The long-term sustainability of commercial vegetable production requires increased fertilizer and irrigation efficiency. Three vegetables growers recognized as leaders in fertilizer and irrigation management in North Florida were selected to demonstrate how irrigation and fertilizer management are linked together and how management may prevent water movement below the root zone of two muskmelon and one watermelon field, all grown with plasticulture. The approach was to create irrigation rates by using drip tapes with different flow rates, inject colored dye in the irrigation water three times during the growing season, and digging the dye to determine its position. Similar results were found at all three locations: water movement was greater early in the season (1 to 5 weeks after establishment) and moved below the root zone (20 to 30 inches deep). Vertical movement was greater on a loamy soil than on the two sandy soils. The uniformity of water movement decreased as depth increased. Overall these results show that some leaching is likely to occur on light-textured soils, even when recommended practices are followed. Educational efforts should focus on fertigation management during the first weeks after crop establishment. Based on these observations, cooperators are considering improving their fertigation practices by using two drip tapes, reducing preplant fertilizer, using a 100% injected N/K program, and/or adding organic matter to the soil. The efficiency of these additional practices on further reducing water movement and fertilizer leaching will be studied next with these farmers and with organic growers who use different nutrient sources. This project shows that vegetables growers are more likely to try and adopt sustainable practices when they actively participate in the process than when these changes are mandated through legislation.
Irrigation management is directly linked not only to yield and economical value of vegetable crops, but also to long-term sustainability and environmental impact of vegetable production. Precise knowledge of where irrigation water goes has direct implications not only on irrigation management, but also on fumigant application (Hochmuth et al., 2002; Santos et al., 2003) and fertilizer leaching (Simonne et al., 2002). The recommendations of UF/IFAS for irrigation management for vegetable crops include using a combination of target irrigation volume, a measure of soil moisture to adjust this volume based on crop age and weather conditions, a knowledge of how much water the root zone can hold, and an assessment of how rainfall contributes to replenishing soil moisture (Simonne et al., 2003). Improving irrigation management in vegetable crops has been limited by the fact that water movement in soil is a process that cannot be easily seen because it occurs under ground.
A direct knowledge of how much water can be stored in the root zone can be gained by visualizing water movement in the soil using soluble dye (German-Heins and Flury. 2000). A blue dye and controlled irrigation conditions were used to visualize the wetting pattern of drip irrigation using different drip tapes on sandy soils representative of vegetable producing areas of Florida (Santos et al., 2003; Simonne et al., 2003, 2004). As research tools, these dye tests were used to describe the shape of the wetted zone for several water volumes applied by drip irrigation, determine height, width and depth of the wetted zone, and determine if soluble fertilizer and the water front represented by the dye move together in the soil (Simonne et al., 2004). As educational tools, these dye tests have been used to show growers how deep water moves into several soils and how drip tape flow rate and emitter spacing affects wetted zones. While novel in their approach, these dye tests have used single irrigation events and were done without an actively transpiring vegetable crop.
Past educational efforts and fertilization recommendations generally attempted to reduce environmental impact by reducing fertilizer application rates. While this approach is theoretically valid, it is not practical since fertilizer costs only represent 10% to 15% of the overall pre-harvest production costs. Fertilizer are often applied at rates above the crop nutritional requirement as a means to decrease the risk of reduced yields due to shortage of fertilizer, especially close to harvest. We believe that it is possible to follow a different approach to improving fertilizer management. As water is the vehicle to soluble nutrient movement in the root zone and below, it is possible, in theory, to improve nutrient management by improving irrigation management. If irrigation water stays in the root zone, smaller amount of fertilizer are likely to be leached. If growers are shown how their current irrigation schedule affects water movement in their fields, they are more likely to understand how water and nutrients are linked. With this integrated approach, sustainability becomes more compatible with economical profitability. Therefore, the goals of this project were to demonstrate to cooperating growers how irrigation and fertilizer management are linked together and how management may prevent water movement below the root zone.
More specifically, the objectives of this project were to (1) establish a partnership with three key growers and discuss fertilizer and irrigation management, (2) determine the position of the water front throughout the growing season, (3) diagnose crop nutritional status, and (4) determine nitrate distribution in the soil profile at the end of the season. From a producer’s stand point, this information will be used to increase sustainability by reducing water used and environmental impact of vegetable production. From a regulatory stand point, this information will contribute to demonstrate the efficacy of possible nutrient/water Best Management Practices and set practical management expectations.
