Final Report for OS05-026

Project Type: On-Farm Research
Funds awarded in 2005: $14,663.00
Projected End Date: 12/31/2005
Region: Southern
State: Florida
Principal Investigator:
Eric Simonne
University of Florida
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Project Information

Abstract:

The environmental impact of intensive vegetable production may be reduced through the implementation of Best Management Practices (BMPs) that integrate water and nutrient management together. This multi-faceted project used a combination of visualization of water movement in mulched beds, nutrient profile below the plants, diagnosis of plant nutritional status to help eight cooperating growers better understand the relation between fertilization, irrigation, and nutrient leaching. Although growers used sophisticated irrigation schedules (split drip irrigations), the vertical movement of the water front ranged from 0.5 to 2.5 inches/day (and reached 5.3 inch/day on one occasion). Nutrient management in drip-irrigated, plastic mulched organically-grown bell peppers proved particularly challenging. All cooperators increased their understanding and knowledge of nutrient and water management, and they indicated they planned to reduce their preplant fertilizer and irrigation rates. This type of project that involves growers is likely to have a positive impact on water quality.

Tables and Figures mentioned in this report
are on file in the Southern SARE office.
Contact Sue Blum at 770-229-3350 or
sueblum@southernsare.org for a hard copy.

Introduction

Most vegetable growers in the Suwannee Basin region of North Florida are small growers who have adopted drip irrigation and plastic mulch over the past twenty years to produce vegetable such as tomato, bell pepper, eggplant and watermelon. More recently, the availability of short-vined tropical pumpkin varieties (‘La Estrella’ and ‘El Dorado’) and increased demand for tropical pumpkin, has resulted in the emergence of a small tropical pumpkin industry. The dominant production system currently used for tropical pumpkin is bare ground with drip irrigation. Soils in the area are sandy with low water holding capacity (<10%) and low organic matter content (<1.5%). Hence, vegetable production in North Florida requires intense irrigation and fertilization management. 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., 2005a).

Previous educational efforts in North Florida have focused on plant establishment and fertilizer management (Hochmuth et al., 2003). The recent development and adoption of state-wide Best Management Practices in Florida in rule 5M-8 of the Florida Administrative Code (Florida Department of Agriculture and Consumer Services, 2005) and the increase in production costs, have emphasized the need for improved irrigation practices and a better understanding of water movement in mulched bed. Growers’ understanding of the interdependence between fertilization, irrigation, and nutrient leaching below the root zone was increased through a targeted effort supported by a 2003-2004 SARE on-farm project with three cooperators (Simonne et al., 2005b). While cooperators’ irrigation and fertigation schedule were adequate throughout the season, it appeared that irrigation in excess of crop water use necessary for plant establishment early in the season resulted in early nutrient leaching. Hence, monitoring nutrient in the soil was needed not only at the end of the season, but also at the beginning. Therefore, the goal of the 2005 project was to address the effect of irrigation on nutrient movement early in the season.

Project Objectives:

More specifically, the objectives of this project were to (1) increase the number and diversity of growers/production systems involved in the “dye” project, (2) optimize our portable dye rig for on-farm dye injections, (3) visualize the movement of irrigation water in the soil (3) determine nitrate and ammonium distribution in the soil profile early in the season, and (4) collect feed-back information on the lessons learned

Research

Materials and methods:

The project required the development of a simple, versatile system that could be easily towed from field to field and that would allow the injection of dye into irrigation systems of all sizes. In the past, dye was injected by using the farm water as the water source. This approach was cumbersome (required a farm operator to be present) and technically challenging (field laterals ranging from 1 to 3 inches in diameter). The “dye-mobile” (patent applied for!!) was developed to perform dye injections totally independently from the grower’s existing irrigation system. Using an 8-ft trailer as a platform, the dye-mobile consisted of two 30-gallon water containers, a gas-powered electrical generator, a peristaltic pump, and a ¾-inch submain line. One end of the submain line was connected to the grower’s drip irrigation using drip-tape fittings. Approximately 100-ft long sections of beds were isolated from the field by inserting on-line on/off connectors so that dye was not used over the entire field. The other end of the submain line was connected to the peristaltic pump. Initially, clear water was pumped from the tank to pressurize the drip tape, followed by soluble blue dye (Terramark SPI High Concentrate, ProSource One, Memphis, TN) injected at a rate of approximately 1 gallon/500 ft of drip tape, followed by water to flush the system. Then, the on/off connectors were re-opened, and the ends of the drip tape were tied.

