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Irrigation and Fertigation of Fresh Market Tomatoes

The Tomato Magazine
Feburary 2005

A warm season crop, fresh market tomatoes can be produced in approximately 60 days. While the crop does require a large investment in time and labor, attention to proper management can pay off in increased yields and profits.

So reported Dorota Z. Haman, Ph.D., Department of Agricultural and Biological Engineering, University of Florida-Gainesville, speaking during the Great Lakes Fruit, Vegetable and Farm Market Expo tomato session, held Dec. 7, 2004, at the DeVos Place Convention Center in Grand Rapids, Mich.

Tomatoes can be irrigated using a variety of irrigation systems, she reminded the group, mentioning drip, seepage, surface and sprinklers. Sprinkler irrigation can be portable, self-propelled (traveling gun) or solid set system.

Seepage irrigation (also referred to as subirrigation) for fresh tomatoes can work if the conditions are right, she said. Natural high water tables help as does a semi-impermeable spodic layer 18 to 36 inches below the soil surface, common in Florida soils. High hydraulic conductivity of the soils is also required so that the water can quickly migrate into the planting beds. Sandy or organic soils with a spodic horizon or other shallow restrictive layer also work well.

"Typically, in open or semi-open systems, water is conveyed by open ditch or pipe to lateral ditches and pumped continuously, except during rainfall, to maintain a water table 18 to 24 inches below the soil surface," Haman reported. "The closed system uses pipes for water delivery and distribution similar to typical drainage pipes that are blocked at the outlets and deliver the water to the field in place of draining it. Any seepage system can also be used for drainage during heavy rainfall."

Plasticulture Advantages
Plasticulture (plastic-covered raised beds with drip irrigation lines under the plastic) is frequently used on fresh market tomatoes, the speaker said. Polyethylene plastic film, used for mulch, accelerates plant growth by increasing soil temperature and stabilizing moisture.

"Properly scheduled drip irrigation, used in conjunction with plastic mulch helps maintain optimum soil moisture, aids plant establishment and promotes excellent crop growth throughout the season," Haman said. "Drip irrigation also allows for 'prescription' applications of fertilizer and other chemicals."

Well-managed early spring vegetables on plastic mulch generally mature one to two weeks earlier than those on bare ground, she added. High value crops such as tomatoes, cantaloupe, peppers, cucumbers, squash, eggplants, watermelon and okra mature earlier, produce higher yields and have improved quality. Other advantages of plasticulture include added protection against decay (preventing contact with contaminated soil) and reduced weed growth.

When considering irrigation and fertilization under plasticulture, it is important to think in terms of rows, not field surface, the speaker cautioned. In bare ground vegetable production scenarios, fertilizer and irrigation rates are typically expressed in lbs/acre and gallons/acre, respectively. Under plasticulture, however, the number of linear feet of beds in an acre becomes more important that the actual surface of the field. It is helpful to think in terms of lbs/100 linear bed feet (lbf) for fertilization injections and gallons/100 lbf for irrigation and take into account the bed spacing.

"Soil should be tested and the test recommendation carefully followed," Haman emphasized. "A soil sample has to be recent, representative and large enough to ensure valid results. The soil test recommendation has to be understood and properly implemented. Typically, 20 to 50 percent of N and K2O and 100 percent of P2O5 and micronutrients recommended are applied as pre-plant fertilizer. The remaining 50 to 80 percent of N and K2O are injected through the drip system."

It is important to monitor plant nutritional status through the season, she stressed. Early in the season (from transplanting to fruit set), this can be done with a sap test or foliar analysis. For petiole and leaf analysis, a representative sample should be made with at least 20 leaves selected randomly throughout the field from recently collected, fully mature leaves.

Importance of Adequate Irrigation
"Efficient water application is critical to successful production of vegetables," she explained. "Irrigation scheduling is knowing when to start irrigation and how much to apply-in a way that satisfies crop water needs, conserves water and does not leach mobile nutrients. Irrigation scheduling requires: (1) a target water volume to apply; (2) guidelines on how and when irrigation should be split to several applications; (3) a method to account for effective rainfall; and (4) a practical method to monitor soil moisture.

