8 Fig. 8.1 shows soil particles of three different sizes found in a sample of agricultural land. (a) Name the type of particle labelled R, S and T, given th...

Labelled answer diagram:

(a) The three particle types, identified by size (R is largest, T is smallest):

(b) The relationship between particle size and water movement through the soil:

1. Large particles (sand) leave large pore spaces (macropores) between them. Water moves rapidly through these wide channels under the force of gravity, so sandy soils drain quickly and rarely become waterlogged. [1]
2. Small particles (clay) pack closely together, leaving only tiny pore spaces (micropores). Water is held tightly in these narrow spaces by adhesion to particle surfaces and by capillary forces, so it drains very slowly. [1]
3. The practical consequence is that sandy soils may dry out too fast for crops (poor water retention), while clay soils may hold water so tightly that drainage is impeded and roots become waterlogged. A mixture (loam) balances drainage and retention. [1]

(c) Capillarity is the upward movement of water through narrow spaces (pore spaces between soil particles) against the pull of gravity. It occurs because of two molecular forces acting together:

1. Adhesion - water molecules are attracted to the surfaces of soil particles and cling to them. [1]
2. Cohesion - water molecules are attracted to each other, so as the molecules at the soil-particle surface are pulled upward by adhesion, they pull neighbouring water molecules along with them, creating a continuous column of water that rises through the narrow pores. [1]

Capillarity is critically important for plant growth because it draws water upward from the water table (or from deeper, wetter layers) into the root zone where plants can absorb it. During dry periods when surface soil dries out, capillary rise is the main mechanism that keeps the root zone supplied with moisture between rainfall or irrigation events. [1]