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Irrigation made easy: Part 1 – Principles of irrigation

by Kevin Scott

It is important to understand the fundamental principles of irrigation to become a good farmer. In this series, we shall discuss all the steps and general practices to ensure that your crops will be satisfied and healthy, and reach their full potential.

A plant is similar to an animal and human and also needs the following to survive:

An animal can always travel to find
water.
Plants rely on water that comes to
them.

Without water both plants and animals will die. Soil is an important component in the irrigation process. During irrigation, many interesting and vital processes are happening in the soil underneath the crops.

What happens in the soil?

Soil has texture

Soil can consist of rough sand particles…
…or of finer clay.

Most soils have a mix of sand and clay particles.

Soil provides a home for the plant.

Roots are firmly anchored in the soil
and support the plant, holding the
plant upright and in place.

Soil holds water and air

A layer of moisture is held around each soil particle, where water is freely available for the plant to use. Like animals, plants need to breathe. Too much water held in the root zone will lead to some roots suffocating and dying.

Clay soils tend to hold and store more water, and less air (per unit soil depth) than sandy soils. Soils that have more sand particles and less clay will store less water in a fixed soil depth than a clay soil. The soil texture will largely determine how much water the soil can hold in the root zone of the plant.

15 mm water applied to the surface
of dry sandy soil will penetrate to a
depth of about 300 mm, saturating
that soil volume.
15 mm water applied to the surface
of a dry clay soil will penetrate only
to a depth of about 100 mm, saturating
that soil volume.

Plant roots only reach down to a certain depth in the soil. Water that has penetrated below this depth is not normally available to the plant.

Too much irrigation will wet the soil
so that water penetrates to a depth
where the plant roots can no longer
reach this water. Water held in the
soil below the normal plant root zone
will be lost to the plant.
If drainage within the soil profile is
blocked by a layer of less pervious
soil or a ‘plough pan’, water will fill
the soil profile, forcing out air.

The plant will drown!

Excessive irrigation can harm your plants. Photo: Joris de Vente 2007, available at www.desire-his.eu.

Too much irrigation will always waste water, and can harm the plant. Next month we shall look at what happens with water. Published with acknowledgement to the ARC Agricultural Engineering for the use of their manuals. Visit www.arc.agric.za for more information.

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