University of Florida Institute of Food and Agricultural Sciences. Gulf Coast Research and Education Center 5007 60th Street East Bradenton, FL 34203

University of Florida Institute of Food and Agricultural Sciences Gulf Coast Research and Education Center 5007 60th Street East Bradenton, FL 34203 FINAL REPORT Submitted to the Southwest Florida Water Management District May Project B-36 IMPACT OF WATER TABLE DEPTH ON VEGETABLE PRODUCTION USING A SEEPAGE SUBIRRIGATION SYSTEM C. Stanley, Ph.D., Soil Science G. A. Clark, Ph.D., Engineering Gulf Coast Research and Education Center 5007 60th Street East Bradenton, FL 34203

University of Gulf Coast Research and Education Center 5007 60th Street East FL 34203 May Report to the Southwest Florida Water Management District FINAL REPORT TITLE: Impact of water table depth on vegetable production using a seepage subirrigation system RESEARCHERS: D. Stanley, Ph.D., Soil Science G. A. Clark, Ph.D., Agricultural Engineering REQUESTED FUNDING LEVEL: $70,000 DURATION: BACKGROUND: 3 years Conventional seepage subirrigation where water is conveyed within the field through lateral ditches is the most common irrigation system used for tomato production on flatwoods soils. The naturally high water table on these soils is raised to a level where water moves into the production bed by capillary action. Control over the actual level at which the water table stabilizes in any particular field is very limited with this system and is often determined by soil characteristics, level of the natural water table, and a producer s application schedule. Because control is difficult, field determination of a desired water table level which would provide for optimum crop production and minimal application of water has not been accomplished. Determination of this optimum level is important because higher water tables, require more water for establishment and maintenance. The development of the fully enclosed seepage system has allowed more control over a stabilized water table level in the field as well as significant reduction in application amounts required compared to conveyed seepage. Since water is applied through microirrigation tubing, this systems offers a greater degree of application uniformity, increased control over application amounts, and ultimately, when using automated sensors, control over the depth at which the water table is maintained.

SCOPE OF The primary goal of this proposed research is to determine the deepest water table level that could be used and still provide adequate water for normal tomato or green pepper production levels. The fully enclosed seepage system would be designed to float switches set to maintain water tables at predetermined levels of and 30 inches below the bed surface. Water quantities used by the tomato crop grown with each water table level treatment would be monitored throughout the season, and yield and quality of the crop determined at the end of the season. Water table level recorders would be installed in each treatment plot area to verify actual water table fluctuations and the impact of rainfall events on water table behavior. This study would be carried out at the Gulf Coast Research and Education Center, Bradenton. Although one soil fine sand) will be used in the study, this soil is representative of many flatwoods soils where tomatoes or peppers are grown with seepage irrigation, making the results applicable over a large area. The primary soil characteristic that would influence the optimum water table height would be soil texture affecting capillary rise above the water table. Since most flatwoods soil types are sand in the top 20 inches, the capillary rise characteristics would likely be similar among soils. 1 ) Determine the effect of the depth of water table level on subirrigated season fresh market tomato (spring season) and green pepper (fall season) production grown using the fully enclosed seepage system Determine the relative differences in application amounts that are required to maintain water table level at different depths. BENEFITS: The determination of a water table level lower than what is conventionally used which provides adequate amounts of water so as to not affect tomato production, would prove to be an invaluable management tool, and at the same time, provide growers several options to consider for reductions in water use. Although the fully enclosed seepage system would be used to determine optimum water table levels for tomato and pepper crops, the results from this study will be directly applicable to improving the management of ditch-conveyed seepage systems, also. Widespread adoption of this improved management practice could have a significant impact on more efficient use of available water resources. Growers presently using ditch-conveyed seepage irrigation and attempting to manually maintain a target water table level should experience reduction of amounts of water

applied by switching to the FES system. The results also may encourage more widespread adoption of the fully enclosed seepage system (shown to use 3040% less water than conveyed seepage when the water table is maintained near 24 inches below the bed surface), either automated or manually operated. Finally, a design of an automated float switch system (for electrical-powered pumping systems) to control applications will be available. Task 1. DESCRIPTION Field plot preparation and equipment installation This task involves land preparation, the installation of microirrigation tubing, water table control and monitoring equipment prior to each season of crop production. Crop establishment and production. This task involves the practices required for proper crop establishment including soil preparation, fumigation, transplanting, pest management, pruning, and fruit harvest and grading. Tomatoes will be grown as the spring crop and green peppers will be grown as the fall crop (due to the susceptibility of tomatoes to virus problems in the fall). 3. Data collection and analysis. This task involves the collection of field data throughout the season and statistical analysis and interpretation of the data at the end of each production season. 4. 5. Mini-field day and workshop on the use of water table management for improved irrigation management. Final analysis and project conclusions. This task involves the compilation of all data and a final analysis which summarizes the data collected 1) to determine the effect that maintaining a target water table level has on application amounts, and 2) the effect of water table level on crop production.