Eight commercial fields (six conventional and two certified organic) were included in the Spring of 2005 dye project (Tables 1, 5). Cooperators from previous years who were eager to continue improve their management techniques were joined by two certified organic growers. The approach was similar at all sites. Growers prepared the field with raised bed, drip tape and plastic mulch according to their usual practices. Educational activities throughout the season consisted in dye injection, digging the soil profile, taking 1-ft increment soil samples to the 5-ft depth, monitoring crop nutritional status by petiole analysis, and holding on-farm field-days (Table 2). Soil samples were dried, sieved to pass a 2-mm screen and sent to the University if Florida Analytical Research laboratory for NO3-N and NH4-N analysis using methods 352.3 and 350.1, respectively (US EPA, 1983a,b). Cooperators were constantly informed of activities and results.

Research results and discussion:

Visualization of the movement of irrigation water in the soil. Cooperators used flexible irrigation schedules that were as low as 1 hr daily when plants are small. Average water front movement ranged from 0.5 to 5.3 inches/day (Table 3). In previous dye tests, the vertical water movement of the water front was 0.75 to 1.5 inches/day (Simonne et al., 2005b; 2006). On several occasions in 2005, heavy rainfalls resulted in the loss of dye, thereby showing the effect of leaching rains. Cooperators looked closely at the results of the dye tests and were overall surprised by how fast the water front moved despite the little water they used in each irrigation cycle. Two lessons can be learned from these observations. First, from a BMP standpoint, this confirms that sophisticated irrigation schedules may not be enough to prevent leaching in the absence of rain. Consequently, fertilization (preplant and early injections) should be kept minimal until transplants are fully established. Second, new strategies in stand establishment such as using larger transplants, amending the central part of the beds to increase water holding capacity should be investigated. Third, a new look may be needed at irrigation system designs. We need short and frequent irrigations, but the shortest length of irrigation is conditioned by irrigation design. Typically, charging time should not take more than 25% to 30% of the irrigation cycle or field uniformity will be reduced. Hence, a 30 min irrigation cycle may require a charging time of less than 7 min. Currently, most systems have 10 to 15 min. charging times, with some longer (>30 min.) when the water source is not near the field.

Assessing plant nutritional status with in-season sap petiole testing. Sap testing was performed on most farms during the growing season (Table 4). One cooperator purchased and used his own meters and did not require assistance. Overall, sap test levels were above the sufficiency ranges, especially for K. Although UF-IFAS recommendations generally include a 1:1 N:K2O ratio, vegetable growers typically use a 1:2 or 2:3 ratio on the assumption that higher K rates increase produce quality and shelflife. Because no environmental issues are currently associated with K, high K rates are strictly a production economics decision.

Sap testing was instrumental in helping diagnose and correct a N shortage in field Pepper-O. One of the organic farms had excessive rainfall early in the season during the planting season and shortly thereafter. These rainfall events solubilized much of the preplant fertilizer which was apparently leached out of the root zone (Figs.2,3). Since this organic grower was dependant on that preplant fertilizer application for the seasonal N supply, the crop became severely deficient by early fruit set. This low nutrient status was determined by the petiole sap testing (Table 4), and this resulted in action on the grower’s part to apply more organic N through the drip system. This process certainly “saved” the crop. Reduced N supply due to leaching usually results in small pepper plants and leaves, exposes pepper pods, and increases risk of sunscald. Organic production systems that depend on most, or all, of the fertilizer applied preplant are very vulnerable to early season heavy rains and/or excessive early season irrigations.

