Drainage

The time it takes water to drain away will depend on  the size of the soil pores that hold the air and water.

If  the pore spaces are large the water will drain fast. When they are small the  water will drain slower.

For  example, clay swells on wetting, the pores are closed off and  water cannot move through. Where  there is no air the soil water will not move to carry nutrients to the roots of  plants.

The wet soil in this paddock has made germination patchy and plant growth uneven.

Poorly  drained soils are a problem because they:

• lack air required by plants for growth and respiration. Roots of plants will be unable to carry nutrients to the plants
• cause seeds to have poor germination and uneven growth because they don’t have enough air to respire
• are slow to warm up in spring, which can slow down plant growth
• contain nitrogen that is unavailable to the plants. Plants growing in these conditions will have yellow leaves
• are difficult to cultivate which can lead to a delay in sowing and delayed harvesting dates
• are  easily damaged by machinery.

All these things can combine to decrease soil  fertility and crop and pasture yield.

This is sometimes also called ripping or aerating. This makes space for roots, water and air. Subsoiling is expensive because of fuel and labour costs.

The water content of the soil affects the success of subsoiling. Soil that is too wet will not break up but will smear instead. Dry soil will break into coarse blocks.

The subsoiler has chisel type blades that are pulled though the soil, which lifts and cracks.

Soil aerator machines can be useful where there is surface soil compaction. These are useful on turf and sports fields. The aerator is attached to a tractor. They can be coulters that have spikes or tines on them. The spikes or tines are punched in and out of the soil. The spikes make holes in the soil, which helps remove excess water from the soil surface. This increases root growth and improves crop yield because more air is available to the plant roots for respiration.

When  the water table is very high it needs to be lowered by drains.

Open  drains, tile drains or plastic piping can be used to lower the water table. The  type of system to use will depend on the drainage problem.

• Open drains or ditches

  Ditches can be of any    shape or size and they can take large volumes of water.

  These drains do use up land and do need looking after. Excessive weeds should be cleared so they continue to work properly.

• Graded banks

  These  are formed using a grader or bulldozer. Sloping land is terraced so the water  is allowed to flow down the slope and drain away.

• made of clay or plastic piping.

The pipes are laid under the soil surface. The depth and spacing of the pipes depends on the soil type. The depth of the drain is the level the soil will drain to. When trees are close to the drains their roots can grow into the tiles. This stops them working properly. Tree roots should be pruned regularly.

These are formed with a mole plough. A series of cracks and channels are formed using a metal plug pulled through the soil. Water can drain through these gaps. Mole drains are generally used along with tiles, and run at right angles to them. They are not permanent and need to be re-done every 5–7 years. These types of drains are not suitable on sandy soils, as the channels would collapse. They can be used on clay soils and loams.

This is sometimes referred to as humps and hollows. The idea is to shape  the land into humps and hollows. Surface runoff moves into the hollows and  drains away. You can see this method on flat low-lying areas such as the  Hauraki Plains. This technique does not remove water from within the soil but  moves water from the surface.