Fig. 2 shows three magnified views of soil samples taken from different locations on a farm. Each sample contains particles of different sizes. (a) Define t...

Labelled answer diagram:

(a) Definition of soil texture [2]

1. Soil texture refers to the relative proportions (percentages) of different-sized mineral particles present in a soil sample [1].
2. The three size categories are sand (the largest particles, 0.05-2.0 mm), silt (medium, 0.002-0.05 mm) and clay (the smallest, less than 0.002 mm) [1].

The ratio of these three fractions determines the textural class of the soil (sandy, silty, clay, loam, etc.).

(b) Soil type with the largest particles [1]

Sandy soil [1]. Sand particles range from 0.05 to 2.0 mm in diameter, making them the largest of the three mineral particle types. They are visible to the naked eye and feel gritty when rubbed between the fingers.

(c) Using the feel test to distinguish sandy soil from clay soil [4]

The feel test is a simple field method performed by hand:

1. Take a moist sample of soil and rub it between the fingers and thumb [1].
2. Sandy soil feels gritty and rough because the individual sand grains are large enough to be felt against the skin [1]. Clay soil feels smooth and sticky because the particles are too small to be felt individually.
3. When an attempt is made to roll the soil into a ball or ribbon shape, sandy soil crumbles apart and will not hold together [1]. Clay soil can be rolled into a smooth ball or a long ribbon and holds its shape when moulded because the tiny particles stick to each other.
4. After handling, sandy soil does not stain the fingers, while clay soil leaves a shiny, smeared surface when rubbed smooth [1].

(d) How particle size affects water-holding capacity and drainage [4]

Particle size is the fundamental factor controlling how soil interacts with water:

1. Sand has large particles with correspondingly large air spaces (pores) between them [1]. Water passes quickly through these large pores under gravity, so sandy soil drains rapidly.
2. Because water drains so freely, sandy soil has a low water-holding capacity - it dries out quickly after rain [1].
3. Clay has extremely small particles with tiny pore spaces between them [1]. Water is held tightly in these narrow pores by capillary forces, so clay soil drains very slowly and may become waterlogged after heavy rain.
4. The strong capillary retention gives clay soil a high water-holding capacity, but the water can be held so tightly that plants sometimes struggle to extract it [1]. A loam soil, with a mixture of all three particle sizes, provides the best balance of adequate drainage and sufficient water retention for crop growth.