Issue link: https://read.dmtmag.com/i/418083
68 DECEMBER 2014 GOOD FRUIT GROWER www.goodfruit.com P recise management of nutrients is an important part of precision horticulture, Dr. Gerry Neilsen, soil scientist with Agriculture and Agri-Food Canada in Summerland, British Columbia, Canada, tells growers. "We've really got to start understanding the resources and sinks at a much more sophisticated level than we did before," he said. He spoke during the International Fruit Tree Association's annual meeting. "We've got to match inputs to demand—there are a lot of environmental and economic advantages to that." Advantages include less risk of leaching nutrients in the ground as well as less wasted fertilizer from applying more than the tree needs or applying it at the wrong time. Nitrogen was one of the first nutrients discovered to be important for tree growth. When growers applied it, there was an obvious growth response from the tree. It's taken scientists much longer to understand the role of other nutrients in tree fruit production, Neilsen said, par- ticularly those that are less mobile and are needed only in smaller quantities. Phosphorus It used to be thought that deficiencies of phosphorus were rare, but when Neilsen and his colleagues applied nitrogen and monoammonium phosphate as a replant treatment for apples, the trees grew better than those that received nitrogen alone. "We were quite shocked by that," Neilsen said. Following the lead of a Washington State grower, he also tested phos- phorus fertigation at bloom in an established orchard with five cultivars and found it increased the cumulative yield. Evidently, the phosphorus went into the fruit, as it also had some quality effects on some varieties. For example, fruit from Silken trees that received high levels of phos- phorus didn't brown as easily when cut, possibly because it improved the stability of cell membranes, and fruit of Fuji and Silken had less watercore. In cherry trials, phosphorus boosted yields in the first three years, improved fruit size, and increased the stem pull force, but it reduced soluble solids and seemed to delay maturity. It had a similar effect as a gibberellic acid treatment apart from the fruit not being as well colored, Neilsen said. Like GA-treated fruit, these cherries could be harvested later when color was darker and the fruit larger. Potassium It was once thought that Pacific Northwest soils were rich in this ele- ment because it was not being washed through the soil by rainfall, as occurs in the wetter eastern growing regions. However, it now appears that levels can be lower than suspected, perhaps because of irrigation. Symptoms of a potassium deficiency include scorched edges of leaves, particularly in foliage near fruit, which itself is a strong sink for potassium. This can reduce photosynthesis, resulting in poor tree growth. In a trial with Fuji, Neilsen found it also resulted in poorly colored fruit. Neilsen did a fertigation trial on a very sandy soil in which he made daily applications of soluble forms of potassium at various rates during the six weeks when the fruit was sizing (mid-July to mid- August). He found that no matter which form of soluble potassium he used, leaf levels increased. Since then, in blocks where potassium was applied, he has run into a problem of declining magnesium because the two elements are antagonistic, and in a current cherry trial he's thinking of switching to a potassium-magnesium (K-mag) treatment. The ratio of potassium to calcium is also important, and one of the concerns about applying potassium is that it could negatively affect fruit qual- ity. However, Neilsen said where soils are deficient in potassium, applications do not increase susceptibility of the fruit to bitter pit. In another trial with Lapins cherries on Gisela 5, potassium was applied either through a drip system or microsprinklers. The trees irri- gated with microsprinklers had a wider water distribution and higher potassium levels, which indicates that if the root volume is reduced too much in a sandy soil, it can limit potassium uptake, and that follows through to the fruit, Neilsen said. Calcium Calcium is the nutrient that most affects the postharvest quality of apples. Bitter pit is just one of many calcium- related disorders. Studies show that fruit calcium concentrations start out relatively high in the growing season, then drop, reaching their lowest level at harvest. However, fruit concentrations in the same orchard can fluctuate from one year to another, partly because the calcium concentration is related to fruit size. Larger fruit have lower concentrations. Fruit with a high calcium concentration store well. Fruit with a high ratio of potassium to calcium does not, which is a reason not to fertigate with potassium unnecessarily. Critical calcium levels vary by variety. For example, at six weeks before harvest, in British Columbia, the recommended fruit calcium level for McIntosh is 8.5 milligrams per 100 grams. For Golden Delicious, it's 9.5 milligrams. At harvest, the recommended level for all varieties is 4 milligrams per 100 grams. The standard recommendation for apples in British Columbia is to apply calcium spray four to five times at ten-day intervals starting mid- July at a rate of 8 to 12 kilos per hectare (7 to 11 pounds per acre). The idea was to target the fruit when it's larger and has more surface area to Manage nutrients PRECISELY There are both environmental and economic advantages to matching fertilizer applications to the trees' needs. by Geraldine Warner "We've really got to start understanding the resources and sinks at a much more sophisticated level than we did before." —Dr. Gerry Neilsen