Getting Irrigation Right

Irrigation using Tensiometers and Evaporation Pans

Introduction

In Tasmania, a lack of summer rainfall is a major factor limiting crop production with potatoes being one of the most sensitive crops to moisture stress in relation to yield. The aim of irrigation is to prevent periods of water shortage and so ensure maximum plant and tuber development as well as minimising plant stress which may predispose plants to attack by pests and diseases.

If crops are not irrigated, financial penalties result from lost potential yield as well as reduced quality. For example, when potatoes are stressed they produce more smaller tubers plus uneven sugar distribution. Both result in reduced payouts to farmers.

To decide when to irrigate and how much water to apply, the amount of water in the soil needs to be monitored. Without monitoring, it is easy to over or under water soils.

Over watering is applying too much water, or watering too often, causing soils to become over wet. This results in poor aeration, restricted root growth, higher incidence of root diseases and powdery scab, leaching of fertilisers out of the root zone and soil erosion when irrigation rates are higher than infiltration rates. Continuing to irrigate potatoes unnecessarily at the end of the season when plant water use is minimal results in soils being too wet at harvest resulting in higher harvest costs and potential soil degradation.

Underwatering occurs when irrigations are spaced too far apart or when insufficient water is applied at irrigation. Dry soil conditions for potatoes results in poor growth, reduced tuber size, too many shoots per plant, knobbly tubers, increased incidence of common scab and greater infestations by potato moth when soils crack.

How Do I Know When to Water?

A number of options are available to farmers to decide when to water including:

1. Hit or Miss

Usually means watering too frequently "to make sure the crop gets plenty". Farmers "hit" the problem but "miss" out on some money from excess pumping costs, leaching of fertilisers and possible yield reductions.

2. Careful Observation

Over the years some growers have developed a sense of deciding when to water based on subtle changes in plant colour and appearance resulting from water stress, an appreciation of the effects of different weather conditions on water loss, and a knowledge of their soils.

Feeling the soil in the hand is often used to determine when to water. If the soil is powder dry or crumbly and will not hold together it obviously needs watering. If the soil is a bit crumbly but will hold together it probably needs watering, and if it forms a ball and sticks together it is wet enough.

Problems with the observation methods include:

  • By the time we see visual symptoms of stress, plants have often been under yield influencing stress for some time.
  • It is impossible to know how much water is in the soil just by feeling the surface soil. Plant roots generally extend to at least 60 cm (2 ft) below the surface and, in friable red krasnozem soils, many crops send roots down to 100 cm (over 3 ft).
  • An individual's assessment of conditions may vary over time and may not be an objective assessment of plant growth conditions.
  • We can not tell if too much irrigation has been applied to subsoils.
3. Evaporation pan with rainfall records

An evaporation pan is used to measure the amount of water evaporated from a free-water surface. The reading closely approximates the amount of water lost from a growing crop with full ground cover and can be used as a guide to the need for irrigation. Losses increase with greater sunshine intensity.

4. Monitoring soil water with instruments


These all provide information on soil water conditions in the plant root zone allowing the farmer to make a more objective decision as to when to irrigate and how much water to apply. Instruments can range from relatively inexpensive tensiometers, which are easy to operate, to expensive systems, such as neutron probes, which require considerable knowledge and skill to operate or else the services of a consultant to operate them and interpret the information.

Other Options

Soil water can be monitored using gypsum block sensors, a capacitance probe or a neutron probe. Capacitance probes and gypsum blocks provide a convenient and relatively low cost instrument for measuring soil moisture level.

All commercially available soil moisture-monitoring instruments are relatively accurate. It is primarily ease of data access and interpretation that influences the usefulness of the soil moisture monitoring tool in assisting daily irrigation management decisions. Most soil moisture monitoring systems store soil moisture data that is recorded at multiple soil depths several times a day.

If a farmer wishes to operate these instruments some initial training and support by the supplier or within an industry discussion group can be very valuable.

Some processing and rural service companies do actively encourage soil moisture monitoring and it is worth discussing options with company field officers.

Soil moisture monitoring systems can be supplied through most major agricultural suppliers.

How Much Does it Cost?

Jet Fill Tensiometer

Approximately $220
Gypsum block sensors (set of 4)

Data logger and data display for
connection to sensors


Enviroscan
Approximately $300 for 4 units

At least three commonly used options are
available in Tasmania - the one logging unit
required will cost between $700 and $1000

$8,000

The returns in crop yield and quality, and the savings in water and pumping costs will quickly convince you that soil water monitoring for irrigation management is a very worthwhile and profitable investment.



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