We know that foliar absorption of turfgrass occurs through channels in the cuticle, but researchers have recently used isotopes to more accurately measure the rate of nitrogen absorption.

Although all nutrients are capable of being absorbed by the foliage, plants absorb nitrogen the most easily. Research has also shown that some chelates are inappropriate for foliar absorption; however, organic chelates such as citrate and glucoheptanate enhance foliar absorption of micronutrients.

Urea nitrogen is the most suitable source of N for foliar absorption due to its neutral charge, low salt index, and high solubility. It does have some potential for volatilization, however. Ammonium sulfate is also absorbed well, but it has the potential for phytotoxicity. Potassium nitrate, on the other hand, is not a good source for foliar absorption.

Studies report a range of 30-60% for foliar absorption of urea nitrogen, with results in the 50-55% range for both bentgrass and bermudagrass. 50% of absorption occurs within the first hour and is enhanced by higher humidity and the use of a surfactant to help coat the leaf tissue.  Foliar absorption of nitrogen will continue for up to 24 hours after application. 

For best results, applications should be made in temperatures between 65 and 85 degrees in the early morning or at night. Nitrogen absorption declines a little in the summer for cool season turf.  

Water volume has minimal effect on foliar absorption. Increasing the dilution rate from 20 gals/A to 80 gals/A results in only slightly less foliar absorption. Liquid nitrogen that runs off is still readily available to the plant through other entry points.

Harrell’s MAX and Bio-MAX products have been ideally formulated for maximum foliar and liquid feeding:

  • All of the N products (except 6-0-0) are urea based
  • All of the urea N products have half of the nitrogen stabilized with UMAXX to prevent volatilization and to provide a more consistent feed over the application interval
  • All of the micronutrients are chelated with organic chelates

Five tips to get the most out of your foliar program:

  1. Use stabilized urea sources
  2. Use products with organic chelates
  3. Use an adjuvant like Harrell’s Activator
  4. Use flat fan nozzles to increase leaf surface coverage
  5. Since foliar absorption doesn’t decline that much with higher volumes of water, go ahead and use up to 2 gals/1,000 sf dilution to facilitate time-saving tank-mixes

References:

  • Branham, USGA Turfgrass Environmental Research Online, Vol. 9, #19, Oct 1, 2010
  • Richardson et al., University of AR, Dept of Horticulture, Research Series 568, 2008