Weather conditions and supplemental fertilizer applications.
Overall, South Florida was hot and dry throughout the fall, and cool and dry during the winter and spring of 2006-2007. Rainfall recorded by growers during the 2005-2006 season showed accumulations of 5, 0.5 to 13 and 10 inches for fall, winter and spring, respectively (Table 3). The IFAS tomato fertilizer recommendation allows supplemental N and K fertilizer applications in specific situations (Maynard et al., 2003), as does the BMP manual (Simonne and Hochmuth, 2003). Under this recommendation, 30 lb/acre of N can be added for each leaching rain event. Therefore, using fall/winter/spring 2006-07 as an example, a supplemental application of 30 lbs/acre of N fertilizer was permissible in two trials (7 and 8) in Palm Beach and four trials (10,11, 12 and 13) in Manatee due to three leaching rains. No fertilizer addition due to leaching rain was justified in the rest of the trials, so N fertilizer application consisted of the base 200 lbs/acre rate only (Olson et al., 2005). These results suggest that analysis and prediction of leaching rain frequency and timing would be valuable for Florida’s vegetable growing areas.
Irrigation management.
The BMP trial acreage was irrigated 80% by seepage and 20% by drip systems. The water table in the seepage-irrigated trials fluctuated between about 16 to 20 inches deep and tensiometer readings were between 4 and 8 kPa. In the drip-irrigated fields, water was applied daily at a volume estimated from the Weather Service Class A Pan evaporation combined with a crop coefficient. In trial 6, there was a higher late blight incidence in the IFAS than grower’s rate, probably due to a higher water tables or higher soil moisture and not to N rate.
Plant nutritional status.
Petiole sap NO3-N concentrations were above the UF-IFAS sufficiency threshold throughout the season in all thirteen locations and under all N treatments, except for the lower N rates in the multiple N rate trials (Figure 1). After seven, nine, ten, eleven, thirteen, fifteen and sixteen week after transplants (WAT) the 20, 60, 120, 180, 240, 300, 360 and 420 lb/acre treatment were lower than sufficiency range to the end of the season. In general, the higher N rates produced tomato sap NO3-N concentrations that were greater compared to the lower rates. Petiole sap K concentrations tended to be above the UF-IFAS sufficiency threshold during the all season (Figure 2). The sap data suggest a weak N and K relationship in a dry year.
Yield response to N rates.
In this dry season, IFAS and higher N rate produced significant higher yield in first harvest of extra-large tomatoes (80% of the total harvest) and total yields in 1 and 2 out of 13 trials [ Figure 3 (P<0.05)] respectively. In general, during the season when soluble fertilizer was used there were between 90 to 300 boxes/acre more in total yields with higher N rates, although the differences were not significant [Figure 3 (P<0.05)]. At the highest prices during the season of $23/box growers revenues will be $2,070 for 90 boxes/acre and $6,900 for 300 boxes/acre to off-set $20 to $45 in cost of extra fertilizer. Regression analysis of first and total harvest extra-large yields and total yields indicated a quadratic response to the multiple N rates in trial 10 (Figure 3). The trend indicated an increase in total yield and first harvest extra-large and total extra-large fruit from 20 to 240 lb/acre N, but a plateau with higher rates of N. There was no response to N treatment by other tomato size categories at first, second and third harvest or all harvest combined. These results show that it may be possible to reduce N rates especially when the risk of rainfall is low (winter, spring and dry year), or when only two harvests are expected (late spring).
Grower participation in the project.
We would like to thanks the growers participating in the project for their in-kind contribution and valuable inputs. The BMP trials are a popular on-arm research project were growers and IFAS cooperators work as a team. Together the cooperating farms represented 16,000 acres (80%) of staked tomato production in southern and eastern Florida and 310 acres under BMP experiments.
