Land practice questions.

Nitrate dissolved in percolating rainwater is carried down below the rooting zone and into the groundwater. Within the soil system this movement is best described as

• an output that crosses the soil system boundary
• a transformation within the soil organic storage
• an input to the soil system from the parent material
• a transfer between two organisms in the soil

Answer: an output that crosses the soil system boundary

Leaching carries the nutrient across the system boundary and out of the soil altogether, so it is an output rather than an internal transfer; weathering of parent rock and the fall of litter are the corresponding inputs.

Compared with a sandy soil, a soil with a high clay content is most likely to have

• high water-holding capacity and high permeability
• low water-holding capacity and low permeability
• high water-holding capacity and low permeability
• low water-holding capacity and high permeability

Answer: high water-holding capacity and low permeability

Clay particles are extremely small, so the pore spaces between them are numerous but very narrow: water is held tightly by capillary forces and drains slowly. The usual error is to assume that many pores mean fast drainage, which describes sand and its large pores.

During the 1930s the southern Great Plains of the United States lost vast volumes of topsoil in the event known as the Dust Bowl. The immediate cause of that soil loss was

• acidification of the topsoil by sulfur deposition from coal-fired power stations
• ploughing out of the deep-rooted prairie grasses before a severe drought
• salinisation of the surface horizon by irrigation water drawn from deep aquifers
• compaction of the subsoil by heavy machinery, which prevented infiltration

Answer: ploughing out of the deep-rooted prairie grasses before a severe drought

Wind erosion needs a bare, dry, poorly structured surface, and the prairie sod had previously held the soil in place with a dense root mat. Acidification and salinisation degrade a soil chemically but do not physically remove it.

Which statement about the horizons of a mature temperate soil profile is correct?

• The B horizon gains the clay and minerals leached from above
• The C horizon is the layer richest in humus and rooting biomass
• The O horizon consists of weathered fragments of the parent rock
• The A horizon lies directly beneath the B horizon when undisturbed

Answer: The B horizon gains the clay and minerals leached from above

Eluviation strips soluble ions and fine clay out of the O and A horizons and illuviation deposits them in the B horizon. The C horizon below is weathered parent material with very little organic matter, and the O horizon at the surface is litter and humus.

A farming region on a flat, dry plain loses topsoil during the windy season between harvest and sowing. The most directly appropriate conservation measure is

• applying lime to raise the pH of the surface horizon
• installing a network of sub-surface drains across the field
• deep ploughing the field to break up the compacted subsoil
• planting shelterbelts of trees across the prevailing wind

Answer: planting shelterbelts of trees across the prevailing wind

Shelterbelts cut wind speed at ground level over a distance many times their own height, and leaving stubble or a cover crop in place would do the same job. Liming, drainage and deep ploughing address chemical or structural problems, not wind erosion.

One hectare of arable land feeds far more people when the cereal grown on it is eaten directly than when the same cereal is fed to cattle. The main reason is that

• cattle graze land that has a lower net primary productivity than arable land
• only around 10 % of the energy in the feed is converted into cattle biomass
• cereals reach a higher gross primary productivity when they are left unharvested
• cattle occupy the same trophic level as the people who eat the cereal

Answer: only around 10 % of the energy in the feed is converted into cattle biomass

Adding a trophic level costs roughly an order of magnitude of energy, because most of the feed energy leaves as respiration, movement and egestion. The error to avoid is treating this as a difference in the productivity of the crop rather than a loss in the transfer.