The development of the fully-enclosed subirrigation (FES) system for fresh market vegetable production has improved the ability to establish and maintain water table at specific and desired levels when compared to conventional ditch-conveyed subirrigation. This is possible due to a more precisely controlled application system and a greater uniformity of application throughout the field (Clark et al., Stanley and Clark, 1991; Clark and Stanley, 1992). The FES system uses microirrigation tubing rather than open ditches to convey and apply water needed to raise the water table to desired levels (Fig. 1). When manually controlled, the system saves 3040% (Stanley and Clark, 1991) in irrigation applications through elimination of irrigation tailwater runoff from ditches, minimization of surface evaporation, and the ability to apply water more uniformly throughout the field. The system is well-suited for automated control of water applications to maintain static water table levels by using field-located sensors and system controllers, thus, further increasing irrigation efficiency. With subirrigation, lateral subsurface losses increase as maintained water table levels increase because field edge flow gradients increase resulting in a need for require greater irrigation applications. If desired crop production levels could be achieved while maintaining lower water table levels, substantial amounts of irrigation water could be saved. At the same time, increased effective use of rainfall and reduced potential for field flooding could be achieved through greater available soil water storage above the water table. Because water tables can rise to within 8 in. of the bed surface during heavy rainfall periods, applied fertilizer is vulnerable to leaching through solubilization and movement downward as the water table drops to its original position. Growers commonly compensate for this potential loss by applying nitrogen (N) in amounts lb N/acre. The recommended N application rate (Hochmuth et al., 1988) for fresh market tomato production is 190 to 240 lb N/acre based on 8712 bedded row ft/acre (equivalent to 22 to 28 lb ft of bedded row. If fruit yield and quality levels can be maintained with reduced fertilizer application amounts combined with managed lower water table levels, both nutrients and irrigation water could be conserved, and the potential for nutrient loading into water resources would be decreased. MATERIALS AND METHODS This study was conducted at the University of Florida s Gulf Coast Research and Education Center near Bradenton, FL during the 1992, 1993, and 1994 spring (tomato) and fall (pepper) growing seasons. A raised, plastic-mulched bed culture was used to grow each crop. The study area was a contiguous block subdivided for installation of three water table level treatments: and 30 in. below the top of the bed. Plot areas 40 wide and 300 long were used for each water level treatment (6 beds spaced 6 center to center), each separated by a 40 wide buffer to negate water table influences one treatment plot to another. tubing (GEOF LOW, Inc.) with in-line emitters (0.5 with an line emitter spacing of 24 in. was installed 16 in. deep with laterals spaced every 20 A. The system was subdivided into irrigation zones to allow for individual control of water table levels in and around each treatment area. Each water table level treatment was instrumented with observation wells, water level recorders and loggers, and an electronic float switch irrigation

Ditch Ditch-conveyed subirrigation Ditch Natural water table Raised water table level Fully enclosed subirrigation eds! Microirrigation Natural water table level Raised water table level Figure 1

