The diagram below illustrates an experimental set-up on soil. Study it and answer the questions that follow. a) (i) State the aim of the experiment. (ii)Nam...

Answer Details

The set-up is a soil sedimentation (mechanical analysis) experiment. Soil has been shaken with water in a tall transparent cylinder and left to stand. Because particles settle in order of size and density, the largest and heaviest fall first and the finest settle last, so the material grades from coarse at the bottom to fine at the top, with organic matter floating on the water surface.

(a)(i) Aim of the experiment

To separate a soil sample into its constituent fractions (gravel, sand, silt, clay and organic matter) according to particle size, and so to determine the texture and composition of the soil by sedimentation.

(a)(ii) Parts labelled I and II

The pointers pick out the two fine bands lying just under the floating organic matter and the clear water, above the coarser sand and gravel at the base. In a settled column the finest fraction rests highest:

• I = Clay (the finest particles, still partly in suspension, forming the uppermost, densely stippled band of settled sediment just below the water).
• II = Silt (the next, slightly coarser band lying directly beneath the clay).

For orientation: the wispy material floating on the surface is decayed organic matter (humus), and below silt lie sand and then gravel/stones at the very base.

(b) How the experiment is carried out

1. Take a sample of dry, crushed soil and pick out large roots and stones.
2. Put the soil into a tall transparent glass jar or measuring cylinder until it is about one-third full.
3. Add clean water (and a little dispersing agent such as sodium carbonate or caustic soda) until the jar is about three-quarters full.
4. Cover the mouth and shake vigorously so that all the clods break up and every particle is suspended in the water.
5. Stand the jar on a level bench and leave it undisturbed for about an hour (or longer).
6. Observe the layers as they settle: gravel and stones at the bottom, then sand, then silt, then clay, with humus floating on top and the water clearing above.
7. Measure the depth of each layer and express it as a percentage of the total settled soil to estimate the proportion of each fraction. For example, if sand is \(4\ \text{cm}\) of a total \(10\ \text{cm}\) of settled soil, \(\dfrac{4}{10}\times 100\% = 40\%\).

(c) Four properties of sandy soils

1. Large particles with wide pore spaces, giving very good aeration and free (rapid) drainage.
2. Low water-holding capacity, so the soil dries out quickly and needs frequent watering.
3. Poor in plant nutrients and low in humus, and nutrients are easily leached (washed) out.
4. Light, loose and gritty; it warms up quickly and is easy to till.

(d) Test for soil acidity using litmus paper

1. Put a little of the soil sample into a clean test tube or beaker.
2. Add distilled (or rain) water, stir well to make a suspension, and allow it to settle so a clear solution forms above the soil.
3. Dip a strip of blue litmus paper into the clear liquid; test a fresh portion with red litmus paper as well.
4. If the blue litmus paper turns red, the soil is acidic. If red litmus turns blue, the soil is alkaline (basic); if neither paper changes colour, the soil is neutral.