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Horticulture 1000
HT1033 - Soils 3 - Drainage
Irrigation/Mulches Soil management and plant growth


Poor drainage

Poor drainage can show up as puddles of surface water that can’t drain away. Excess water pushes out air in the soil, which is  essential for plant growth. Soils need to be aerated to ensure oxygen is  available for respiration of plant roots – otherwise the roots will die.

Water logged soil

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.

Water logged soil

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

Good drainage

Well-drained soils are warmer and produce earlier  crops in spring. They contain more soil organisms, and may increase nutrient  availability.

Drainage problems

Weeds will grow in very wet soilsWeeds will grow in very wet soils.

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.

What causes poor drainage?

  • A hard pan

    Iron, clay or humus can be removed (leached) by  water from the upper layers of the soil and settle to form a hard layer in the  subsoil. This is called a hard pan. Water will stay on this layer and not drain  away. It is hard for water to drain though this layer.

  • A  plough pan

    When machinery like ploughs and rotary hoes are used  to cultivate the soil at the same depth soil particles will pack together. This  compacted layer called a plough pan will stop water draining.

  • Fine textured soils with poor structure

Signs of poor drainage

  • In clay soils a pan will look a patchy grey/blue colour.
  • Surface water, ponding and flooding.
  • Yellow leaves on plants caused by a lack of nitrogen and oxygen.
  • Machinery tracks.
  • The soil particles pack together into clods.
  • Rushes, buttercups and other weeds.

Improving drainage


A compacted layer of soil can be broken up and  loosened by subsoiling.

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

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.


A tine aerator on a roller pushes and leaves holes in the  ground.

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.

Artificial  drainage systems

The  removal of excess water from the ground surface or the root zone is called drainage.
  The  aim of a drainage system is to carry surface water away or to lower the water  table.

Water tables

Some of the water that falls on the soil runs off  but a lot of it goes into the soil through wormholes and cracks. It takes up  pore spaces until it reaches rock, a hard pan, or heavy clay subsoil. The water  collects and forms a waterlogged layer. The top of this layer is called the  water table. The water table may be only a few centimetres from the surface or  many metres down. Its depth will vary with the season.

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

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.

Surface drains

Machinery is used to shape the land to make  somewhere for excess water to go. This is often considered a cheap option.

This open drain is an example of surface drainageThis open drain is an example of surface drainage.
Surface drains include:
  • 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.

Subsurface  drains

These  drains are formed underground. They can be:

Clay tiles are buried in trenches under the ground for drainageClay    tiles are buried in trenches under the ground for drainage.
Subsurface drains can be:
  • 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.

Mole drains

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.

Other methods that can be used

Raised beds

These are useful when the water table is high. The soil is raised above ground level. The soil can be raised in a structure of wood, brick, or cement blocks.


Bedding increases surface run-offBedding increases surface run-off.

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.

Complete Activity 9A in your workbook

Key points   Key points

Poorly drained soils usually have poor aeration. They  may have nutrients leached from them. Wet soils will be slow to warm up in the  spring. Soils need to be aerated to ensure oxygen is available for  transpiration and respiration.

Well drained soils are:

  • warmer and produce earlier crops in spring
  • more likely to have readily available nutrients
  • better for soil organisms.

Drainage can be improved with: cultivation, adding  organic matter, raising the garden or seedbed and/or providing drainage systems  such as field tiles to remove excess water into waterways.

What's next?

Go to: 10 Hydroponics.

Irrigation/Mulches Soil management and plant growth