/A 3 .-)-
IRRIGATION OF FALL TOMATOES
F. M. Rhoads and S. M. Olson
Florida Agricultural Experiment Station
Institute of Food and Agricultural Sciences
University of Florida, Gainesville
NFREC, Quincy Res. Rpt. 92-3
The objective of crop irrigation is to produce maximum yield with the least possible
amount of water. Achievement of this objective becomes more important as energy costs
increase and competition for water between agriculture and urban areas intensifies. Water use
permits are required for irrigation in North Florida whether from surface or ground water
sources. Growers need reliable information on which to base requests for permission to use
water for irrigating tomatoes. If too much water is permitted acreage would be reduced,
however, with too little water permitted profits would be reduced or eliminated. Previous
research has shown that drip-irrigation is more efficient than sprinklers for tomato production
in North Florida (Rhoads, 1990).
There are several procedures available for scheduling irrigation. Among them are
tensiometers, electrical resistance blocks, pan evaporation, and computer models. Evaporation
from a free water surface integrates many climatic factors, such as solar radiation, temperature,
wind and humidity, that influence evapotranspiration by crops. A standard evaporation pan costs
several hundred dollars and it would be quite expensive to place one in each field of tomatoes.
However, research has shown that a No.-1 wash tub can be used for acceptable measurement
of evaporation (Westeson and Hanson, 1981).
The crop coefficient is the ratio of potential evapotranspiration to pan evaporation and
depends upon crop species, growth stage, and available soil moisture. It is difficult to determine
the crop coefficient for tomatoes because of the wide row spacing making it a monumental task
to monitor soil moisture for determining total plant water use. In previous research, pan
evaporation was used to schedule irrigation of spring planted tomatoes in North Florida with the
following irrigation rates:. 0, 1/4, 1/2, 3/4 and 1.0 times the amount of pan evaporation
(Locascio et al. 1989, and Rhoads, 1990). Highest yield was achieved with 3/4 times pan
evaporation. Further refinement is needed because the crop coefficient is not constant
throughout the growing season, although, pan evaporation is lowest in the spring when plants
are small. Irrigation data from spring tomatoes does not apply to fall tomatoes because pan
evaporation is greatest for fall tomatoes when plants are small and not using much water.
The purpose of our research was to use pan evaporation to schedule irrigation of fall
tomatoes and determine which schedule produced highest yields. Electrical resistance blocks
were also installed to monitor soil-water depletion for each irrigation schedule. In addition,
tensiometer scheduled irrigation was compared with pan evaporation schedules.
MATERIALS AND METHODS,
Staked tomatoes (Solar Set cultivar) were grown with drip-irrigation under plastic mulch
in rows six feet apart to simulate commercial production practices. Transplanting dates were
July 23, 1990 and August 1, 1991. Daily irrigation was applied through drip tubes placed under
the mulch. Amount of irrigation for each treatment was controlled by a time clock. Application
time for each treatment was calculated from manufacturer's flow rate specifications for the
irrigation tubing. A flow meter was installed to verify irrigation amount. Irrigation treatments
were 0, 1/4, 1/2, 3/4 and 1.0 times standard pan evaporation (PE) rate. An additional treatment
received 0.2 inches of irrigation when tensiometer readings averaged 20 centibars (cb) in 1990
and 15 cb in 1991. Tensiometers were placed in each replication of this particular treatment in
the row six inches from a plant with sensors at the six and 12 depths in 1990 and six inch depth
only in 1991. Electrical resistance blocks (made from gypsum) were placed at six and 12 inch
depths in 1990 only, to measure soil moisture depletion. Resistance blocks were located three
inches from the irrigation tube between the plant row and tube. Readings from tensiometers and
resistance blocks were recorded on Monday, Wednesday, and Friday each week during the
growing season before applying daily irrigation. Tomatoes were harvested four times in 1990
and only twice in 1991 because of an early freeze in 1991. Fruit were divided into medium,
large, and extra large grades at each harvest and the yield of each grade was determined. The
experimental design was a randomized complete block with four replications. Regression
analyses were used to determine the significance of response to irrigation.