The project was conducted in the Spring of 2004 on three commercial vegetable fields with three cooperating growers who had participated in previous irrigation management projects (Simonne et al., 2001). These growers are recognized as leaders in water and nutrient management. For educational purposes, the three fields will be referred to as ‘Site 1-cantaloupe’, ‘Site 2-watermelon, and ‘Site 3-cantaloupe’. The approach at the three sites was similar. Growers prepared the field with raised bed, drip tape and plastic mulch. Sections of beds were replaced with drip tapes with three different flow rates (Table 1). Other cultural practices were conducted by the cooperating grower throughout the growing season (Table 2). Soluble blue dye (Terramark SPI High Concentrate, ProSource One, Memphis, TN) was injected three times at each site and was traced through three or four digs (Table 3). Petiole NO3-N and K concentrations were also determined throughout the crop (Table 3) and compared to published sufficiency ranges (Maynard et al., 2003). Soil samples were taken in one-foot increments up to the 6 foot depth at each location after final harvest. Soil samples were dried, sieved to pass a 2-mm screen and sent to the University if Florida Analytical Research laboratory for NO3-N analysis using methods 352.3 (US EPA, 1983).
Spring 2004 was warm and dry in North Florida; rainfall marginally contributed to replenishing soil moisture and did not interfere with the irrigation schedule. Cooperating growers were eager to participate in this project and showed continuous interest and support. Their respective fertilizer and irrigation schedules were considered to be sophisticated as they took full advantage of the flexibility of drip irrigation to split fertilizer applications and to change irrigation schedules based on plant growth. Yet, each grower had his own approach to fertilizer management, as the ratio of preplant:injected and the starting date of injection varied widely. These different approaches are consistent with current UF/IFAS fertilizer recommendations. Nitrate-nitrogen and K concentrations in petioles were all at or above the sufficiency ranges. Drip-tape flow rate had no practical influence on crop nutritional status. As drip irrigation flow rates ranged from 59% to 100% of all cooperating growers’ rates, this suggests that crop nutritional status could be maintain while reducing fertigation inputs.
Soil types were different at the three sites. Soils were sandy at the 1-cantaloupe and 2-watermelon sites, and was relatively heavier (loamy) at the site 3-cantaloupe. Hence, the positions of the water front as represented by the dye were also different and are discussed separately (Table 4). At the 1-cantaloupe site, the depth of the first dye ring ranged between 30 and 38 inches and averaged 34 inches on April 28. From transplanting to that date, irrigation applied was for transplant establishment and was only 50 min/day (Table 2). Yet, 34 inches is well below the root zone. On the next dig two weeks later (May 14), the dye injected on April 6 (1st dye) had moved only an average of 5 inches deeper. On May 14, the dye injected on April 28 (2nd dye) had a depth ranging between 16 and 23 inches, with a 19 inches average. The second dye had moved less than the first dye. This is most likely due to differences in cantaloupe water use. Small plants (between April 6 and 14) used less water than larger plants (between April 28 and May 14). This example confirms the prediction that irrigation water needed early in the season for plant establishment may push the water front well below the root zone. Changing the existing irrigation schedule from 1 x 50 min/day to 2 x 30 min/day may not be currently practical as it take approximately 15 minutes to charge the drip irrigation system. If this 2 x 30 min/day schedule were adopted with the current irrigation system, a large (approximately 50%) portion of the irrigation cycle would be used for system charge, which is likely to decrease uniformity of application. A costly possibility to reduce the charging time would be to modify the drip irrigation system to keep it continuously pressurized. If this is not feasible (for economical reason and no cost-share), two alternative practices may be used to reduce the risk of nutrient leaching. First, it would be possible to modify the fertilizer program to include a smaller amount of preplant nitrogen and increase proportionally that injected after plant establishement. While this approach is theoretically valid, the feasibility of a 100% injected fertilizer program needs to be demonstrated first before growers are likely to adopt it. The second alternative is to change water distribution in the bed by using two drip tapes, each with lower nominal flow rates. For example, if the existing 24 gal/100ft/hr drip tape is replaced by two, 16 gal/100ft/hr drip tapes the same amount of water may be applied by reducing irrigation time by 25%. Using two drip tapes would reduce by approximately half the vertical movement of water, but would slightly increase production cost. However, this may become a cost-shareable practice. On June 2, the position of the third dye (injected on May 14) ranged between 14 and 28 inches, and averaged 22 inches. Although irrigation was at that time several hours daily, large cantaloupe plants that were setting fruits used a large amount of water. The effect of drip tape flow rate was detectable only between digs 2 and 3. Reducing drip tape flow rate by 33% (from 24 to 16 gal/100ft/hr), reduced the position of the water front on the date of dig 3 by approximately 50% (28 vs. 14 inches). Cantaloupe roots were found mainly in the plough zone (top 12 inches) but several actively growing roots were found in the top 42 inches. These results suggests that reducing irrigation amount by 25% (by using a drip tape with reduced flow rate) may be instrumental in keeping the wetted zone within the root zone. Therefore, these findings and observations together suggest that it may be possible to keep the wetted zone within the root zone of cantaloupes on this sandy soils by using two drip tapes and reducing current grower’s schedule by 25%.
