Soil practice questions.

A soil is drawn as a system. Which of these is an input to that system, rather than a transfer within it or an output from it?

• Leaf litter falling onto the surface from the vegetation
• Nitrate leached downward into the groundwater below
• Clay particles washed from the upper into the lower horizon
• Water drawn out of the soil through the roots of a crop

Answer: Leaf litter falling onto the surface from the vegetation

Inputs cross the boundary into the system: litter, precipitation and weathered parent material. Leaching and root uptake carry material out, and the downward movement of clay is a transfer between two storages inside the same soil.

By volume, a productive topsoil in good condition is made up approximately of

• 90 % mineral matter, 5 % organic matter, 5 % water and air
• 45 % mineral matter, 5 % organic matter, 25 % water, 25 % air
• 45 % mineral matter, 45 % organic matter, 5 % water, 5 % air
• 25 % mineral matter, 25 % organic matter, 25 % water, 25 % air

Answer: 45 % mineral matter, 5 % organic matter, 25 % water, 25 % air

About half of a good soil by volume is pore space, shared between water and air, and it is that space which allows roots to respire and water to move. Compaction destroys the pore fraction without changing the mineral or organic content at all, which is why it damages a soil that still looks normal.

Over several decades of acid deposition and heavy nitrogen fertiliser use, the pH of a farmed soil falls from 6.5 to 4.5. The most direct consequence for the crop is that

• the soil holds a considerably greater mass of water at field capacity
• nitrogen fixation by free-living bacteria rises sharply
• aluminium becomes soluble and several nutrients less available
• the rate of physical weathering of the parent rock falls

Answer: aluminium becomes soluble and several nutrients less available

Nutrient availability depends on pH: below about 5.5 phosphorus is locked up and aluminium ions come into solution at concentrations that damage roots. Liming works because it reverses the chemistry rather than because it adds a nutrient.

Rain removes a thin, fairly even layer of topsoil across the whole of a cultivated slope, without cutting any visible channels. This is

• gully erosion
• rill erosion
• downslope mass movement
• sheet erosion

Answer: sheet erosion

Sheet erosion is the most damaging form precisely because it is invisible: it takes the finest and most fertile particles first and leaves no feature to notice. By the time rills and gullies appear the loss has usually been running for years.

A grower stops ploughing and instead drills seed directly through the residues of the previous crop. The most direct effect on the soil system is that

• the mineral fraction of the soil is enriched with clay
• the surface stays covered, so erosion and organic matter loss fall
• the soil pH rises towards neutral within a single season
• evapotranspiration from the whole field increases very substantially

Answer: the surface stays covered, so erosion and organic matter loss fall

Ploughing exposes bare soil to rain and wind and aerates the organic storage so that decomposers consume it faster. Leaving the residues in place protects the surface from raindrop impact and slows that loss, which is why no-till is treated as a conservation measure rather than merely a labour saving.

A student rolls a moist soil sample into a thread, bends it into a ring without it cracking, and writes: 'the sample must be high in sand, so it will drain quickly'. Identify the error in that conclusion.

• A soil can only be moulded once it has been oven-dried, so this test can support no conclusion at all.
• Drainage rate depends only on the depth of the water table, never on the texture of the soil itself.
• The ring test measures organic matter content rather than the proportions of mineral particles.
• Soil that moulds into a ring without cracking is high in clay, which drains slowly.

Answer: Soil that moulds into a ring without cracking is high in clay, which drains slowly.

The observation is sound but the inference is reversed: plasticity comes from the fine, plate-like clay fraction, and clay soils have small pores that hold water tightly and drain slowly. Sandy soils crumble rather than mould and drain rapidly.