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Fluid Journal : Fluid Journal 1999-2001
1 Fluid Journal Summer 2001 Summary: Urea applied as a foliar is a very efficient source of N for citrus growth. Leaf age is of little conse- quence to good uptake. The biuret lev- els in urea should be less than 0.8 per- cent and, if applications are repeated, biuret contamination should be less than 0.4 percent. Potassium uptake is less efficient than urea, but in young leaves uptake was still good, particu- larly in K deficient plants. Phosphorus uptake was much less compared to K or urea uptake, but a P spray raised leaf P the same percentage as an equal spray of K raised leaf K. Little uptake of P and K by leaves with high levels of these elements occurred. Further work on the effect of different rates of NPK, timing of these sprays, and spraying without P is planned for the coming years. Field experience still indicates that sprays of N and K at bloom and post-bloom enhance fruit set and yields. Earlier work in California (Ali and Lovatt, 1988) and in Florida (Albrigo, 1999) found that winter urea sprays en- hanced flowering and fruit yields. Bloom and post-bloom sprays of N as urea with P and K sources increased fruit set in Florida (Albrigo, 1997). These results led to several questions about best sources of these nutrients for citrus foliar applications. Year one During the first year of this project supported by the FFF, emphasis was placed on citrus leaf reaction to biuret levels as urea was applied to old and young citrus leaves. Biuret levels of 0.2, 0.5, 0.8, and 1.05 percent were used with applications of 10, 20, or 30 lbs/A of N as urea. At least 0.5 percent biuret was necessary to cause leaf tip yellow- ing and only the two higher rates/A of N with 5 percent biuret resulted in an aver- age 5 percent of leaf area with a light yellowing. At 0.8 percent biuret, the same level of injury occurred at the 10- lb/A N rate in half the treatments and at the 30-lb/A rate about 10 percent of the leaf surface showed a moderate yellow- ing in half the treatment leaves. With the 1.05 percent level of biuret, all three N levels developed some tip yellowing (biuret symptoms). The leaf area af- fected was still in the 5 to 10 percent range but the yellowing was more pro- nounced as the biuret level increased. The leaf yellowing appears to persist indefinitely without recovery of chloro- phyll. A second goal was to evaluate sources of P for foliar uptake. Three products---ammonium polyphosphate (APP), mono-potassium phosphate (MKP), and a PK humate (PKH)---were evaluated at two rates, with and without urea as a booster. Applications on limb units were equivalent to 5 or 10 lbs/A of P, with or without urea at 5 lbs/A of N. Drs. L.G. Albrigo and J.P. Syversten What about Foliar NPK On Citrus? Florida researchers are finding that these sprays at bloom and post-bloom enhance fruit set and yields. APP increased P levels about 10 percent in young leaves three days after applica- tions with the 10-lb/A rate. If urea was added, the increase was the same at ei- ther the 5- or 10-lb/A rate. Levels of P in young leaves were not different from the controls six days after treatment, sug- gesting that redistribution to other plant parts may have taken place. Leaf K level increases for mature leaves were inconsistent after applica- tion of K-containing products. Some treatments caused increases of up to 17 percent. These tests suggested that better methodology was needed to de- tect any foliar uptake of P and K. Major problems appeared to be movement of nutrients away from sprayed limb units, and leaf to leaf variation in original nutri- ent level from one shoot to another. These problems led to 1) reduced values for detecting increases the longer time went by before subsequent sampling and 2) variability because the same shoots and leaf position were not sampled each time. Year two In year two, these problems were par- tially controlled by spraying whole trees to minimize nutrient loss from treated leaves to nearby areas of lower nutrient levels, by using adjacent leaves for se- quential sampling, and by doing follow- up sampling within 3 to 6 days of appli- cation, before applied nutrients could be translocated from treated leaves. Using these techniques and repeating the APP
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