3 Fig. 3.1 shows a cross-section of farmland with two types of drainage system. The drainage system at the soil surface is labelled X. The drainage system b...

Labelled answer diagram:

(a) Meaning of drainage in agriculture [1]

Drainage is the removal of excess water from the soil or from the land surface. [1] It aims to lower the water table to a depth where crop roots can access air and grow properly.

(b) Type of drainage labelled X (at soil surface) [1]

Surface ditch (also called open drain or surface drain). [1] These are channels dug along the soil surface that collect and carry away excess surface water by gravity.

(c) Type of drainage labelled Y (below soil surface) [1]

Subsurface drain (also called tile drain, underground drain, or pipe drain). [1] These are perforated pipes or tiles buried below the soil surface that intercept and carry away water moving through the soil profile.

(d) Three effects of poor drainage on crop growth [3]

Any three of:

1. Roots cannot obtain oxygen for respiration in waterlogged soil, causing them to suffocate and eventually rot. [1]
2. Essential nutrients are leached away from the root zone by the excess water. [1]
3. Waterlogged soil stays cold longer in the growing season, slowing plant growth and reducing the rate of seed germination. [1]
4. Soil structure is damaged or compacted under prolonged saturation.
5. Plant growth is stunted and crop yield decreases significantly.

(e) Two advantages of subsurface drains over surface ditches [2]

Any two of:

1. Subsurface drains do not occupy land that could otherwise be used for growing crops - the entire field surface remains available for cultivation. [1]
2. Farm machinery can pass freely over the field without the obstruction of open ditches, making cultivation and harvesting easier. [1]
3. There is less risk of accidents to livestock or farm workers who might fall into open ditches.
4. Subsurface drains are not blocked by weed growth or debris and require less ongoing maintenance.

(f) How excess irrigation leads to salt build-up [2]

1. When too much irrigation water is applied, it percolates deep into the soil profile and dissolves mineral salts from deeper layers of rock and subsoil. [1]
2. In hot, dry conditions this salt-laden water rises back toward the surface by capillary action. When it evaporates at or near the surface, the dissolved salts are deposited and accumulate in the topsoil (salinisation). [1]

(g) Two ways to reduce salt build-up caused by poor drainage [2]

Any two of:

1. Install subsurface drains to flush dissolved salts downward and away from the root zone before they can accumulate. [1]
2. Apply extra irrigation water (leaching fraction) to wash salts below the root zone, provided drainage can carry the salty water away. [1]
3. Add organic matter to improve soil structure and drainage capacity.
4. Switch to drip irrigation instead of flood irrigation to avoid over-watering.
5. Grow salt-tolerant crop varieties while the soil is being reclaimed.