Month Field J (sandy soil) temperature / °C Field K (clay soil) temperature / °C January 4 3 February 5 3 March 9 6 April 13 9 May 17 14 June 20 18 A farmer...

(a) Two factors that affect soil temperature [2]

Any two (1 mark each):

• Soil moisture content - wet soils are slower to warm up because water has a high specific heat capacity [1].
• Soil colour - dark soils absorb more solar radiation than light-coloured soils [1].
• Air temperature / climate / season [1].
• Amount of sunlight / solar radiation received by the soil surface [1].
• Soil texture / particle size [1].
• Vegetation cover / shade from canopy [1].
• Organic matter content [1].

(b) Why sandy soil (Field J) warmed up faster than clay soil (Field K) in spring [4]

The data shows that by April, Field J was at 13 degrees C while Field K was only at 9 degrees C - a 4-degree difference. The physical reasons are:

1. Sandy soil contains less water than clay soil because its large pore spaces allow water to drain freely [1].
2. Water has a high specific heat capacity, meaning it absorbs a large amount of energy before its temperature rises. Clay soil holds much more water, so it requires significantly more solar energy input to achieve the same temperature increase [1].
3. Sandy soil has larger air-filled pore spaces, and air warms up much faster than water, so the overall soil mass heats more quickly [1].
4. Energy reaching the surface of clay soil is partly consumed by evaporating the surface moisture rather than raising the soil temperature, further slowing the warming process [1].

(c) How soil colour influences warming rate [2]

1. Dark-coloured soils absorb a greater proportion of incoming solar radiation (sunlight energy) than light-coloured soils [1].
2. Light-coloured soils reflect more sunlight back into the atmosphere and therefore absorb less heat energy, so they gain temperature more slowly [1]. In the data, however, Field K (dark clay) still warms more slowly than Field J (light sand) because the high water content of the clay overrides the colour advantage.

(d) Two methods to raise clay soil temperature in early spring [2]

Any two (1 mark each):

• Cover the soil surface with plastic mulch or polythene sheeting, which traps heat beneath it and raises soil temperature (the greenhouse effect at soil level) [1].
• Improve drainage by installing drains or forming raised beds, which reduces the water content and allows the soil to warm faster [1].
• Add organic matter to improve soil structure and drainage, indirectly reducing the water-holding that slows warming [1].
• Remove weeds or vegetation cover that shades the soil surface and blocks incoming sunlight [1].

(e) Why soil temperature is important for germination and root growth [3]

1. Seeds require a minimum soil temperature before germination can begin; below this threshold the seed remains dormant and will not sprout [1].
2. The enzymes that control the biochemical processes of germination (breaking down starch reserves, cell division) are temperature-dependent and work faster at warmer temperatures up to their optimum [1].
3. Root growth depends on active cell division and elongation, both of which slow dramatically in cold soils because metabolic reactions proceed more slowly at low temperatures [1]. Nutrient uptake by roots is also reduced in cold conditions because active transport across root cell membranes requires energy from respiration, which is temperature-limited.

(f) How organic matter improves clay soil structure [2]

1. Organic matter (humus) acts as a binding agent that sticks individual clay particles together into larger clumps called aggregates or crumbs [1]. This process is called aggregation.
2. The spaces between these larger aggregates are much bigger than the tiny pores between individual clay particles, so drainage and aeration both improve significantly [1]. The soil also becomes easier to cultivate because the crumb structure resists compaction and reduces the sticky, heavy character of raw clay.