A huge change in tree fruit production and irrigation systems over the last 20 to 30 years means growers need to change the way they manage nutrition.
Cherry growers are beginning to adopt dwarfing rootstocks and plant trees closer together, as apple growers did earlier. When applying water and nutrients, growers need to keep in mind what the root systems are like, Dr. Denise Neilsen said during the International Fruit Tree Association’s annual conference in British Columbia, Canada.
Rootstock vigor affects root growth, which can limit uptake of immobile nutrients, said Neilsen, a researcher with Agriculture and Agri-Food Canada in Summerland, British Columbia. Roots of Malling 9, for example, tend to be concentrated around the emitter, whereas trees on larger rootstocks explore a much greater area of the ground.
“When we change production systems, root systems really change,” she said.
Tree fruits in general have very efficient, but small root systems. Apple trees have the smallest root length density of all tree fruits, with cherries not far behind, Neilsen said. For example, deciduous fruit trees have only 1 to 10 centimeters of roots per square centimeter of soil surface compared with grasses that have 1,000 centimeters or more.
Nitrogen is very mobile in soil and water, and can be difficult to control and to keep in the root zone. Neilsen and her husband and colleague, Gerry Neilsen, have done many trials using fertigation to apply nutrients.
“This allows us to control the application of nutrients into the root zone,” Neilsen said. “It’s very precise with respect to how much we apply and when we apply it. Water management is key to nitrogen management in these irrigated systems.”
Whereas broadcast nitrogen applications boost the soil level for a short period, fertigation allows growers to maintain the nitrogen level at whatever concentration they might want through the growing season with frequent, small applications.
So, when should nitrogen be applied?
During the growing season, the tree stores nitrogen in its leaves. In the fall, nitrogen is withdrawn from the leaves and is stored in woody tissue. The following spring, the stored nitrogen fuels new growth, and then the cycle repeats.
Neilsen said an experiment with Lapins cherry trees on Gisela rootstocks showed that yields decreased when leaves were stripped from the tree in the fall, confirming that fruit bud development and early fruit development depend mostly on stored nitrogen and carbon. However, studies have also shown that fall applications of nitrogen are only worthwhile if the tree has insufficient nitrogen. Beyond a certain point, no more nitrogen will be stored.
Shoot growth, on the other hand, is more dependent on the current season’s nitrogen supply. Studies have shown that trees don’t take up much nitrogen from the soil until about 30 days after bud burst, so spring applications of fertilizer are unnecessary. Uptake of nitrogen by roots into shoots and leaves begins just after bloom in apple, and fruit uptake doesn’t begin in earnest until the end of cell division, Neilsen said. Nitrogen applications later in the season appear to increase soluble solids and acids and promote maturity.
Neilsen said the amount of nitrogen removed by the crop gives an indication of the replacement amount that might be needed. A crop of cherries removes anywhere from 11 to 45 kilograms per hectare (10 to 40 pounds per acre), depending on the variety, rootstock, and size of the tree. The higher the yield, the more nitrogen is required.
However, it’s possible to have too much of a good thing. The Neilsens conducted a five-year experiment in which cherry trees were fertilized daily at a season rate of 63, 126, or 254 kilograms per hectare (56, 112, or 226 pounds per acre). The higher rates suppressed growth, reduced fruit yield in some seasons, and were detrimental to fruit size.
Both apples and cherries need a relatively small amount of nitrogen, but apples are not quite as sensitive as cherries to changes in nitrogen rates.
In a five-year trial when rates of between 33 and 198 kilograms per hectare (56, 112, and 226 pounds per acre) were applied, the higher rates resulted in only a small increase in yield, along with slightly less color and less firmness (which could have been due to advanced maturity).
Good nitrogen management depends on increasing nitrogen availability in the root zone by good water management and timing applications to match demand, Neilsen concluded. •