control system. A solenoid valve which controlled irrigation applications was activated or deactivated depending on the float switch position and water table control height. A filtered and chlorine-treated (to prevent tube clogging) pressurized water supply pumped from a surface water source was used for irrigation. The soil is an fine sand haplaquods, sandy, siliceous, with a natural water table level normally between 36 and 48 in. from the ground surface. Residual N in the rooting from previous cropping seasons is generally very low ( 2 because interseasonal rainfall leaches remaining nutrients very quickly through this soil which is approximately sand. Replant soil N determinations were made prior to each seasori to this condition. Gravimetric soil moisture determinations were periodically made during the season to determine the relationship between water table position and soil moisture gradient with depth to the water table. Pest management was accomplished with standard recommended practices for tomato or pepper production. Commercial fertilizer used for supplying N and was a blend of ammonium nitrate, calcium nitrate, and potassium nitrate. rate treatments were selected to range the recommended rate to the high end of grower practice. The rate treatments were (in of P-K) 1) 2) and 3) Even though the objective was to focus on N, K rates were adjusted with N to simulate current grower practice. The experimental design for the study was a split-plot with water table level as the main plot, split by fertilizer rate, and replicated in time by season. Within each season, 4 subplots were used for each treatment combination to increase data collection and confidence in the results. Commercial fresh market tomato or pepper varieties were used as the test crop in each season. Prior to field preparation, the system and float switches were activated to establish and maintain an deep water table (below the top of the beds) in all plots. Preparation of the production beds (32 in. wide and 8 in. high) was accomplished in the following sequence: beds were formed on centers (8712 bedded ft/acre); P was broadcast on the beds and lightly incorporated; N and K fertilizer was applied in grooves 1 in. deep on the bed surface either as two bands 8 in. on either side of bed center for tomato production or a single band in the center of the bed for pepper production; fumigation with methyl was injected at a rate of 350 lb/treated acre; and beds were then covered with 1.5 mil black plastic two weeks prior to planting. Each crop was planted using transplants approximately six-weeks old. Tomato transplants were planted with an in-row spacing of 24 in. (single row in center of bed) resulting in a plant density of 4356 plants/acre, and pepper transplants were planted spaced 12 in. apart in double rows for a plant density of 17424 plants/acre. Initially, a water table level of 18 in. was used to establish the transplants and the float switches were adjusted to the plot treatment levels of and 30 in. water table positions approximately two weeks after transplanting.

RESULTS AND DISCUSSION Spring Tomato Results Results summarizing the effect of water table level and fertilizer rate on total marketable yield for each individual season and all seasons combined are shown in Tables and Figures Regression analysis of the data revealed no significant interactions between fertilizer and water table level treatments, There were no significant differences for marketable yields among N fertilizer rates at individual water table levels, indicating that lower fertilizer rates were adequate to achieve acceptable production. Although no differences in marketable yield were measured between fertilizer treatments, occasional visual differences were observed with respect to plant condition at the lowest N rate treatment (especially in the water table level treatment). Nitrogen deficiency became visually apparent late in the season after the second harvest. Similar plant nutritional deficiencies have been reported as a result of higher water table management levels. The highest N-rate treatment showed a significant quadratic response with respect to water table level for the combined seasons indicating the optimum water table level when using 360 lb N/acre is near 24 inches (Table 4). With respect to total marketable production, again there was a significant quadratic response for water table level treatment indicating that the water table level was more favorable than either the treatments. Marketable fruit size yield results as affected by fertilizer rate or water table level showed no consistent significant response in fruit size yield among fertilizer treatments for any fruit size category or total marketable fruit production. A significant quadratic response was observed among water table level treatments for total marketable fruit production, but not for any fruit size category. Fall Bell Pepper Results Yield results for the fall peppers were very similar to that of the tomatoes. Tables and Figures 6-9 contain the summarized data for the individual and 1994 seasons, and combined results. While the 1993 season showed significant yield differences with respect to water table level (lower yield for the l&inch treatment), these were due to transplant and salinity problems which resulted in loss of plants and poor growth of those that survived. Although the combined season analysis reflected the yield response measured in the 1993 season and caused a overall significant water table level effect, the problems experienced that season need to be considered when drawing conclusions. Water Table Level Results Figures show the water table level fluctuations for the 18.24, and 30 in. treatments during each growing season. Rainfall events occurred where sharp upward peaks are evident. In the spring seasons (Figs. and 14) there was development of a fluctuating (2-5 in.) diurnal cycle of the water table level for all treatments (during mid- to late season) that seemed to indicate that even though irrigation applications were made

Table 1. Marketable tomato fruit yield and size as a result of N fertilizer and water table treatments for the 1992 spring growing Water Table Level (inches below bed surface) N-rate (lb/acre) (lb/acre x 1000) -- 192 89.8 89.1 88.9 276 83.0 83.8 78.3 330 84.1 94.2 813 N-rate Medium Fruit Size Total 192 276 (lb/acre x 1000) ------- 12.7 22.7 53.7 89.1 14.6 21.5 45.7 81.8 360 22.2 25.3 39.0 865 -MARKETABLE FRUIT Fruit Size FOR FRUITSIZE (BY Water Table Medium Extra Lame Total Level (inches) --------- 18 19.9 24.8 40.9 85.6 24 17.7 22.0 493 89.0 30 11.9 15.1 48.3 75.3 Quadratic! # $ %!, Significant linear or quadratic relationship at &'&( and 0.01 levels of probability, respectively; ns = not significant.

