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"Size Year" Result Tree How to Fertilize
In a "big year" for apple trees, the tree uses up a significant amount of nutrients, which leads to reduced accumulation of nutrients in the later stages of growth. This also alters the nutrient composition: soluble sugar levels tend to be higher, while starch content is lower. Additionally, protein levels, along with the proportions of starch and total nitrogen, are decreased, and both organic phosphorus and available nitrogen are lower. In contrast, inorganic phosphorus levels are higher. These imbalances result in delayed flower bud differentiation, a slower rate of differentiation, fewer flower buds, and overall poor quality.
When fertilizing a "big year" tree, three key considerations should be taken into account. First, it's essential to apply sufficient fertilizer based on the expected yield to prevent the tree from becoming nutritionally deficient, which could negatively impact its growth and fruit development. Second, because a large number of flowers are produced in a "big year," early spring fertilization before bud break and flowering should be adjusted depending on the amount of basal fertilizer applied in autumn and the tree’s nutrient reserves. If the basal fertilizer was adequate and the tree has high nutrient storage, there's no need to apply additional fertilizer before sprouting or blooming, as this could lead to excessive fruit set and increase the pressure of thinning, worsening the "big year" effect. However, if the basal fertilizer was insufficient or the tree's nutrient reserves are low, a small amount of readily available nitrogen can be applied before bud break to support flowering.
Third, it's important to take advantage of the timing before flowering by applying a topdressing of available nitrogen to encourage better flower bud differentiation, increase the number of flower buds, and ultimately boost the production for the following year, which is typically a "small year."
On the other hand, during a "small year," when the fruit load is lighter, the tree grows more vigorously, leading to greater nutrient accumulation in the middle and late growing seasons. Flower buds begin to differentiate earlier, at a faster rate, and in larger numbers. Therefore, applying nitrogen fertilizer early, before bud break and flowering, helps improve fruit set and increase the yield for that year. However, it's generally not advisable to apply fast-acting nitrogen fertilizer before flower bud differentiation, as this could lead to an overproduction of flower buds, further intensifying the cycle between "big years" and "small years."