Nutrient Management Research Database

Urea applied as a foliar is a very efficient source of N for citrus growth.  Leaf age is of little consequence to good uptake.  The biuret levels 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, particularly 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.

This study investigated the use of foliar sprays in citrus trees.

The role of biruet concentration in urea and the effectiveness of 3 sources of P foliar spray.

Urea was applied at rates of 10, 20, and 30 lbs N/acre, each of these rates was tested with biuret levels of 0.2, 0.5, 0.8, and 1.05 percent.

P was applied as either ammonium polyphosphate (APP), mono-potassium phosphate (MKP),and PK humate (PKH). P was applied at rates of 5 and 10 lbs P/A with or without 5 lbs/A of N as urea.

At least 0.5% biuret was needed at the 20 and 30 lbs N/ac rates before leaf tip yellowing was observed. At 10 lbs N/ac, yellowing occurred only above the 0.8% biuret level. At the 1.05% rate, all urea rates showed yellowing.

Year 1

APP increased P levels about 10% in young leaves 3 days after applying at the 10 lbs P/ac rate. When added along with urea this leaf P increase was observed at both P rates. After six days, leaf P levels returned to baseline, suggesting that P is relocated within the tree.

Year 2

In year 2, sprays were applied to the whole tree instead of just the leaves. In this year, APP and MKP showed similar levels of P and K uptake between 3-6 days after application. Young leaf uptake values increased 60-100% when trees were nutrient deficient. However, old leaves did not show signs of P uptake, but did show signs of K uptake. For trees that were not nutrient deficient, these uptake rates were reduced.