At the 2-watermelon site, the depth of the first dye ring (injected on April 6 and dug on April 28) ranged between 12 and 24 inches, and averaged 20 inches. At this site, the depth of the dye ring tended to decrease as drip tape flow rate decreased. These results suggest that water used for watermelon establishment may be reduced by approximately 20%. A valve malfunction shortly after April 28 resulted in a non-scheduled 6-hour irrigation event which pushed the water front below the 45-inch depth on May 14. The depth of the third dye ring (injected on May 2) and dug on June 2 ranged between 11 and 19 inches, and averaged 15 inches. These results show that the grower’s schedule during fruit set and enlargement was adequate and did not result in a dye front moving deep below the root zone. Lessons from the 2-watermelon site are similar to those from the 1-cantaloupe site. In the absence of rain, the risk of the water front moving below the root zone is greatest during crop establishment and while plants are small (1 to 5 WAT).
At the 3-cantaloupe site, the depth of the first dye ring (injected on April 6, dug on April 28) ranged between 16 and 18 inches. While roots may be found at the 18-inch depth when cantaloupe plants are fully grown, this depth was below the root depth when the plants were at the 6-inch long vines. On May 14, the dye injected on April 6 could not be found, and the depth of that injected on April 28 ranged between 22 and 38 inches, and averaged 30 inches. On June 2, the depth of the dye injected on May 14 ranged between 17 and 20 inches, and averaged 18 inches. On June 30, the depth of the dye injected on May 14 was similar to that found on June 2: it ranged between 17 and 20 inches, and averaged 18 inches. Because of the heavier soil texture, water tended to move less at this site than at the two other sites. However, it was also observed at this site that the greatest dye movement occurred when the plants were small. Grower’s schedules when the plants were fully grown seemed adequate.
German-Heins, J. And M. Flury. 2000. Sorption of brilliant blue FCF in soils as affected by pH and ionic strength. Geoderma 97:87-101.
Hochmuth, R.C., W.E. Davis, W.M. Stall, E.H. Simonne and A.W. Weiss. 2002. Evaluating nutsedge control (Cyperus spp.) with various formulations and rates of 1,3-dichloropropene chemigated using drip tape under two polyethylene mulches. Proc. Fla. State Hort. Soc. 115:195-196.
Maynard, D.N., G.J. Hochmuth, C.S. Vavrina, W.M. Stall, T.A. Kucharek, S.E. Webb, T.G. Taylor, S.A. Smith, E.H. Simonne, and S.M. Olson. 2003. Cucurbit production in Florida, pp. 159-182 In: S.M. Olson and E. Simonne (Eds.) 2003-2004 Vegetable Production Guide for Florida, Vance Pub., Lenexa, KS.
Santos, B.M., J.P. Gilreath, and T.N. Motis. 2003. Length of irrigation and soil humidity as basis for delivering fumigants through drip lines in Florida spodosols. Proc. Fla. State Hort. 116:85-87.
Simonne, E., D. Studstill, and R. Hochmuth. 2004. Understanding water movement in mulched beds on sandy soils: An approach to ecologically sound fertigation in vegetable production. Acta. Hort. (In press)
Simonne, E.H., D.W. Studstill, R.C. Hochmuth, G. McAvoy, M.D. Dukes and S.M. Olson. 2003. Visualization of water movement in mulched beds with injections of dye with drip irrigation. Proc. Fla. State Hort. Soc. 116:88-91
Simonne, E.H., M.D. Dukes, and D.Z. Haman. 2003. Principles and practices for irrigation management, pp. 33-39 In: S.M. Olson and E. Simonne (Eds.) 2003-2004 Vegetable Production Guide for Florida, Vance Pub., Lenexa, KS.
Simonne, E., M. Dukes, R. Hochmuth, D. Studstill and S. Kerr. 2003. Are you dyeing to see where the irrigation water goes? Proc. Fla. Agric. Conf. Trade Show (FACTS X). April 29-30. Lakeland, FL, pp.21-23.
