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Fluid Journal : Fluid Journal 1999-2001
2 Fluid Journal Summer 2001 and MKP applications, we determined that both sources were equal for P and K uptake three to six days after applica- tion. Young leaf nutrient uptake values increased 60 to 100 percent when trees were in the deficient nutrient range (0.6 percent P and 0.59 percent K). Old leaves did not take up P well even if de- ficient (11 percent increase), but K up- take was still high (135 to 254 percent increase). In less deficient leaves (P and K, 0.12 and 1.20 percent, respectively), uptake was more modest with P increas- ing 15 to 22 percent in young leaves and 10 to 13 percent in older leaves (more than five months old). K increases were only 5 to 10 percent when leaf K values were 1.2 percent before spraying. Up- take of P was only one-eighth to one- tenth that of K uptake or proportional to the internal leaf levels of the elements (.14 percent P versus 1.4 percent K rec- ommended for citrus). But this smaller uptake raised P in the leaves as much, proportionately, as did the K uptake. Uptake needs to be evaluated for pos- sible beneficial effect on plant yield and other responses. Year three The third year studies were commercial field tests to see if various NPK sprays, when applied by commercial sprayer, resulted in increased leaf N, P, and K levels and increased yields if applied at bloom and post-bloom. Moreover, the relative effects of APP versus MKP were evaluated further. Urea biuret lev- els of 0.4 and 0.8 percent were compared to see if severe toxicity symptoms oc- curred when the 0.8 percent biuret level was used twice per season. A mature Valencia block was selected in Central Florida on a deep sandy soil, and plots were single rows (0.9 acres) replicated four times. Urea was applied at 14 lbs/A of N with either 0.4 or 0.8 percent biuret. P was supplied as either APPorMKPat7lbsP2O5/A. Kat7lbs K2O/A was either in the nitrate form ap- plied with APP or part of the MKP form. Sprays were applied at 5 percent open flowers and again five weeks later. Leaf samples for mineral analysis were taken from 20 tagged shoots at three locations before spraying in each plot, and five days after application. The center leaf of each of the 20 shoots was selected before spraying, and an adjacent leaf to this position was selected five days later. This sampling procedure was used each spray time. An additional sample was take 180 days after the last spray. Results Leaf values of N, P, and K were not different between treatment plots at the beginning of the experiment. Five days after the first treatment, leaf N was higher in the treatments receiving APP and K nitrate, along with either the 0.4 or 0.8 percent biuret urea (Table 1). No difference was detected in P or K levels after this first spray (Tables 2 and 3, re- spectively). Five weeks later, before the second spray, the two treatments using APP were different in N (0.8 percent bi- uret urea + APP was highest and 0.4 percent + APP was lowest). The leaf K level of trees receiving the APP and urea with 0.8 percent biuret was higher at this time. Five days after the second spray, all treatments had higher N levels than the non-sprayed, but P and K levels were not different between treatments. No significant changes in P occurred after either spray, but the natural decline Table 1. Leaf nitrogen levels at various times of sampling before or after sprays of 14-7-7 lbs/A on Valencia oranges. % nitrogen Treatment Pre- Spray+ Pre- Spray2 Spray2+ spray1 5 days spray2 +5 days 180 days Control 3.43 3.32 3.19 3.23 2.71 Urea 0.4% with APP 3.59 3.57 2.97 3.39 2.83 Urea 0.8% with APP 3.49 3.55 3.42 3.40 2.91 Urea 0.4% with MKP 3.51 3.50 3.17 3.45 2.75 Urea 0.8% with MKP 3.42 3.45 3.26 3.43 2.69 Table 2. Leaf phosphorus levels at various times of sampling before or after sprays of 14-7-7 lbs/A on Valencia oranges. ----------------% phosphorus----------------------- Treatment Pre- Spray+ Pre- Spray2 Spray2+ spray1 5 days spray2 +5 days 180 days Control 0.26 0.23 0.43 0.20 0.17 Urea 0.4% with APP 0.28 0.25 0.19 0.20 0.17 Urea 0.8% with APP 0.28 0.24 0.21 0.20 0.18 Urea 0.4% with MKP 0.27 0.24 0.18 0.20 0.17 Urea 0.8% with MKP 0.26 0.24 0.19 0.20 0.17
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