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The ins and outs of vegetable crop irrigation

In the Garden

People often water their garden when the soil looks dry. This is the wrong approach as the roots are not at the soil surface. Vegetable plant roots vary from 6 to 36 inches deep. Soil characteristics and delivery systems also affect irrigation practices.

Use the following information to help become a better irrigator.

Root depth

Effective root depth (half total root depth) of common vegetable crops.

root depth chart

The height of the plant (or length of the vine) is a good indicator of root depth. When plants of different root depths are grown together, focus on the plants with the greatest need.

Plant-available water

The plant-available water depends on soil texture and soil organic matter. Soil texture is based on the percent of sand, silt and clay. Sandy soils only hold 1-inch or less of plant-available water per foot of soil depth. That is the plant’s water “fuel tank.” Loamy soils, (nearly equal parts of sand, silt and clay) can hold up to 2.5 inches of plant-available water. Every one percent increase in SOM in the top six inches of the soil will increase the water holding capacity by another 0.75 inches of plant-available water.

It is best to use only one-half of the plant-available water before irrigating. A fine sandy soil with high SOM would supply over 0.75 inches of water ((1.0 + 0.75)/2) per foot of root depth. Loamy soil with high SOM will be closer to 1.6 inches of water ((2.5 + .75)/2) per foot of root depth.

Delivery system

Sprinkler and drip irrigation are the most common vegetable garden water delivery systems.

With sprinklers, place a few soup cans in the garden area and turn on the sprinklers. Run them for a given amount of time and average the depth of the water in the cans.

Drip irrigation is more complicated. First, determine the application rate of your emitters. They can range anywhere from 0.2 gallons per hour to 1.5 gph. The most common are 0.6, 0.9 and 1.0 gph, and 0.623 gallons is one inch of water over one square foot. Most emitters are twelve inches apart.

Use this formula to calculate how long to run your drip system to supply one inch of water.

(60 minutes x 0.623)/0.9 gph = 41.53 minutes (41 minutes 32 seconds)

Another concern with drip irrigation is that because you are not watering between the rows the roots will not spread out. This means you will need to water more frequently with drip irrigation.

For 0.9 gph emitters, spaced 12 inches apart, water every other day for 45 minutes for shallow-rooted crops. Every third time, water an extra 45 minutes for the deep-rooted crops.

irrigation 2
Ron Patterson | University of Idaho

Irrigation frequency

The easiest way to determine soil moisture is to take a long screwdriver and poke it into the soil. If it goes in easily, you do not need to water. If it is difficult to push in, you should probably water soon. Also, watch your crops.

Improper irrigation is one of the most common causes for the plant problems we see in the office.

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