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On-Farm Demonstration of Soil Water Movement In Vegetables Grown with Plasticulture

The long-term sustainability of commercial vegetable production requires increased fertilizer and irrigation efficiency.  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 and fertilizer leaching.  However, 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.

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 using soluble blue dye as a water tracer.  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 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; Fig. 1) 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. 

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 (Figs. 2, 3, 4).  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 (1^st dye) had moved only an average of 5 inches deeper.  On May 14, the dye injected on April 28 (2^nd 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 establishment. 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 (Fig. 5).  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 (Fig. 6).  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.

The irrigation and fertilizer schedules used by 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 simultaneously at the end of the experiment, showing 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 horticulturally adequate for the remainder of the season, but could be reduced by 20%.  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.

In summary, 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 approximately 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.

(Eric Simonne and David Studstill, Horticultural Sciences Dept.; Bob Hochmuth, NFREC-Suwannee Valley; Justin Jones, NFREC-Quincy; and Cliff Starling, Suwannee County)