According to Kerry Harrison, Extension Engineer
Irrigation is essential to produce consistent yields of high quality tomatoes. Rainfall amounts are often erratic during the growing season, and tomatoes are often grown in sandy soils with low water holding capacity. This combination of factors makes supplemental irrigation necessary for commercial tomato production.
Irrigation studies in the southeast show that irrigation increases annual tomato yields by an average of at least 60 percent over dryland production. Quality of irrigated tomatoes is also much better. Irrigation eliminates disastrous crop losses resulting from severe drought.
Tomatoes are potentially deep rooted, with significant root densities up to 4 feet deep. In Georgia soils, however, the effective rooting depth is generally much less. Actual root depths vary considerably depending upon soil conditions and cultural practices. The effective rooting depth is usually 12 to 18 inches with half of the roots in the top 6 inches. It is important not to allow these roots to dry out or root damage will occur.
Moisture stress in tomatoes causes shedding of flowers and young fruit, sun scalding and dry rot of fruit. The most critical stages for watering are at transplanting, flowering and fruit development.
Several types of irrigation may be used successfully on tomatoes in the southeast. Ultimately, the type chosen will depend on one or more of the following factors:
1) Availability of existing equipment
2) Field shape and size
3) Amount and quality of water available
4) Labour requirements
5) Fuel requirements
These systems include center pivot, linear move, traveling gun, permanent set and portable aluminum pipe with sprinklers. Any of these systems are satisfactory if they are used correctly. There are, however, significant differences in initial cost, fuel cost and labor requirements.
Any sprinkler system used on tomatoes should be able to deliver at least an inch of water every 4 days. In addition, the system should apply the water slowly enough to prevent run-off. In sandy soils, the application rate should be less than 3 inches per hour. In loamy or clay soils, the rate should not exceed 1 inch per hour.
Sprinkler systems with a high application uniformity (center pivot, linear move and permanent set) can be used to apply fertilizer. This increases the efficiency of fertilizer utilization by making it readily available to the plant and reduces leaching.
Drip irrigation has become the standard practice for tomato production. Although it can be used with or without plastic mulch, its use is highly recommended with plastic mulch culture. One of the major advantages of drip irrigation is its water use efficiency. Studies in Florida indicate that drip irrigated vegetables require 40 percent less water than sprinkler irrigated vegetables. Weeds are also less of a problem, since only the rows are watered and the middles remain dry. Some studies have also shown significant yield increases with drip irrigation and plastic mulch when compared with sprinkler irrigated tomatoes. The most dramatic yields have been attained by using drip irrigation and plastic mulch, and supplementing nutrients by injecting fertilizers into the drip system (fertigation).
Drip tubing may be installed on the soil surface or buried up to about 1.5 inches deep. When used in conjunction with plastic mulch, the tubing can be installed at the same time the plastic mulch is laid. Usually one line of tubing is installed on each bed. A field with beds spaced 5 feet center to center will require 8,712 feet of tubing per acre (one tube per bed). The output rate of the tube is specified by the user. For discussion purposes, however, you can determine the per acre water capacity by multiplying the output rate of the tube (per 1000') by 8.712 (i.e., on a 5' bed spacing a 4.5 gpm/1000' output rate tube will require 39.2 gpm per acre water capacity).
The tubing is available in various wall thicknesses ranging from 3 mils to 25 mils. Most growers use thin wall tubing (10 mils or less) and replace it every year. Heavier wall tubing can be rolled up at the end of the season and reused; however, take care in removing it from the field and store it in a shelter. Labour costs for removing, storing and reinstalling irrigation tubing are often prohibitive.
Excellent results have been achieved by injecting at least half of the fertilizer through the drip system. This allows plant nutrients to be supplied to the field as needed. This method also eliminates the need for heavy fertilizer applications early in the season, which tend to leach beyond the reach of root systems or cause salt toxicity problems. Only water soluble formulations can be injected through the drip systems. Nitrogen and potassium formulations tend to be more water soluble than phosphorous and, consequently, are more easily injected. These nutrients also tend to leach quicker and need to be supplemented during the growing season. Thoroughly flush drip systems following each fertilizer injection.
Water used in a drip irrigation system should be well filtered to remove any particulate matter that might plug the tubing. Test the water for minerals that could precipitate and cause plugging problems.
The combined loss of water by evaporation from the soil and transpiration from plant surfaces is called evapotranspiration (ET). Peak ET rates for tomatoes are about 0.2 inch per day. Factors affecting ET are stage of crop growth, temperature, relative humidity, solar radiation, wind velocity and plant spacing. Transplant tomatoes into moist soil and irrigate with 0.3 to 0.5 inch immediately after transplanting to settle the soil around the roots.
Once a root system is established, maintain soil moisture to the 12-inch depth. The sandier soils in South Georgia have an available water holding capacity of about 1 inch per foot of soil depth. You should not deplete more than 50 percent of the available water before irrigating; therefore, when you use 0.5 inch, it should be replaced by irrigation. Soils having a higher clay content may have water holding capacities as high as 2 inches per foot. In these soils you can deplete as much as 1 inch before irrigating. This means net application amounts should be between 0.5 and 1.0 inch per irrigation. The actual amount applied should be 10 to 20 percent higher to account for evaporation losses and wind drift. The irrigation frequency will depend on daily evapotranspiration. In general, for sprinkler irrigated tomatoes during peak water use periods, sandy soils should receive 0.6 inch two or three times a week, and clay soils should receive 1.25 inches about every 5 days.
Irrigation can best be managed by monitoring the amount of moisture in the soil. This can be done with soil moisture blocks. For best results on tomatoes, maintain soil moisture below 30 centibars. Drip irrigation systems need to be operated more frequently than sprinkler systems. Typically, they are operated every day or every other day. Do not saturate the soil with water, especially when using plastic mulch. Plastic mulch will tend to keep the soil from drying out and tomatoes grow poorly in waterlogged soil.