Table 2. Marketable tomato fruit yield and size as a result of N fertilizer and water table treatments for the 1993 spring N-rate Water Table Level (inches below bed surface) ----- (lb/acre x 1000) 192 69.0 79.2 75.1 ns 276 77.5 90.4 85.6 ns 360 66.8 93.2 80.5 ns ns Fruit Size N-rate Medium Extra Large Total 192 276 360 Water Table Level (inches) 18 (lb/acre x ----- 23.6 22.1 71.2 27.1 28.5 28.9 84.5 26.9 27.9 25.4 80.2 FRUIT Medium 27.0 Fruit FOR Extra Large (lb/acre x 1000) ------- 24.1 19.9 ns Total 71.0 24 29.4 30.1 28.0 87.5 30 24.4 27.6 28.4 80.4 Quadratic Significant linear or quadratic relationship at 0.05 and 0.01 levels of probability, respectively; ns = not significant.

Table 3. Marketable tomato fruit yield size a result of N fertilizer and water table treatments for the 1994 growing N-rate (lb/acre) Water Table - - - - (lb/acre x (inches below bed surface) 192 79.6 76.8 65.8 276 89.8 82.8 69.9 360 83.0 82.4 69.6 ns ns Fruit Sii N-rate Medium Extra Large Total 192 276 360 --------- ------- 14.6 24.5 35.0 74.1 13.6 25.5 41.8 80.9 12.0 27.1 39.2 78.3 ns! # MARKETABLE FRUIT FOR (BY Fruit Size Water Table Medium Extra Large Total Level (inches) 18 13.0 26.4 44.7 84.1 24 12.7 26.3 41.8 80.8 30 14.6 24.4 29.4 68.4 Quadratic Significant linear or quadratic relationship at 0.05 and 0.01 levels of probability, respectively; ns = not significant.

Table 4. Marketable tomato fruit yield and size as a result of N fertilizer and water table treatments for combined spring growing seasons. Water Table Level (inches below bed surface) N-rate ------ (lb/acre 1000) --- 192 79.5 81.7 76.6 276 83.4 85.7 77.9 360 78.0 89.9 77.1 ns ns! # Fruit Size N-rate Medium Extra Large Total --- (lb/acre -------- 192 18.1 24.3 36.9 276 18.4 25.2 38.8 360 20.4 26.7 34.5 Water Table Level (inches) FRUIT FOR (BY Fruit 793 82.4 81.6 Medium Extra Large Total ------- ------- ns 18 24 30 Quadratic 20.0 35.1 80.2 19.9 26.1 39.7 85.7 17.0 24.9 35.4 773 ns!,! Significant linear or quadratic relationship at 0.05 and 0.01 levels of probability, respectively; = not significant.

1992 Spring Tomatoes Figure 2 Water Table 0 20 0

1993 Spring Tomatoes Figure 3 2 4

1994 Spring Tomatoes Figure 4 t Table I 120 1 2 0 loo 2 4

Table 5. Bell pepper marketable fruit weight, fruit number, and average weight/fruit. for the 1992 fall production N-rate 276 Water Table Depth (inches] 28.60 29.44 28.09 28.52 30.83 25.04 27.33 28.70 27.61 N-rate 192 276 360 Water Table Depth (inches) 65.67 69.04 62.29 65.01 73.07 5739 60.76 65.99 61.75 N-rate (lb/a} 192 276 360 Water Table Depth (inches) 24 0.44 0.43 0.45 0.44 0.42 0.44 0.45 0.43 0.45 * Significant linear or quadratic relationship at 0.05 and 0.01 levels of probability, respectively; = not significant.

Table 6. Bell pepper marketable fruit weight, fruit number, and average weight/fruit for the 1993 fall production season. N-rate (lb/acre) 192 276 360 Water Table Depth (inches) 11.16 27.66 8.71 26.92 32.97 7.30 27.48 33.65 ns Water Table Depth (inches) N-rate 192 31.25 72.96 77.10 276 24.83 71.22 85.16 360 21.34 73.40 85.60 ns ns ns N-rate (lb/a) 192 276 360 PEPPER Water Table Depth (inches) 24 0.36 0.38 0.37 ns 0.35 0.38 0.39 034 037 0.39 ns ns! * Significant linear or quadratic relationship at 0.05 and 0.01 levels of probability, respectively; ns = not significant.

