Knowing when crops need water and how much they need are the key elements to effective irrigation management. With basic knowledge of the soil-moisture crop relationship, an irrigator can easily schedule more scientifically and anticipate irrigation requirements.

Effective irrigation practices require the development of a good irrigation schedule. Irrigation scheduling is simply knowing when and how much to irrigate. An effective irrigation schedule helps to maximize profit while minimizing water, energy and fertilizer use. 

The following factors influence an efficient irrigation schedule:

The first step is to establish initial soil-moisture content. Once initial soil-moisture content and crop water requirements are known it is possible to calculate the application required for optimum growth. There is a variety of scheduling aids available on the market today: C-probes, enviroscans, neutron probes, soil electrodes, tensiometers and soil augers.

Scheduling with tensiometers is easy and inexpensive.

A tensiometer is a sealed, water-filled tube with a porous ceramic tip (2) at the bottom and a vacuum gauge (1) at the upper end. The tube is placed into the soil with the ceramic tip at the desired root zone depth and the gauge above ground. 

Fig. 12.1 Tensiometer

If the soil is dry, water will move from the tube via the ceramic tip into the soil. This creates a vacuum inside the tube that is shown on the gauge meter. A tensiometer reading reflects soil-moisture tension. High readings reflect soil-moisture tension. High readings indicate a dry soil while low readings indicate a wet soil. 

It is advisable to install two tensiometers at different depths – one in the root zone and one below the root zone of the plants.

The tensiometer installed at the root zone (A) indicates when to irrigate. This tensiometer reading should be kept at a low soil-moisture tension i.e. between 10-20 kPa.    The tensiometer installed below the root zone (B) indicates the irrigation time, or the duration of the irrigation cycle. This tensiometer reading should be kept between 20-50kPa to ensure optimum conditions which will result in higher oxygen content in the soil.

Placement of tensiometers 

By applying this concept when irrigating sugar cane it has been found that while it is standard practice to irrigate 50mm once a week, this quantity can be reduced down to three applications of 12mm, or only 36mm per week.

This principle applied resulted in a 20%-40% increase in yield compared to hand movable systems. The increase in yield is due to less water applied more frequently, which eliminates stress conditions between irrigation cycles, creating a continuous optimum growth environment. Furthermore, there is a reduction of water and fertilizers moving past the root zone. Thus, savings on water consumption and fertilizers is another advantage of the system. It is therefore safe to conclude that effective scheduling will result in lower water and energy input costs and more efficient fertilizer use whilst increasing yield. 

Interpreting tensiometer readings