Simonne, E., M. Dukes, R. Hochmuth, G. Hochmuth, D. Studstill and W. Davis. 2002. Long-term effect of fertilization and irrigation recommendations on watermelon yield and soil-water nitrate levels in Florida’s sandy soils. Acta Hort. 627:97-103 (http://www.actahort.org/books/627/627_11.htm).
Simonne, E., A. Andreasen, D. Dinkins, J. Fletcher, R. Hochmuth, J. Simmons, M.. Sweat, and A. Tyree. 2001. On-farm irrigation scheduling for vegetables using the watermark soil moisture sensor. Proc. Fla. Agric. Conf. Trade Show. Oct. 1-3. Lakeland, Fla., pp.17-22
U. S. Environmental Protection Agency. 1983. Nitrogen, Nitrate-Nitrite. Method 353.2 (Colorimetric, Automated, Cadmium Reduction). pp.353-2.1 — 353-2.5. In Methods for Chemical Analysis of Water and Wastes, EPA-600/ 4-79-020. U.S.E.P.A., Cincinnati, OH, USA.
Educational & Outreach Activities
An EDIS version of this report is under preparation. (EDIS is the electronic repository of the extension recommendations of the Univeresity of Florida. EDIS is accessible at http://edis.ifas.ufl.edu.
Results will be presented at the on-coming Suwannee Valley workshops held at the North Florida Research and Educaton Center- Suwannee Valley on November 10, 2004 (see http://nfrec-sv.ifas.ufl.edu for program details)
Growers will present their views of the project at a round table included in the program of the Fourth North Florida Drip Irrigation School scheduled for January 20, 2005 at the North Florida Research and Education Center – Suwannee Valley
Pictures of dye movement will be added to the dye collection at the NFREC-SV web site. These pictures will be used in county extension educaiton programs. A copy of the presentation on CD will be sent to SARE offices
The irrigation and fertilizer schedules used buy cooperating growers well followed UF/IFAS splitting and scheduling recommendations and well represented proposed nutrient BMPs. It was not possible to observe the three dye rings at the end of the experiment, suggesting that these near-optimal fertigation schedules did not keep the water front within the root zone for the entire season. At the three sites, greatest water movement was observed at the beginning of the growing season between 1 and 5 WAT. This period should be the focus of educational efforts. Cooperating growers’ irrigation schedule appeared to be adequate for the remainder of the season. Using tapes with flow rates ranging from 59% to 100% did not practically affect crop nutritional status, and water movement. Cooperating growers’ fertigation schedule maintain crop nutritional status within the recommended range.
As observed in previous dye tests, the uniformity of water distribution in the soil profile decreased with depth, as water may found paths of preferential flow. Hence, leaching may not be uniform in a field even when the uniformity of the drip system exceeds 90%. Consequently, no consistent practical benefit was found in reducing irrigation rates as an attempt to reduce leaching. However, theoretically, reducing irrigation rates should reduce leaching. Another consequence of field heterogeneity is that growers tend to irrigate based on the ‘dry spots’. This often results in increasing irrigation on the other parts of the field.
This project has demonstrated again the importance of soil texture in water movement. Water moved vertically faster on sandy soils than on the loamy soil. Lateral water movement was also less on the sandy soil than on the loamy soil.
These three cooperating growers have improved their irrigation and fertilizer management every year, including this year. This demonstration has allowed them to personally see and understand how water and fertilizer management are linked. One grower indicated his intention to reduce his fertilizer rates next year by approximetely 10%. Another one is considering using two drip tapes with lower flow rate to increase the width of the wetted zone. Another grower has stated his intention to add organic matter to the soil. Lastly, cooperating growers now show interest in determining in their fields the distribution of nitrate in the soil. This is very encouraging, as on-farm monitoring has always be an unpopular practice. This project is a good illustration of the fact that BMP demonstration and implementation are possible with vegetable growers when they are directly involved in it.
Areas needing additional study
These encouraging results point at a additional set of possible BMPs that these growers could use: two drip tapes, fully injected N/K fertigation schedule, and increased organic matter. All three cooperators were ‘conventional growers’ this year, and much of the approach and methods used this year apply to all vegetable growers. Organic grower would definitely benefit from participating in a similar demonstration: they often incorrectly believe that leaching does not occur with organic production and organic production is not subjects to BMP legislation. In addition, conventional and organic growers use different fertilizer sources with different levels of solubility. Nutrient leaching pattern is likely to be different with organic sources than with conventional ones.