Theory question 102
Rocks and Soil Formation(a) Describe each of the following horizons in a soil profile. (i) horizon A; (ii) horizon B; (iii) horizon C; (iv) horizon D.
(b) Using the table below, classify each of the following rocks based on their mode of formation: marble, gabbro, dolomite, limestone, slate and coal. (Columns: Sedimentary rock, Metamorphic rock, Igneous rock.)
(c) State four effects of temperature on agricultural production.
Show the model answer
(a)(i) Horizon A: the topsoil; dark, rich in humus and living organisms, most weathered and leached, most fertile; where most roots are.
(ii) Horizon B: the subsoil; reddish-brown, contains minerals (clay, iron and aluminium oxides) washed down from A; little humus and less fertile.
(iii) Horizon C: weathered parent material; partly broken rock fragments, little biological activity.
(iv) Horizon D: the bedrock; hard, unweathered parent rock with no living organisms.
(b) Sedimentary: limestone, dolomite, coal. Metamorphic: marble, slate. Igneous: gabbro.
(c) Any four: decides which crops and animals can be raised in an area; affects germination and breaking of dormancy; high temperature increases evaporation and transpiration, causing wilting; affects growth rate and ripening; high temperature causes heat stress, lower feed intake and production in animals; affects activity of soil organisms; influences pests and diseases.
Put each rock in exactly one column.