Even after years of scientific research, getting the correct level of nitrogen into apple trees remains in many ways an art.

Lailiang Cheng, a researcher at Cornell University since 2000, has dedicated his career to helping growers make the right choices. He does the scientific studies, and he also helps growers interpret what their trees are telling them, because orchards are all different in their inherent ability to generate nitrogen. In general, for orchards in New York State, where soils are mostly fertile, fine-textured, and well watered, growers may need to add no nitrogen fertilizer and still have the 50 to 80 pounds per acre that a modern high-density orchard will need each year for fruit, shoot, and leaf production.

Late summer is a good time to evaluate. Not only can growers then see what happened in the current season, but they can also do leaf tissue analysis to determine the nitrogen status of their trees. Cheng says that leaf tissue analysis is really a good tool that helps growers diagnose tree nitrogen status.

“Because each orchard soil is unique and all the fertilizer field trials are site specific, the best way to fine-tune the amount of nitrogen fertilizer you should apply is to have your own nitrogen rate trial on your farm based on leaf analysis and tree indicators,” Cheng said.

Leaf analysis

“Determining a tree’s nitrogen status is important for making decisions about whether and how much nitrogen fertilizer should be applied,” Cheng said. “Leaf analysis is highly recommended for this purpose.  If leaf samples are taken correctly and the results are interpreted properly, it provides a good tool for developing an effective ­fertilization program.”

Optimal ranges of leaf nitrogen status have been worked out, and Cheng recommends the following:

  • Young nonbearing apples should have leaf nitrogen levels of 2.4 to 2.6 percent.
  • Young bearing trees need levels of 2.2 to 2.4 percent.
  • Mature soft apple varieties need levels of 1.8 to 2.2 percent. (Soft varieties include Cortland, Empress, Golden Delicious, Honeycrisp, Jerseymac, Jonagold, Jonamac, Jonathan, Macoun, McIntosh, Mutsu, Paulared, Spartan, Tydeman Red, and other early ripening varieties.)
  • Mature hard apple varieties and processing apples need 2.0 to 2.4 percent nitrogen in the leaf tissue. (Hard varieties include Red Delicious, Empire, Gala, Idared, ­Liberty, Melrose, Rhode Island Greening, Rome, Stayman, and York Imperial.)
  • As a rule of thumb, he said, every 10 percent increase in nitrogen fertilizer application results in a 0.1 percent increase in leaf nitrogen. “If nitrogen is provided via fertigation, less nitrogen is needed, as nitrogen uptake efficiency is higher in fertigation than in regular soil application,” he said.

What nitrogen does

Nitrogen that supports tree growth and development comes from three sources. “The first source is the reserve nitrogen that has accumulated in the tree from the previous growing season,” Cheng said. “This pool of nitrogen is readily available for the initial growth during spring.”

The second source is soil organic matter—and the key problem is that it is released by soil microbial activity, which increases as soil warms. This release pattern is not related to tree need.

The third source, nitrogen fertilizer, is what growers apply to achieve the proper level.

“In commercial orchards, some soils with high organic matter provide a substantial amount of nitrogen during the summer, so heavy nitrogen fertilization late in the spring combined with natural release of nitrogen from the soil during the summer can elevate tree nitrogen ­status to excess levels, leading to vigorous vegetative growth, poor fruit color development, and storage quality problems,” Cheng said.

“At the other extreme, lack of nitrogen supply on soils with low organic matter can result in poor fruit set, small fruit size, low yield, and alternate bearing. Because the effect of nitrogen on fruit set and size is just opposite to that of fruit color, flesh firmness, and storage quality, orchard nitrogen management has to be optimized to balance these opposite effects with the ultimate goal of producing a high yield of quality fruit.”

Ideal

An ideal pattern, he said, is for trees to have a relatively high nitrogen status early in the season to promote rapid leaf area development and early fruit growth. Then, nitrogen status should decline gradually to guarantee fruit quality development and wood maturity.

“This provides a basic framework for guiding nitrogen management in apple orchards,” he said. “Nitrogen management is all about matching tree nitrogen demand with the three supply sources in an environmentally sound way.”

The annual nitrogen requirement is estimated to be about 50 to 80 pounds for mature apple trees on dwarfing rootstocks in high-density plantings, Cheng said.

On high organic matter soils (3 percent), soil mineralization will release 50 to 70 pounds, he said, but perhaps only 60 percent will be captured by the tree.

There are two windows for regular soil nitrogen application that would fit the tree nitrogen demand pattern: one is from bud break to the beginning of rapid shoot growth and the other is late in the season when soil nitrogen application no longer affects fruit quality (just before or shortly after fruit harvest).

Nitrogen applied early in the season contributes directly to the spur and shoot leaf development and fruit growth in the current season, while nitrogen applied late in the fall helps to build up nitrogen reserves, which is used to support leaf development and fruit growth the following year.