Vegetables are 80 to 95 percent water. It stands to reason, then, that their yield and quality suffer dramatically during periods of drought.
“Think of vegetables as sacks of water with a small amount of flavoring and some vitamins,” says Joe Kemble, horticulturist with the Alabama Cooperative Extension System. “For good yields and quality, irrigation is essential to the production of most vegetables.”
If water shortages occur early in the crop's development, he says, maturity may be delayed and yields reduced. If a moisture shortage occurs late in the growing season, quality often is reduced although total yield may not be affected, he adds.
“Most vegetables are rather shallow-rooted,” says Kemble. “Even short periods of two to three days of moisture stress can damage marketable yields. Irrigation likely will increase the size and weight of individual fruit, in addition to preventing defects such as toughness, strong flavor, poor tip-fill, cracking, blossom-end rot and misshapen fruit.”
On the other hand, he continues, too much moisture reduces soluble solids in muskmelons (cantaloupes) and capsaicin (what makes peppers hot) in hot peppers when it occurs during fruit development.
Growers often wait too long to begin irrigating, says Kemble, thinking, “It'll rain tomorrow.” “This often results in a severe stress for that portion of the field that dries out the quickest or receives irrigation last. Another common problem is trying to stretch the acreage that reasonably can be covered by the available equipment. Both of these practices result in all or part of a field being water stressed. It's better to do a good job on some of the acreage rather than a halfway job on all the acreage,” he says.
Drought stress, says Kemble, can begin in as little as three days after a 1-inch rain or irrigation in such crops as tomatoes and in soils like those found in the Piedmont of Alabama. Thus, frequent irrigation is necessary to maximize yields.
“Soil moisture requirements differ with each crop and with each particular stage of development. Soil moisture availability varies with the amount of water in the soil and with the type of soil. Knowing your soil type is essential in planning for and in using an irrigation system,” says the horticulturist.
Up to 1 1/2 inches of water is needed each week during hot periods to maintain vegetable crops with a plant spread of 12 inches of more, advises Kemble. This need decreases to 0.75 inch per week during cooler seasons.
Droplet size and irrigation rate also are important when irrigating vegetable crops, he says. “Large droplets resulting from low pressure at the sprinkler head can cause damage to young vegetable plants and contribute to crusting when the soil dries.
“Irrigation rate also is important in sandy soils that absorb water more readily than clay soils. However, clay soils have a greater percent of available water. Irrigation rate will depend on soil type, but application rates shouldn't exceed 0.40 inch per hour for sandy soils, 0.30 inch per hour for loamy soils or 0.20 inch per hour for clay soils. High application rates of water will result in irrigation running off the field, contributing to erosion and fertilizer runoff.”
Most vegetables, says Kemble, have small seeds that are planted 3/4 inch deep or less. “The upper layer of the soil can dry rapidly, leaving shallow sown seeds susceptible to drying out. Without enough soil moisture, the seed is left partially germinated. Whenever this happens, no stand or, at best, an incomplete stand will result. An irrigation of 0.50 to 0.75 inch immediately after sowing should be applied to settle the soil around the seeds and to begin seed germination.”
For larger seeded crops, irrigation should begin a few days prior to sowing, he says. If seeds are slow in emerging from the soil due to cool temperatures or slow germination, apply 0.75 to 1 inch of water per acre as needed to encourage emergence.
“Do this to keep the area around the seed moist until seedlings emerge. Irrigation is a valuable tool in producing a good, uniform stand which will help to insure high yields. Good, uniform stands mean uniform harvest dates and greater production efficiency.”
Vegetable transplants, he adds, also require good soil moisture. A light irrigation of 0.50 to 0.75 inch per acre will help in the establishment of young transplants by providing a ready supply of water to young, broken roots.
“In addition to hastening seedling emergence, irrigation at planting time can reduce soil crusting. If 0.50 to 0.75 inch of irrigation is applied slowly — either with low rates or by turning the irrigation system off long enough to allow water to soak in — crusting can be reduced and stands will be improved.”
Product development & fruit set
A wide fluctuation in soil moisture injures fruit crop vegetables such as tomatoes and peppers, says Kemble. These fruits contain large amounts of water and depend on this water to expand and grow.
“When soil moisture is allowed to drop below the proper level, fruits don't expand to produce the maximum size before they ripen. Thus, their yield is reduced. If moisture is allowed to fluctuate too much, blossom-end rot can occur and fruits no longer are useable.”
If moisture occurs during the fruit expansion stage, fruit cracking will occur, he says. This typically occurs when the application of inadequate water has been followed by heavy rainfall. The best way to prevent cracking, notes Kemble, is to apply a steady supply of moisture. In addition, moisture fluctuation causes secondary growth, or knobs, in Irish potatoes.
It's important, says Kemble, that the soil profile be filled with water during each irrigation event. Otherwise, frequent light irrigation events result in the formation of shallow root systems.
“Shallow root systems result in plants being stressed even in short periods of water deficit. In addition, plants with shallow root systems are more prone to lodging and nutrient deficiencies. Shallow root systems neither explore nor exploit all of the available nutrients in an area.
“On the other hand, excessive irrigation can leach nutrients from the soil and encourage the development of diseases and nutrient deficiencies. It's important that shallow-rooted crops receive more frequent irrigations.”