Distribution of nitrate and ammonium in the soil profile. Soil samples were taken early May and late June, which typically represented the “shortly after plant establishment” and “mature plants” stages of growth, respectively. Nitrate and ammonium distribution in soil profiles was linked to the amount of preplant fertilizer incorporated into the bed and the irrigation program used. Early in the season, two fields (Watermelon-2-Well and Pepper-O) showed NO3-N concentrations greater than 40 mg/kg in the top foot of the soil profile (Fig. 1). Nitrate concentrations in the other fields were less than 10 mg/kg in the top foot of soil (Fig.2). Ammonium concentration was less than 1 mg/kg in all fields on all dates except for Watermelon-1-10 (2 May), Watermelon-2-Well (2 May), and Calabaza-1 (9 May). In Watermelon-1-10 (2 May) and Watermelon-2-Well (2 May), ammonium concentration ranged between 1 and 4 mg/kg between the 20-40 in depths. In Calabaza-1 (9 May), ammonium concentration increased linearly with depth from 1 to 5 mg/kg. These numbers are slightly above the typical residual ammonium and nitrate concentrations in the sandy soils of North Florida which are typically less than 5 mg/kg.
Growers have mixed feelings when they look at nutrient profiles in their fields. On one side, they are eager to see the relationship between fertilizer application, soil water movement, plant health and leaching risk. On the other hand, soil nutrient concentrations may be highly variable within a field, and their interpretation may be complex. While the scientific merit of such samplings may be limited, it should be continued and refined as it reflects a genuine grower desire in knowing what happens below their fields.

Cooperators feed-back on lessons learned. All cooperators were polled at the end of the season as an attempt to summarize their knowledge gain and identify changes in cultural practices (Table 5). All cooperators reported a gain in knowledge, all recognized that early irrigation management is critical (especially for the organic fields), and all had identified specific changes to their irrigation/fertigation programs. Changes mainly focused on reducing preplant fertilizer rates and early season irrigation rates. The comments provided through the post-project poll were very encouraging to us as they well reflected the educational value of this type of project. Growers engagement in hands-on, in-field education showed more promise than regulation in an attempt to reduce the environmental impact of vegetable production.

Participation Summary

Educational & Outreach Activities

Participation Summary

Education/outreach description:

See tables

Tables and Figures mentioned in this report
are on file in the Southern SARE office.
Contact Sue Blum at 770-229-3350 or
sueblum@southernsare.org for a hard copy.

Project Outcomes

Project outcomes:

On-farm injection of dye using the dye-mobile together with regular visits, petiole sap testing, soil sampling and follow up with cooperators was an efficient educational tool to improve irrigation management of conventional and certified organic vegetable growers. The link between irrigation and fertilization has become even more important for organic production. When nutrients are applied at bed formation, the risk of nitrogen loss is high. A similar risk exists for conventional growers when a large portion (>50%) of the fertilizer is applied preplant. This type of project when conducted over several years is instrumental in growers developing sophisticated irrigation practices that take into account total daily irrigation amounts and amount of water the root zone can hold. A typical schedule may be 2x 30 min /day when the plants are small up to 3 x 2hr per day when plants are fully grown. However, even 2 x 30 min/day may have been enough excessive water to result in nutrient leaching during this period. The second main accomplishment of this project is the open desire and acceptance cooperators have shown in wanting to see and know what the nutrient profile was early and late in the season. Such voluntary approach is much more likely to be successful in the long term in achieving the TMDL goals than regulatory programs.

Farmer Adoption

See tables

Tables and Figures mentioned in this report
are on file in the Southern SARE office.
Contact Sue Blum at 770-229-3350 or
sueblum@southernsare.org for a hard copy.

Recommendations:

Areas needing additional study

This project has highlighted the need to improve irrigation practices not during the crop itself, but during transplant establishment and shortly after when the plants are small. This project also illustrated some of the challenges organic growers face when they need to supply nutrients throughout the crop – not just up front in the bed.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.