RESULTS AND DISCUSSION
Highest yield occurred in 1990 at the 1/2 pan evaporation irrigation rate (Fig. 1). The
tensiometer scheduled irrigation treatment produced slightly less yield. Although total yield was
related to irrigation rate, yield of extra large fruit was the only single grade correlated with
amount of irrigation. Medium and large fruit yields were not related to irrigation. Yield
response to irrigation in 1991 was very similar to that in 1990 with maximum yield occurring
also at 1/2 pan evaporation irrigation rate (Fig. 2). The unirrigated treatment produced the
highest yield of medium and large fruit and the lowest yield of extra large fruit. As in 1990,
only the yield of extra large fruit was related to irrigation rate.
Fall Tomato Response to Irrigation
0 5.3 6.5 10.6 16 21.3
Total Irrigation (inches)
5.3" 1/4 PE, 10.6' 1/2 PE, 16' 3/4
PE, 21.3' 1.0 PE, 6.5" 0.2" 20cb
Fig. 1. Response of fall tomatoes to irrigation in 1990. The six irrigation treatments were:
non-irrigated, 5.3" = 1/4 Pan Evaporation (PE), 10.6" = 1/2 PE, 16.0" = 3/4 PE,
21.3" = 1.0 PE, and 6.5" = irrigation at tensiometer reading of 20 cb with 0.2" or
5425 gallons per acre per application.
Fall Tomato Response to Irrigation
0 3.97 6.4 7.93 11.86 15.86
Total Irrigation (inches)
3.97'- 1/4 PE, 7.93' 1/2 PE, 11.86-3/4
PE, 15.86'" 1.0 PE, 6.4' 0.2' 15cb
Fig. 2. Response of fall tomatoes to irrigation in 1991. Irrigation was scheduled the same as
in 1990 except the season was shorter and the tensiometer treatment was irrigated at 15
cb and received 6.4" of water.
Regression analyses revealed a highly significant correlation between total yield of
tomatoes and amount of irrigation applied in both years of the test (Fig. 3). Predicted yield at
10 inches of irrigation was near the maximum yield produced each year. Also, predicted yields
were similar between years in the range of 0 to 10 inches of irrigation. The rapid drop off in
predicted yield for 1991 is probably related to the short picking season which allowed only two
harvests. Generally, excessive irrigation or rainfall results in delayed harvest of tomatoes with
a greater portion of the yield occurring in later harvests as compared with optimum irrigation.
Irrigation scheduled with tensiometers is expected to approximate the water use demand
of the crop, however, scheduling 0.2 inches at 15 cb rather than 20 cb made a difference in
water use rate of tomatoes between years (Fig. 4). There are several factors influencing
tensiometer readings in drip-irritated tomatoes under plastic mulch. Irrigation emitters can
become clogged causing apparent higher water demand. Uneven root distribution may cause
either higher or lower apparent water demand. Electrical resistance blocks showed that soil
water in the tensiometer scheduled treatment at the six inch depth was maintained relatively
stable while soil water at the 12 inch depth was being depleted (data not shown). This suggests
that the total plant water demands were not met with tensiometer scheduled irrigation. The
tensiometer schedule revealed that onset of rapid water demand by tomatoes occurred both years
at about three weeks after transplanting (Fig. 4).
Soil-water depletion in the non-irrigated treatment at the six-inch depth became rapid
about three weeks after transplanting, while at the 12-inch depth it became rapid about six weeks
after transplanting (data not shown). Rapid soil-water depletion at the six-inch depth started
about 20 days after transplanting in the 1/4 PE treatment, and about 45 days in the 1/2 and 3/4
Yield Response of Fall Tomatoes
to Inches of Irrigation
Y 954 + 148X 5.09X^2 -- 1990
500 Y 1091 + 15X 7.33X2 1991
Y=1091+150X- 7.33X^2 1991
0 2 4 6 8 10
Inches of Irrigation
12 14 16
Fig. 3. Regression analysis for yield response of fall tomatoes to irrigation rate in 1990 and
1991. 1990 R2 = 0.891, P < 0.001; 1991 R2 = 0.577, P < 0.001.
