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IRRIGATION OF FALL TOMATOES

IN

NORTH FLORIDA


F. M. Rhoads and S. M. Olson


1932


Florida Agricultural Experiment Station
Institute of Food and Agricultural Sciences
University of Florida, Gainesville


NFREC, Quincy Res. Rpt. 92-3


9~ 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
1990


Yield (boxes/acre)


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
1991


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


Total Boxes/Acre
2500


2000- ...

1500-

1000 Crop-Year---
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.


Tensiometer


Irrigation Fall Tomatoes
1990-91


Irrig. (in)


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

accumulation.

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)
Fall 1990

% Soil Water(relative)
100

80

60

40 --- -1/4-PE--
-- 1/2 PE


0 I I I I
0
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.









SUMMARY

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.



REFERENCES

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.