Table 7..... Bell pepper marketable fruit weight, fruit number, and average weight/fruit for the 1994 fall production season. Water Table Depth (inches) N-rate (lb/acre) 192 21.52 20.30 20.73 276 19.38 2330 21.63 22.15 22.83 24.12 N-rate (lb/a) Water Table Depth (inches\ 192 63.60 62.5 1 63.38 276 59.90 71.22 66.86 360 67.05 69.04 75.15 PEPPER AVERAGE Water Table Depth (inches) N-rate (lb/a) 24 30 192 0.34 0.32 0.33 276 032 033 0.32 360 0.33 0.33 0.32!, Significant linear or quadratic relationship at 0.05 and 0.01 levels of probability, respectively; = not significant.

Table 8. Bell pepper marketable fruit weight, fruit number, and average weight/fruit. for a l l N-rate 192 Water Table Depth (inches) 20.42 25.8 276 18.87 27.02 360 18.93! # Water Table Depth (inches) N-rate 192 276 53.51 68.16 49.91 71.84 69.8 360 49.72 69.48 74.17! ) Water Table Depth (inches) N-rate (lb/a) 192 18 24 0.38 0.38 0.38 230 0.37 0.38 0.38 360 0.37 0.38 039!,! * Significant linear or quadratic relationship at 0.05 and 0.01 levels of probability, respectively; ns not significant.

1993 Fall Bell Peppers Figure 7 Water Table 276 102 Table 1 0 0.1 0.2 0.3 0.4 Fruit 0.5

1994 Fall Bell Peppers Figure 8 50 Water Table 10 0 102 Water Table Trt I 0 0.1 0.2 0.3 0.4 Weight 0.5 0.6

Spring Tomatoes-All Seasons Figure 5 120 18 24

1992 Fall Bell Peppers Figure 6 0.1 0.2 0.3 0.4 0.5 0.6 Fruit Weight

All Seasons Bell Peppers Figure 9 so Water Table Table * 18itlCh * 24inch * +,-!!.! 0 0.1 0. 2 0. 3 0. 4 0. 5 Weight

continuously during that period, the application rate was not high enough to overcome the plant demand for water and the amount of water needed to maintain the water table level. Recovery to desired levels (indicated by the flattening of the lines) was always achieved overnight. During the 1993 season (Fig. a system malfunction which caused an interruption in irrigation for the treatment occurred for about 7 days (approximately between 1300 and 1550 hrs) showed that even when irrigation didn t occur and the water table level declined, the diurnal cycle continued to be evident. An indication of water savings due to effective use of rainfall for the water table level treatment can be seen in Fig. 13 where for nearly 27 days virtually no irrigation was required to maintain the target level, while irrigation applications continued for the inch, and especially for the treatments. Seasonal Water Application Amounts In preface to the discussion of water application amounts, it must be stressed that this study was never intended to be a quantitative water use study so the reader must be aware that the information presented in this section should be viewed as site specific and qualitative only. This is due to the lack of total control of measuring water losses and movement from the system used. However, the water application amounts do give an indication of how the water table level may affect water use. Table 9 contains seasonal application for each treatment and average over seasons. These amounts do not include water used for field preparation or the first two weeks after transplanting. Figures graphically illustrate the seasonal progression of water application for each treatment in each season. It is very clear that much less water was used in the and treatments relative to season and year. The fall seasons consistently required less water than the spring seasons. The on/off cycles for each water table level treatment as controlled by the field-located float switches were continuously logged throughout the growing seasons. Figure 22 illustrates a typical period (16 March to 20 March 1992) following significant rainfall for the spring tomato season. This figure shows how the 18 in. treatment initially required irrigation applications sooner than the other two treatments and to a greater extent during the high ET demand during daylight hours. Again, although this study was not designed to quantify water use differences among treatments, based on the amount of total time the irrigation applications were made during this period, relative amounts applied compared to the 18 in. treatment were 45% and 30% for the 24 in. and 30 in. treatments, respectively. This figure indicates that more water is required to maintain a higher water table (as a function of frequency of application) than a lower water table. The soil moisture gradient of soil profiles in the water table level treatment areas and surrounding field edge buffer areas (sampled in order to increase collected data at water table depths other than the maintained levels) was measured using gravimetric sampling at soil depths upward from the water table to determine whether differences existed among treatments for capillary movement into the upper portions of the plant bed. fine sand has a volumetric moisture content of approximately 10% at -10 (near field capacity). The capillary rise from the water table is not enough to maintain a soil moisture content above 10%

