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Fluid Journal : Fluid Journal 1999-2001
3 Fluid Journal Summer 2001 in leaf P was numerically less than the control after the second spray. K levels were increased after the first spray while the control decreased over the five-day period. None of these changes was sig- nificant from one another. The K level of leaves of the treatment receiving MKP with 0.4 percent biuret urea was significantly higher after the second spray, but the K level of the 0.8 percent biuret urea with APP treatment leaves was significantly lower. By fall, the two sprays using urea with 0.8 percent biuret resulted in tip leaf yel- lowing toxicity symptoms in about 5 percent of the leaves. This injury level Table 4. Difference in leaf nutrient values five days after spraying an NPK combination and the values just before spraying. Two spray cycles are represented (a bloom and a post-bloom spray). ---Leaf N %--- ---Leaf P %--- ---Leaf K %---- Treatment Spray1 Spray2 Spray1 Spray2 Spray1 Spray2 +5 days +5 days +5 days +5 days +5 days +5 days pre- pre- pre- pre- pre- pre- spray spray spray spray spray spray Control -0.15 +0.04 -0.03 -0.23 -0.16 -0.04 Urea 0.4% with APP -0.02 +0.42 -0.03 +0.01 +0.20 +0.01 Urea 0.8% with APP +0.05 -0.02 -0.04 -0.01 +0.19 -0.23 Urea 0.4% with MKP -0.01 +0.28 -0.03 +0.01 +0.20 +0.10 Urea 0.8% with MKP -0.01 +0.17 -0.03 +0.01 +0.18 -0.06 Table 3. Leaf potassium levels at various times of sampling before or after sprays of 14-7-7 lbs/A on Valencia oranges. ------------ ------% potassium------------------------ Treatment Pre- Spray+ Pre- Spray2+ Spray2+ spray1 5 days spray2 5 days 180 days Control 2.13 2.33 2.02 1.98 1.39 Urea 0.4% with APP 2.21 2.42 2.01 2.02 1.43 Urea 0.8% with APP 2.17 2.35 2.33 2.09 1.46 Urea 0.4% with MKP 2.22 2.41 2.00 2.09 1.36 Urea 0.8% with MKP 2.04 2.30 1.93 1.89 1.36 should not alter tree growth or yields, but possible effects will be evaluated in subsequent years. Another approach to evaluation of the data is to look at the difference between the leaf nutrient values just before spraying and the values five days after spraying (Table 4). After the first spray at bloom time, there was no significant rise of leaf N, but the natural decline with leaf age was numerically reduced. There was a significant increase in leaf N after the second spray for one of the 0.4 percent biuret urea sources. Three of the four treatments with urea were positive numerically. These data do not indicate a strong response of either source of P and K on increasing leaf nutrient levels when leaf nutrient levels were already high. This is contrary to earlier work, but these trees had luxuriant levels of N, P, and K before treatments started (.26 to .28 P versus 0.6 to 0.9 percent P in the previ- ous year's studies, and 2.04 to 2.22 K versus .59 to 1.2 percent K in the previ- ous year's studies). These third-year data further support the earlier observa- tions that nutrient uptake from foliar applications is less if the pretreatment nutrient levels are higher. A spring foliar N, P, K spray appears to be a good, quick method to get these nutrients into the leaf tissues, particu- larly if leaf values are below optimum levels. Growth and yield responses from foliar nutrient sprays in the spring have been demonstrated, but if tissue values are above optimum, there may be little uptake and benefits may be small. Fur- ther work on this aspect is still required. Dr. Albrigo is horticulturist and Dr. Syversten is plant physiologist at the Citrus Research & Education Center, University of Florida.
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