does not create waterlogging or reduction. C is wrong because gleying is not defined by a volcanic eruption. D is wrong because a sandy sea surface is not the same as a waterlogged soil profile. Memory cue: gleying means grey, poorly drained, oxygen-poor soil; connect it with prolonged saturation.