Irrigation/Mulches

Uses of water

Water is used by plants:

in photosynthesis and growth
to transport materials in plants
to bring nutrients into the soil from the roots
to keep plant cells rigid.

Not enough water

Water is spread over the soil particles as a thin film. Air is in the spaces between the particles. When there is not enough water in the soil the plant will wilt or look stressed. The leaves might turn brown and any growth will be slow. This can happen when there has not been much rain or it is very hot, dry and windy. Irrigation is used to add water to the soil when there is a shortage of rainfall.

Amount of water needed by crops

The amount of water needed will vary with the plant’s growth. Important times are when:

the leaves are growing quickly
flowers are starting to form
seed and fruit is forming, just after flowering.

Tomato plants need adequate water levels at flowering for satisfactory fruit set. In fruit crops if there is not enough water at flowering and fruit set, the yield will be reduced. The fruit may also be much smaller in size.

Fresh harvested crops such as lettuce and cabbage need water throughout the growing season. Because the crop is harvested fresh, water will be needed up to the time of harvest.

Methods used to irrigate

Travelling gun irrigators

Travelling irrigators put water over the general area rather that targeting specific crop rows.

These are high pressure guns mounted on a type of trailer.

The hosepipe or winch is used to pull the gun along the paddock. They are easy to maintain and move from paddock to paddock but this is quite labour-intensive.

When it is windy the water can go in all directions, which can waste water. Because they are high-pressured the cost of running them can be high.

Trickle irrigation

These are often used on horticulture crops and shelterbelts. Drippers are used to apply the exact amount of water at low pressure to individual plants and rows of plants.

Pipes take water from a main line to outlets or drippers at each of these nursery plants.

The outlets are thin tubes or drippers.

Trickle irrigation applies water to the root zone

A trickle irrigation system applies water to the root zone

Trickle irrigation is an efficient method as water is applied directly to the soil surface above the roots at a low rate. This means that all the water is available to the plants, so there is little water wasted.

The system is permanent, which makes it easy to manage once it is installed.
Soil structure is maintained and water is conserved. Liquid fertiliser can also be added.

A problem with this system is that the drippers can become blocked. The drippers need to be cleared or a filter installed so the drippers don’t become blocked. This method is suitable for row crops such as vegetables, nursery plants and fruit trees.

Micro or mini-sprinklers

This sprinkler is used to apply water to a kiwifruit plant.

Sprinkler methods involve applying the correct amount of water at the right time.

These are suitable for most row and tree crops. Water can be sprayed over or under the crop canopy. The water is pumped to the sprinklers and sprayed into the air through the sprinklers.

It breaks into small droplets that fall to the ground, which makes it an efficient system. The initial cost of setting up this system can be high.

Mulches

A mulch is a substance or material that is used to cover the soil. Mulches can be used to help conserve soil water. Organic mulches include bark, sawdust, wood chips and lawn clippings.

Inorganic mulches include crushed stone, plastic and polythene sheet and weed matting.

The mulch material is put over the surface of the soil. This helps reduce evaporation and the amount of water loss from the soil. It can warm the soil, giving better growth in cold seasons.

Using organic mulches can also help increase the organic matter level in the soil, which increases water absorption and breaks the force of rain. Ideal mulches let water and air enter and exit. Sometimes mulches cause problems. Pests such as slugs and snails often flourish in mulches. Some disease organisms can overwinter under a mulch and increase rapidly in spring.

Plastic mulches work well to keep the water in the soil but they also prevent extra water getting in. Putting an irrigation system under the plastic is one way to overcome this problem. Plastic mulches will also stop weeds growing.

Bark mulch around plants help reduce water loss.

Stone chips can be used as mulch.

The table below summarises the advantages and disadvantages of some mulches.

Type	Form	Advantages	Disadvantages
Dark material mulches	Compost	Warms the ground early in spring. Has few pests. Good for micro-organisms and worms. Improves soil structure. Holds moisture.
	Black polythene	Warms the ground in spring. Keeps moisture in. Prevents weed growth.	May hide pests.
Light material mulches	Straw, hay	Keeps soil cool. Holds moisture. Reduces weed growth.	Encourages pests. May create nitrogen deficiency. May contain weed seeds.
	Grass clippings	Suppresses weeds. Adds nitrogen. Encourages worms.	May get too hot.
	Sawdust	Suppresses weeds. Builds soil texture.	Poor aeration. May cause nitrogen deficiency if it rots too quickly. Sawdust from slow-rotting species is best for mulches.

Complete Activity 8A in your workbook

Key points

Water is used by plants for photosynthesis, plant growth, and transporting material in plants such as nutrients.
Different crops have different water needs. The amount of water needed by plants varies with the time of the year and the stage of the crop’s growth.
The season and the climate of an area will affect water usage by plants. In an area where the climate is hot and sunny, the plants’ water needs will be high. Lower water needs will be found where it is cool, humid, cloudy and there is little wind.
Irrigation can be used when there is a water shortage. The total amount of irrigation required depends on the amount of rainfall in an area, and on crop needs.

There are several types of irrigation systems that can be used to add water to the soil:

Trickle and mini-sprinklers apply water to the soil at low pressure. The water is put directly next to each individual plant.

Mulches can be used to help conserve soil water. Materials like sawdust, wood chips and bark are put over the surface of the soil. The mulch helps reduce evaporation and water loss from the soil. Organic mulches can also add organic matter and increase water absorption and break the force of rain.

What's next?

Go to: 9 Drainage.