Stored Nitrogen Key to Early Growth in Apple

The question of timing of nitrogen application in apples is one that appears at first to be very simply answered. As a grower, you go out sometime around bud break when the brush is mulched up from dormant pruning and apply so many kilos of a granular fertilizer per hectare. This is based on cultivar, tree performance, age and yield goals combined with an intimate knowledge of the orchard (through tissue analysis) and its soils. This approach may not be the only way to successful nitrogen use in mature orchards.

Suffice it to say that we know a lot more about timing and usage of nitrogen today than we did when the first high-density plantings were established in Ontario 2 or 3 decades ago. We now know that for the first 3 weeks or so after bud break, the tree depends on stored nitrogen picked up in the previous year and to a lesser extent on its stored carbohydrates. Much of this stored nitrogen is held in the tree spurs.

As a nutrient, nitrogen is one of the keys to successful growth and development of canopy and crop. Overdoing nitrogen application carries as potentially serious a consequence in apples as starving the trees from nitrogen.

Tree demand for nitrogen is highest in the early part of the season. Too much nitrogen and crop quality can suffer. Trees can easily outgrow their assigned space, pruning costs can get out of hand and insect and disease problems can be exacerbated.

Available nitrogen is also critical to the pollination - fruit set period of development in the crop. Pollen by itself is considered to be a nitrogen sink. This may sound a little strange, however, pollen is about 40% pure protein and nitrogen is a key component of the protein molecule. A large high density planting of apple can easily produce several tons of pollen in a single year. Honeybees carry off a large portion of that protein rich pollen to feed colony brood.

Just where that first flow of nitrogen comes from in the 3-4 weeks after bud break has been a subject of debate for some time. If we start with nursery stock planted out in the spring of the year, successful growth of that stock is partially reliant on what the nursery tree was able to capture and store in the nursery before being lifted.

Nursery trees should have a higher nitrogen status than trees in a bearing orchard. This is where the skill of the nurseryman comes in. Trees with good nitrogen reserves perform better in the spring of the year than those with reduced reserves. Dr. Lailiang Cheng of Cornell University in New York State has carried out much of the pioneering work done on reserve nitrogen in apple.

If apple trees have been adequately well fed up to the point of leaf shatter in the fall, much of the nitrogen in the leaves will migrate back into the tree just before the leaves abscise. This is why it helps to keep foliage in good shape through the growing season. Premature leaf shatter or poor canopy health gives the tree little chance to retain or store nitrogen for next year's early season growth. Early spring regrowth is mainly dependent on stored or reserve nitrogen in the tree. Nitrogen applied in the spring of the year does not influence new shoot and leaf growth until about 20 days after bud break.

Many apple growers in North Eastern North America have adopted supplemental foliar feeding programs that appear to be working well. These foliar sprays can contain various sources of nitrogen in combination with other macros like potassium and trace elements.

Foliar feeding mature trees can be remarkably adequate in any one year for the current season crop and for building some of the reserve nitrogen when used in the later part of the growing season. This is especially so where trees are not suffering from lack of water, have relatively good nutrient status and where the canopy and tree structure are well managed. Growers relying heavily on foliar programs should however watch for signs of nutrient shortage, and monitor soils and tissues on a fairly regular basis.