"Irrigation amount (target volume) should aim at replacing daily crop evapotranspiration (Etc). Etc may be determined from actual reference evapotranspiration (ETo), historical ETo, class A pan evaporation or empirical values when no other en no other estimates are available," she continued. "Irrigation amount should not exceed soil water holding capacity. Otherwise, water is wasted and mobile nutrients are leached. How far water moves down the soil profile is a rather abstract concept because it is not visible. The amount of water applied in one irrigation event should not exceed the soil water holding capacity of the wetted root volume.

"When irrigation amounts required (daily crop water use) is larger than the water holding capacity of the wetted root volume, then irrigation should be applied in two or three events (split applications) throughout the day," the speaker advised.

Theoretically, the irrigation amounts that tcan be held in the root zone can be calculated based on the soil's physical properties and the width of wetted area on both sides of the drip tape, the researcher explained. If the wetting width of the soil is 12 inches (6 inches on each side of the drip tape)-assuming a 0.75 in/foot soil water holding capacity and allowing a 50 percent soil water depletion-the theoretical largest water amounts that can be stored in the soil are 24 gal/100 ft within the top 12 inches, 36 gal/100 ft within the top18 inches and 48 gal/100 ft within the top 24 inches. These numbers can be used as guidelines for calculating the time of water application.

"Under plasticulture production, rainfall contributes little to replenish soil moisture because of the plastic mulch," Haman noted. "Since the plastic mulch protects the bed from rainfall, there is no need to apply additional fertilizer after a leaching rain. However, when the field gets flooded, mobile nutrients may be leached out of the root zone or carried out of the field through surface runoff. The need for additional fertilizer may be assessed after field drainage by monitoring sap test levels of nitrate and potassium. Another consequence of using the plastic mulch is that an irrigation may still be needed after a small rain. Soil moisture measurements may be used to assess the need for additional irrigation. "

To understand the stress level imposed on plants between irrigation events, growers are advised to monitor soil moisture levels daily. Soil moisture may be reported in terms of soil water tension (soil matric potential) or volumetric water content (VWC). The recommended range for vegetable production on very sandy soils (like in Florida) is to maintain SWT between 6 to 8 cb (field capacity) and 15 cb. Vegetables may tolerate SWT up to 25 cb without yield reduction on loamy soils.

Measuring Soil Moisture
Instruments available for routine monitoring of soil moisture for vegetable crops are tensiometers, granular matrix sensors (modified gypsum blocks), time domain reflectometry probes (TDR) and dielectric probes, the researcher explained, advising those desiring more information to check http://edis.ifas.ufl.edu/AE266.

Haman cautioned her audience to keep a daily record of all irrigation applications. Growers also are required to keep pesticide records, she said. Fetilization records are usually kept in relation to soil testing and implementing the recommendations. However, vegetable growers seldom document their irrigation practices.

Haman recommended that a daily log contain soil moisture measurements (SWT or VWC) at selected depths, rainfall, an estimate of weather demand for water (evapotranspiration) and irrigation amount (gallons/field or duration of irrigation).

"Application uniformity of 85 to 95 percent is expected from a new, well-designed drip irrigation system," she said. "As the irrigation system is used for water and fertilizer applications throughout the growing season, the application uniformity may remain the same if the system is well managed, but will most likely decline with time," she warned. "A comprehensive maintenance plan will reduce the adverse effects of the agents that reduce application uniformity: small solids in suspension, organic matter, micro-organisms and chemical residues on application uniformity.
Without a maintenance plan, the risk of complete emitter clogging and crop loss becomes real. Every vegetable grower who uses drip irrigation should recognize that prevention of clogging is the most important practice in drip system maintenance. A maintenance plan should include: (1) a filtration system; (2) chlorination and acidification; (3) flushing; and (4) regular inspection of irrigation system components."

Haman advised the group to remember that irrigation and fertilization should always be managed together. Scheduling decisions, regarding time and volume of water application guessing should be avoided.

"Poor irrigation management negates the potential benefits of a well-designed fertilization program," the researcher warned. "The irrigation system should be well maintained and operated at the highest irrigation efficiency and uniformity possible."

© 2005 Columbia Publishing

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