-0.5 Seasonal Water Table Levels Spring -1.5-2 -2.5-3 0 500 1,000 1,500 2,000 2,500 Time (hrs) Figure 10 0 Seasonal Water Table Levels Fall -0.5-1 -1.5-2 -2.5-3.5 0 500 1,000 1,500 2,000 Time (hrs) Figure 11

0 Seasonal Water Table Levels Spring 1993-0.5-1 -1.5-2.5-3 -3.5 0 500 1,000 1,500 2,000 2,500 Time (hrs) Figure 12 0 Seasonal Water Table Levels Fall 1993-0.5-1.5-2.5-3 0 500 1000 1500 2000 2500 Time (hours) Figure 13

-0.5 Seasonal Water Table Levels Spring1994-1.5-2 -2.5-3 0 500 1, 0 0 0 1,500 2,000 Time (hrs) Figure 14 0 Seasonal Water Table Levels Fall 1994-0.5-1.5-2.5-3 0 500 1,000 1,500 2,000 2,500 Time (hours) Figure 15

Table 9. Seasonal water applications (in gallons) for Fall and Spring 1992-1994. Water Table Level SEASON Fall 1992 1 6 6 4 9 4 8 0 1 3 3 46947 Fall 1993 1 2 1 6 7 9 42945 35188 Fall 1994 1 5 2 6 5 0 63970 48610 Fall Average 146941 62349 43581 1992 427063 1 3 2 1 0 8 153807 Spring 1993 227356 Spring 1994 167972 99846 119947

500,000 400,000 Spring 1992 Water Applications 18inch. 24 inch 300,000 30 inch 200,000 100,000,. I 80 80 100 120 140 180 180 Day Figure 16 200,000 Fall 1992 Water Applications 18 inch.i 150,000 24 inch 100,000 30 inch 50,000 I I I 80 280 300 320 340 380 Day Figure 17

2 5 0, 0 0 0 200,000 Spring 1993 Water Application8 18 inch 24 inch. 30 inch 100,000 E l 0, 80 100 120 140 160 180 Day Figure 18 140,000 Fall 1993 Water 18 inch 100,000 24 inch 80,000 30 inch 120,000 60,000 40,000 20,000 60 280 I 300 320 340 Day Figure 19

200,000 Spring 1994 Water Applications 3150,000 24 inch 100,000 30 inch 50,000 0 I I. 70 80 100 110 120 130 140 Day Figure 20 Fall 1994 Water Applications 150,000 280 300 320 Day Figure 21

18 inch 24 inch OFF :. :. 30 inch O N : : : : I-/- : : : : : : : : : :. : : : : : :. OFF i i : : Noon Noon Noon I I I I I I I I Midnight Midnight Midnight Figure 22

-16 16 22 24 26 Water Table Depth (inches) 26 32 Figure 23

in the upper 12 in. of the bed when the water table level is below 24 (Fig 23). This information may help explain why the significant quadratic relationship among water table levels for total marketable yield developed. SUMMARY The results from this study show no particular advantage to maintaining a water table above 24 and applying N above recommended levels. The results do show that more effective N management for subirrigated tomatoes grown under the described conditions can be accomplished with no detrimental effect on production by: 1) using the FES system to maintain a water table level near 24 in. below the bed surface; and 2) using a N rate lower than commercially accepted and near recommended guidelines (190 to 240 lb N/acre). Maintaining a lower water table level may effectively expand the available rooting zone and the ability of the plant to use N within the greater soil volume, and increase effectiveness of rainfall (Stanley and Clark, 1995). CITED Clark, G. A., C. D. Stanley and P. R. Gilreath. 1990. Fully enclosed subsurface irrigation for water table management. Florida Tomato Institute, 19-30. Veg. Crops Special series Rep. FL Coop. Ext. Ser., Univ. of Florida, Gainesville. Clark, G. and C. D. Stanley. 1992. Subirrigation by microirrigation. Applied in Agric. Stanley, C. D. and G. A. Clark. 1991. Water table management using microirrigation tubing. Soil and Crop Sci. Fla. Stanley, C.D and G.A. Clark. 1995. Effect of reduced water table and fertilizer levels on subirrigated tomato production. Applied Engineering in Agric. (in press)