Optimization of Irrigation Practices in Organic and Sustainable Vegetable Production with Soluble Dye as an Educational Tool

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.