Irrigation Fall Tomatoes
Tensi. Irrig. Sched.
20 cb 6' d.
15 cb 6' d.
20 40 60 80 100
Days After Planting
Fig. 4. Accumulative irrigation use of fall tomatoes with tensiometer scheduled irrigation in
1990 and 1991. Tensiometer sensors were six inches from plants at the six-inch depth.
Irrigation was applied at 0.2 inches per application when tensiometer readings were 20
centibars (cb) in 1990 and 15 cb in 1991.
PE treatments (Fig. 5). Depletion of soil water at the 12-inch depth occurred with both 1/4 and
1/2 PE irrigation schedules but not with the 3/4 and 1.0 PE schedules (data now shown).
The first increase in soil-water depletion at the six-inch depth (Fig. 5) corresponds to the
beginning of rapid leaf and stem expansion, while the second increase corresponds to the
beginning of rapid dry-matter accumulation (Clark et al., 1990). The reduction in soil-water
depletion at about 9 weeks after transplanting coincides with the completion of dry-matter
Data from electrical resistance blocks suggest that water-use efficiency could be improved
and plant-water stress reduced if the 1/4 PE irrigation rate is applied for the first three weeks
after transplanting, 1/2 PE the second three weeks, 3/4 PE the third three weeks and 1/2 PE the
remainder of the season. This schedule would not increase or decrease total water use by
tomatoes in the 1/2 PE treatment.
Daily Irrigated Tomatoes (6 in SM)
% Soil Water(relative)
40 --- -1/4-PE--
-- 1/2 PE
0 I I I I
0 20 40 60 80 100 120
Days After Planting
Fig. 5. Soil moisture (electrical resistance) block readings for three irrigation treatments in fall
tomatoes. Irrigation rates were 1/4, 1/2, and 3/4 times pan evaporation applied daily.
Irrigation of fall tomatoes at Quincy was scheduled at 0, 1/4, 1/2, 3/4, and 1.0 times
standard pan evaporation and also with tensiometers in a two-year test. The 1/2 pan evaporation
(PE) schedule produced maximum yield each year. Data from soil moisture (electrical
resistance) blocks suggest that maximum irrigation efficiency could be achieved by irrigating at
1/4 PE rate the first three weeks after transplanting, 1/2 PE the second three weeks, 3/4 PE the
third three weeks, and 1/2 PE the remainder of the season. There are fewer problems
encountered with irrigation scheduled by PE than by tensiometers. Several tensiometer stations
are required for accurate evaluation of soil-water depletion while only one station is required for
PE measurement. Tensiometers also require considerable maintenance and periodic replacement.
1. Clark, G. A., G. N. Maynard, C. D. Stanley, G. J. Hochmuth, E. A. Hanlon, and D. Z.
Haman. 1990. Irrigation scheduling and management of micro-irrigated tomatoes. Fla.
Coop. Ext. Serv. (IFAS) Univ. of Fla. Circular 872.
2. Locascio, S. J., S. M. Olson, and F. M. Rhoads. 1989. Water quantity and time of N and
K application for trickle-irrigated tomatoes. J. Amer. Soc. Hort. Sci. 114(2):265-268.
3. Rhoads, F. M. 1990. Irrigation use by mulched staked tomatoes in North Florida. Fla.
Agric. Exp. Sta. (IFAS) Univ. of Fla. NFREC, Quincy Res. Rpt. 90-17.
4. Westesen, G. L. and T. L. Hanson. 1981. Irrigation scheduling using wash tub evaporation
pans. p. 144-149. In C. J. Phene and E. C. Stegman (ed.). Irrigation scheduling for water
and energy conservation in the 80's. Conf. Proc. Amer. Soc. Agric. Engin., Chicago, IL.
14-15 Dec. 1981. St. Joseph, MI.