Theory question 103
Rocks and Soil Formation(a) Explain the term chemical weathering of rocks.
(b) Discuss four processes of chemical weathering of rocks.
(c) State four ways in which soil temperature is important for crop growth.
Show the model answer
(a) Chemical weathering is the breakdown of rocks by chemical reactions between the minerals and water, oxygen, carbon dioxide or acids, which changes the minerals into new substances.
(b) Any four explained: hydrolysis – water reacts with minerals such as feldspar, replacing their ions with H+; hydration – minerals take up water and swell (e.g. haematite to limonite) so the rock weakens; oxidation – oxygen reacts with iron minerals, forming rusty oxides; carbonation – carbonic acid (CO2 in rain) dissolves limestone to soluble calcium bicarbonate; solution – soluble minerals (e.g. rock salt) dissolve in water; reduction – minerals lose oxygen in waterlogged soils.
(c) Any four: controls seed germination; affects root growth and nutrient uptake; affects activity of micro-organisms and rate of decomposition; affects water evaporation; affects rate of chemical reactions in soil; extremes kill seedlings.
For each process give what reacts and what is formed.