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."
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