Irrigating vegetable crops

Irrigation

Most vegetable crops require a uniform supply of moisture throughout the growing season. The average rainfall in Ontario is 70 mm/month during the growing season. This provides only 65% of the water needed for optimum yield.

Crops, soils and water demand

Soil types differ in their ability to retain water following rainfall or irrigation. Water that is held by the soil and can be taken up by plants is called "available soil moisture" (ASM).

Sands and coarse sandy loams retain approximately 25 mm of ASM in the crop's rooting zone. Under average conditions, a crop will use this moisture in 7 days. On these soils, a weekly irrigation of 25 mm of water would be required to maintain high yields.

Fine sandy loams and silty loams retain 40-60 mm of ASM in the rooting zone. On these soils, heavier and less frequent irrigations (40-60 mm of water every 10-14 days) are required for optimum yields. Application of more water than is required wastes water and can cause leaching of fertilizers and pesticides.

Different vegetable crops also vary in their response to irrigation. Shallow-rooted crops such as potatoes and celery, require frequent, light irrigations. Deep-rooted crops can use water from a greater volume of the soil profile and do not require irrigation as frequently. Table 1, Classification of Vegetable Crops by Rooting Depth, shows the relative rooting depths of a number of vegetable crops.

In addition, most vegetable crops have growth stages during which drought stress can severely reduce yield and quality. Irrigation is especially important during these critical growth periods, listed in Table 2, Critical Periods for Irrigation of Several Vegetable Crops.

Irrigation systems

A number of different types of irrigation systems have been adapted for use in vegetable crops.

Hand-moved sprinklers

This is the least expensive overhead irrigation system. The labour cost to move the laterals through the field is relatively high. Water distribution is very uniform, although tall crops are not easily irrigated. Sprinkler irrigation may also be used for frost protection.

Travelling-gun systems

These systems are relatively expensive. Labour cost is reduced compared to hand-moved sprinklers. They are well suited to irrigating large fields and tall crops. Uniformity of water distribution is greatly affected by wind conditions.

Centre-pivot and lateral systems

These are self-propelled systems located to move through and irrigate large fields from a centrally located well or pump. These systems are expensive. Labour costs through the season are low. They are adapted for use in large, relatively level fields.

Drip (trickle) irrigation

Drip irrigation systems consist of a series of drip lines (plastic tubing with built-in emitters) laid along the crop rows. Crop yields are comparable or slightly higher than with overhead irrigation. If fertilizer injection is combined with drip irrigation, greater yield increases are possible (see the Fertigation section).

The cost of installing a drip system is relatively high. However, the labour cost through the season is very low. A major advantage of drip systems is that less water is required. Drip irrigation provides the crop with a uniform supply of water through the season.

A drip irrigation system consists of a filtration system, flow meter, pressure regulator, main line, header lines and lateral lines with built-in emitters (drip tubing or tape). A fertilizer injector is also required for fertigation.

Growers should consult with a specialist at an irrigation company for information on designing a drip irrigation system.

Irrigation scheduling

Whichever type of irrigation system is used, it is important to use some method of irrigation scheduling to avoid drought stress and provide water when it is most needed. There are two basic methods of irrigation scheduling.

Water budget method

This method uses climatic data to estimate the amount of available water lost from the rooting zone through crop use and evaporation. The amount of crop water use is calculated using the percentage of crop cover and the evapotranspiration rate from climatic data. Further details of this method are provided in this OMAFRA fact sheet Irrigation scheduling for tomatoes.

Measuring soil moisture

There are several tools available for measuring soil moisture content.

Tensiometers measure how strongly water is being held by the soil. As the soil dries out, moisture is more strongly held to soil particles and less available to the crop. Tensiometers can be used to determine when moisture stress occurs, and thus when irrigation should be started. Sandy soils are irrigated when soil moisture tension reaches 15-20 centibars. Irrigate loamy soils at 20-25 centibars. These units require proper installation and regular maintenance. Accuracy may be reduced on soils with high clay content.

Electrical resistance blocks also measure soil water tension in centibars. Compared to tensiometers, they are easier to install. The portable meter and low cost of the units is more conducive to reading soil moisture at several different sites.

Time domain reflectometry (TDR) the most costly tool for soil water monitoring. It is portable and quick. Some units require calibration and may be finicky to work with. Accuracy may be reduced on soils heavier than a clay-loam.

For more irrigation information, refer to: Irrigation Management - Best Management Practices Manual.

Fertigation

Fertigation is a method of applying water and nutrients through a drip irrigation system. It can be used to increase the yield and quality of many vegetable crops.

A stock solution of soluble (greenhouse grade) fertilizer is dissolved in a tank and introduced through a valve into the irrigation system either by suction or pressure. The fertilizer solution should be fed through the system slowly. After the fertilizer has passed through the system, continue to irrigate to flush the system.

Do not mix fertilizers containing calcium, phosphates or sulphates as these can precipitate out and plug emitters.

Broadcast all of the phosphate requirement and approximately 30-50% of the nitrogen and potash requirement prior to planting. The remainder should be injected through the drip irrigation system. Use soil tests to determine phosphate and potash requirements. Table 3, Nitrogen and Potash Injection Schedules outlines recommended application rates for tomatoes, peppers and vine crops.