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Tips on Irrigating Vegetables

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(The authors are H.W. Otto, UC Cooperative Extension, Orange County, and Jewell Meyer UC Cooperative Extension, Riverside. Contents adapted in part from "Solid Set Sprinklers for Starting Vegetables" and "Furrow Irrigation," both UC publications.

Irrigation costs are going up. With irrigation taking a larger share of growing costs, you'll want to find better ways to irrigate. This publication contains pointers that will help you irrigate more efficiently.

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The first irrigation for most vegetables is a deep pre-irrigation--except where rains have already wet the soil as deep as most roots will go. Pre-irrigation leaches salts and can save water by trimming the number of irrigations needed later.

How deep should you pre-irrigate? Wet the soil a little deeper than your crop's rooting depth. Approximate rooting depth for vegetables is shown in Table 1.

Knowing your crop's rooting depth can also help you decide how long to run the water in later irrigations. Using Table 2, you can tell roughly how much water you'll need to pre-irrigate a dry soil to a desired depth.

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Table 1. Approximate rooting depth of vegetables 1

1 The figures are averages for mature plants. Root depth varies, depending on soil profile, crop variety, whether you direct seeded or transplanted, etc.
Asparagus, pumpkin, winter squash, seeded tomato, seeded watermelon, lima beans, sweet potato over 4 feet
Bush and pole beans, cantaloupe, carrot, cucumber, eggplant beets, peas, pepper, summer squash 3 to 4 feet
Broccoli, cabbage, cauliflower, corn, lettuce, radish, garlic, onion, celery, potato 1-1/2 to 2 fee

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Table 2. Inches water to pre-irrigate a dry soil to different depths (approx.) 2

Soil Type 3Inches water per foot soil depthInches water to reach 6' deep
2 Based on available water holding capacity; plants have dried soil to permanent wilting point, 15 ATM.
3 Assumes the soil is uniform throughout irrigation depth.
Clay 1.4 - 1.8 8.6-10.8
Silty clay 1.6 - 1.9 9.6 - 11.4
Sandy clay 1.6 - 1.9 9.6 - 11.4
Silty clay loam 2.2 - 2.3 13.0 - 13.7
Clay loam 2.0 - 2.2 12.2 - 13.0
Sandy clay loam 2.0 - 2.2 12.2 - 13.0
Silt-loam 1. 8 - 2.0 10.8 - 12.2
Loam 1.7 - 1.9 10.1-11.4
Very fine sandy loam 1.7 - 1.9 10.1 - 11.4
Fine sandy loam 1.2 - 1.4 7.2 - 8.6
Sandy loam 1.1 - 1.3 6.5 - 7.9
Loamy very fine sand 1.1 - 1.3 6.5 -7.9
Loamy fine sand 1.0 - 1.2 5.8 - 7.2
Loamy sand 0.7 - 1.0 4.3 - 5.8
Very fine sand 0.7 - 1.0 4.3 - 5.8
Fine sand 0.7 - 1.0 4.3 - 5.8
Sand 0.7 - 1.0 4.3 - 5.8
Coarse sand and gravel 0.4 - 0.7 2.2 - 4.3

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Table 3. Estimating soil moisture by feel.

  How Soil Feels or Looks
Soil Moisture LevelCOARSE (sand)LIGHT (Loamy sand, sandy loam)MEDIUM (fine sandy loam, silt loam)HEAVY (clay loam, clay)
4 Ribbon is formed by squeezing and working soil between thumb and forefinger.
5 Cast or ball is formed by squeezing soil in hand.
No available soil moisture. Plants wilt. Irrigation required. (1st range) Dry, loose, single grained, flows through fingers. No stain or smear on fingers. Dry, loose, clods easily crushed and will flow through fingers. No stain or smear on fingers. Crumbly, dry, powdery, will barely maintain shape. Clods, breaks down easily. May leave slight smear or stain when worked with hands or fingers. Hard, firm baked, cracked. Usually too stiff or tough to work or ribbon 4 by squeezing between thumb or forefinger. May leave slight smear or stain.
Moisture is available, but level is low. Irrigation needed. (2nd range) Appears dry; will not retain shape when squeezed in hand. Appears dry; may tend to make a cast when squeezed in hand, but seldom will hold together. May form a weak ball 5under pressure but will still be crumbly. Color is pale with no obvious moisture. Pliable, forms a ball; will ribbon but usually breaks or is crumbly. May leave slight stain or smear.
Moisture is avail able. Level is high. Irrigation not yet needed. (3rd range) Color is darkened with obvious moisture. Soil may stick together in very weak cast or ball Color is darkened with obvious moisture. Soil forms weak ball or cast under pressure. Slight finger stain, but no ribbon when squeezed between thumb and fore finger. Color is darkened from obvious moisture. Forms a ball. Works easily, clods are soft with mellow feel. Will stain finger have slick feel when squeezed. Color is darkened with obvious moisture. Forms good ball. Ribbons easily, has slick feel. Leaves stain on fingers.
Soil moisture level following an irrigation. (4th range) Appears and feels moist. Color is darkened. May form weak cast or wet outline or slight wet outline or slight smear on hand. Appears and feels moist. Color is darkened. Forms cast or ball. Will not ribbon, but will show smear or stain and leave wet outline on hand. Appears and feels moist. Color is darkened. Has a smooth, mellow feel. Forms ball and will ribbon when squeezed. Stains and Smears. Leaves wet outline on hand. Color is darkened. Appears moist; may feel sticky. Ribbons out easily, smears and stains hand, leaves wet outline. Forms good ball.

