Weather data in the 2004-2005 growing season from the
FAWN station in Immokalee
showed monthly minimum-maximum temperature and rainfall in September,
2004 to May 2005 (table 1). During that period, minimum daily
temperatures below 38° F occurred on 13 Dec. (37.8° F), 15 Dec. (35.1° F),
20 Dec. (35.4° F), 24 Jan. (32.3° F), 12 Feb. (31.4° F), and 13-16 Feb.
Rainfall events greater than 0.80 inch occurred on 25 Feb. (1.1 inch),
27 Feb. (0.97 inch), 3 Mar. (0.85 inch), 9 Mar. (2.12 inch), 17 Mar.
(2.46 inch), and 2 Apr. (0.85 inch). None of those rainfall events met
either of the leaching rain criteria. Because most trials were located
south of the FAWN station located in Immokalee and up to 70 miles away
from it, these weather data should be used only to describe the general
weather conditions in the area and not to precisely assess the local
conditions in each trial. Overall throughout Southwest Florida, the
2004-2005 seasons was cool and dry.
The water table depth for the seepage-irrigated trials
(1,
2,
3,
6 and
7) ranged from approximately 10 to 25 inches in
trial 1. Some fluctuations in the ground water table were observed due
to changes in irrigation volume or rainfall. Wells in trial 2, 6, and 7
showed the least fluctuations in the water table depths. Previous
research trials using seepage irrigation have shown that tomato yields
were not reduced when the water table depth was maintained near the
20-inch depth. While keeping a lower water table resulted in reduced
water use in that trial, water table depth fluctuations are likely to
occur in large fields as observed in trial 1. Trial four was the only
site where the water table reached to the surface between the row
middles. Such fluctuations can cause N flushing from the bed. However,
the risk of nutrient leaching was reduced in trial four with the use of
drip irrigation for fertigation. The fluctuations in water table among
the drip-irrigated experiments (trials
4 and
5) were highest in trial
four where the water table depth varied from 32 to 11 inches. The low
water table depth (10 inches) between 24 Dec. and 7 Jan. was attributed
to a 0.6-inch rainfall event on 25 Dec. (trial 4). Such occasional low
water table conditions are mostly unavoidable and are to be expected in
Southwest Florida. The risk of nutrient leaching caused by temporarily
high water tables due to rainfall or frost protection may only be
reduced by using drip irrigation tubing (mixed system) or a
controlled-release fertilizer. The amount of fertilizer leached by
changes in water table depth was not quantified in this study.
The differences in plant dry weight 30 and 60 DAP for
all trials (1,
2,
3,
4,
5,
6 and
7) and seasons were not significant,
except in trial 4 on 60 DAP. In trial four on 60 DAP; the higher N rate
produced significantly higher tomato plant dry weight (187 g/plant) than
the 250 lb N /acre rate (114 g/plant). Overall, N rates had little
effect on tomato biomass 30 and 60 DAP. It was also observed that
differences in shoot weight when plants are small may be due to
differences in pruning. Although plant size is not necessarily a good
predictor of marketable yield, large and actively growing tomato plants
are often associated with increased earliness and increased nutrient
reserves. Vegetable growers often report an association between
increasing number of yellowing or senescent older leaves with increasing
incidence of diseases on the lower part of the plants. Data that support
this observation are currently not available.
Petiole sap NO3-N and K concentrations tended to be
above the UF-IFAS sufficiency threshold in all trials (1, 2, 3, 4, 5, 6
and 7), N treatments and throughout the season. Changes in petiole sap
NO3-N concentrations were different with seepage and drip irrigation
(Fig. 2a, b). The different phases of petiole sap concentration changes
in seepage irrigated trials showed (1) a period of low petiole response
to fertilize rate (corresponding to the time needed for root
establishment and the solubilization of nutrients in the hot band), (2)
a period of increase well above the sufficiency range, followed by (3) a
period of decrease. Dry weather conditions in 2004-2005 did not result
in petiole sap nutrient concentration during period (3) to fall below
the sufficiency range. Although tomato sap NO3-N concentrations were
higher in the higher N rates than in lower N rates, the N nutrition of
plants receiving either N rate would be “sufficient”. These plant-based
data suggest that tomato plants maintained adequate levels of N (and K)
even at the low N rates when cool and dry weather conditions prevailed.
In
trial one, the symptoms of Fusarium crown rot first
appeared on 12 Jan. 05. The number of plants showing symptoms increased
through 2 Feb 05. The plants in the plots with the lowest N rate of 200
lb N/acre had the highest amount of disease incidence with an average of
53% symptomatic plants. The other three treatments receiving 236, 260 or
260+ biosolids lb N/acre, had 10%, 27%, and 20% average disease
incidence, respectively. These observations support growers’
observations and suggest that plant nutritional status may influence the
susceptibility of tomato to diseases such as Fusarium crown rot. These
results support the need to include the incidence of diseases in the
selection of practical fertilizer rates. However, N rate may need to be
associated with factors in determining the incidence of Fusarium crown
rot symptom because such an association was not observed in all the
trials.
More adult-whitefly days were observed on plants
receiving the highest N rate as compared to the lowest N rate (trials
1,
2,
3, and
4). The trend was consistent among all four individual
farms (replicates) and statistically significant over all farms F =
30.6, df = 1, 19, P < 0.01. Nitrogen in the form of amino acids is the
limiting resource for sternrrhynchous homoptera including whiteflies.
Amino acids are concentrated by these phloem feeders through excretion
of water and sugars as honeydew. Whitefly adults are known to prefer
leaves and plants with higher N concentrations that correspond to higher
amino acid titers in the phloem. Positive response of adult-whitefly day
to N fertilization has also been observed on cotton (Gossypium hirsutum)
in the field.
Significant yield differences in the first harvest were
found in two (1 and
4) out of seven trials (2, 3, 5, 6 and 7) (P<0.05),
whereas differences in total yield were significant only in one trial.
In one of these trials using seepage irrigation, increasing N rate from
200 to 236 lb N/acre resulted in a significant first-harvest yield
increase of 149 boxes/acre (P<0.05) in the tomato size category of 5x6.
Total marketable yield increased by 115 boxes/acre (P>0.90) in this
trial. In the second trial using a hybrid seepage-drip irrigation
system, increasing N rate from 250 to 418 lb N/acre resulted in a
significant first-harvest yield increase of 297 boxes/acre in the size
category 5x6. Total yield was also significantly increased by 552
boxes/acre. These results illustrate that fertilizer efficiency may
increased in dry years and tomato yields may not always respond to high
N rates in such years.
