Foliar applications of nitrogen have a number of advantages, says Dr. Lailiang Cheng, a Cornell University scientist.
Combined with tissue analysis, foliar application lets growers respond in a more precise and immediate way than using soil application. Nitrogen can easily be spoon fed to trees in small doses along with their spray program.
“Early foliar nitrogen spray is beneficial for fruit set and early fruit growth when leaf analysis the previous years shows less than 2.2 percent leaf nitrogen,” he said. “Foliar nitrogen sprays can extend the effective pollination period and promote cell division. The spray concentration has to be low, generally 3 pounds of urea per 100 gallons of water prior to bloom, and this can increase to 5 to 6 pounds of urea per 100 gallons at petal fall and early cover sprays.” Foliar sprays in the spring must be applied at lower rates to avoid damage on the tender foliage early in the season, he said.
Foliar nitrogen can also be applied after harvest to improve the tree’s reserve nitrogen status. While the window of opportunity for postharvest application can be small in the Northeast, another benefit was discovered a decade ago. When a 5 percent urea nitrogen application was made before leaf fall, the number of ascospores of apple scab was reduced by 97 percent.
“This beneficial effect of fall foliar urea application on scab control may provide another incentive for growers to use fall foliar urea application,” Cheng said in a paper he delivered during the International Fruit Tree Association conference last March.
Many growers have the concern that postharvest foliar nitrogen application might decrease tree cold hardiness. However, Cheng found no reduction in winter cold hardiness when foliar nitrogen was applied after harvest. This pool of reserve nitrogen is then used to satisfy the initial tree growth and development in the spring.
In experiments he did in 2002, he found the effectiveness of postharvest foliar urea application on tree nitrogen reserves was dependent on the tree’s background nitrogen status, with trees low in nitrogen being much more responsive than high nitrogen trees.
“It appears that apple trees have a feedback mechanism to regulate nitrogen uptake from foliage,” he said.
Cheng believes that fertigation is also a good method for applying nitrogen in precise amounts—well placed and spoon fed. Some of the newer high-density New York orchards are equipped with fertigation systems, but most apple orchards in New York don’t have fertigation.