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When to Irrigate

After pre-irrigation, later irrigations depend on soil, weather, and the crop.

  1. Coarse or sandy soil needs water more often than loam, silt, and most clay.
  2. Soil dries out faster when the weather warms up.
  3. Wind speeds up soil drying.
  4. Sunny weather dries soil faster than cloudy weather.
  5. When plants are large, they use more water.

Since water needs change through the seasons, irrigating by calendar is not recommended.

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Your Soil as an Irrigation Guide

One way to decide whether to irrigate is to use a shovel or auger and feel the soil where most of your crop's roots are. Table 3 shows you how to tell the soil's moisture by feel. This is a general guide only, but the method has been used for years by many farmers. Of course, you must also consider root depth, recent weather, and when the field was last irrigated.

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Which Irrigation System is Best?

Sprinkler, furrow, and drip irrigation systems each have advantages and disadvantages as shown in Table 4 below:

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Table 4. Which irrigation system is best?

Provides good soil moisture and aeration High initial cost
Leaches excess salts Encourages weed growth between rows
Reduces crusting, making High energy requirement
seedling emergence easier May encourage some diseases
Relatively uniform water Wet soil may interfere with timing of pesticide sprays application
Can be used on unleveled land  
Most commonly used Less even water distribution
Lowest initial cost Crusting problems
  Can concentrate salts near plants
  Requires land leveling
Usually requires less water Less commonly used on vegetables
Can cut irrigation labor cost (except strawberries, pole tomatoes)
Allows picking and spraying while irrigating High capital cost
May stretch limited water supply Resetting system each year costly
Can be used on sloped land Requires some technical expertise

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Sprinklers for starting vegetables

The investment for sprinklers is high, yet a growing number of farmers use them to bring up a stand of vegetables because of the advantages listed in Table 4. They are not used for beans or corn unless they are planted shallow--about one inch deep.

How often to sprinkle vegetables

Growers usually get best results by sprinkling daily until the crop comes up. Brief (one hour) daily irrigations help soften the crust and keep salts moving downward. Of course, seedlings must not be kept waterlogged or they will die from damping off. Once the stand is established, you can move sprinklers to the next field.

How fast should sprinklers apply water?

Your sprinkler system should not apply water faster than it soaks into the soil. Putting water on faster than the soil's intake rate results in puddling, the main cause of soil crusting. Sprinkler systems that apply 0.1 to 0.15 inches per hour have proven successful on a wide range of California soils.

But on some soils--those with strong clay or silt characteristics--even that much water is too much. To prevent crusting on such soils, only about 0.06 inches per hour is applied. These low sprinkling rates don't mean you'll spend too much time sprinkling a field. That's because a system can irrigate a field in the same time at low application rates as high application rates, but you'll have to buy more lateral lines.

Weed control

It is especially important to have good weed control when using sprinklers since sprinkling also germinates a high percentage of weed seeds.

Choosing your sprinkler system

  1. Select sprinklers that wet a circle at least 60 to 65 feet in diameter (conventional pressure systems, 50 to 65 pounds per square inch--psi).
  2. Have a nozzle size that won't put water on faster than your soils can take it. The 5/64 to 3/32 inch size works well for most soils. On heaviest (fine textured) soils, the 5/64 inch size can prevent crusting better than the 3/32 inch size, though the smaller nozzle's pattern is affected more by wind.
  3. Use 50 to 65 psi at the nozzles for uniform sprinkling and to prevent crusts. New low pressure systems at about 30 to 40 psi may have smaller droplets and cause less crusting.
  4. Spacing between sprinklers along the pipe depends on wind. Sprinkler spacing should not be over 65 percent of rated diameter of spray with no wind, 60 percent of the diameter for winds up to five miles per hour, and 50 percent for winds up to 10 miles per hour. Between laterals, a spacing of 40 to 45 feet is often suggested for conventional, high pressure systems. New low pressure systems need closer spacings, usually 40' x 40'.

Choosing a sprinkler system will be easier if you use Tables 5-9. For example, if you want to avoid crusting by applying water at 0.1 inch per hour on quartermile long fields (1,320 feet), Table 3 shows that 5/64 inch nozzles at 50 psi on 30 X 40 foot spacings will give you 0.1 inch per hour.

This setup puts on 1.25 gpm (gallons per minute). See Table 5. With 1,320 foot rows and 30 foot long pipe, you'll need 44 sprinklers/row, 1320 = 44

Table 6 shows that three-inch pipe is more than large enough to put on the 1.25 gpm, but a two-inch line isn't big enough (maximum is 34 sprinklers at 1.25 gpm).

If you put the main line through the center of the field, a two-inch line would handle up to 34 sprinklers running each way (68 total) and that's more than enough for the 44 sprinklers needed in this example. (See Table 7.)

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Table 5. Application rates for various nozzle sizes, pressures, and spacings.

        Application rate at spacings (inch/hour)
Nozzle SizePressure (psi) 6Discharge 7(gpm) 8Diameter 9 of spray (feet)20' x 40'30' x 40'30' x 45'40' x 40'
6 PSI--pounds per inch.
7 Showing a three-digit number here is only to indicate the progression as nozzle size and pressure increase. Sprinkler equipment seldom performs so precisely.
8 GPM--gallons per minute.
9 Shows range of diameters of spray for different makes and models of sprinklers.
1/16 35 0.69 59-61 .080 .055    
1/16 40 0.74 60-62 .086 .059    
1/16 45 0.76 60-72 .091 .061