Orchard Foliar Fertilization

Given the wide variety of rootstocks, cultivars, orchard systems and soil types, it is not easy to make generalized fertilizer recommendations for apples.  In established orchards, the best strategy is a July foliar sample to see how your orchard lines up with the optimum levels of nutrients shown in Table 1.  In-season correction of deficiencies can be successful with the following nutrients. 

Nitrogen

Perhaps the most difficult nutrient to optimize is nitrogen.  It often comes down to a balancing act between yield and size (more nitrogen) and quality (less nitrogen).  Too much nitrogen leaves you at risk for poor colour development, fruit drop, disorders like corkspot and bitter pit, and more storage losses.  ‘Honeycrisp’ appears to be particularly sensitive to high nitrogen.  Most nitrogen goes down in early spring.  A leaf analysis in early July will give you a chance to correct any deficiency in-season.  The quickest way of getting N into the tree is with a foliar application of urea or fertigation.  Do not apply nitrogen beyond the end of July to reduce the chance of quality problems.  The leaf analysis will also help you pin down a nitrogen rate for next spring.

If you are using fertigation through drip-lines, bear in mind that in crops where it has been studied, a fertigation N rate of somewhere around half the granular N rate gives a similar response.  This difference does vary quite a bit depending on the crop and site studied and there have been few direct comparisons done in apples.  Cutting your N rate in half if it is supplied by fertigation is a good starting point.  Also, research is indicating that getting fertigation nitrogen on earlier and tapering it off by the end of June is a good scheduling strategy for maintaining quality.

Potassium

On sites with low soil-test K, addition of potassium

has been shown to improve fruit colour development, size titratable acidity and taste.  Excessive N worsens any K deficiency.  Little response has been seen where soil tested medium for potassium.  Since magnesium and potassium compete for uptake, too much K will induce a deficiency in Mg.  Foliar applications of potassium nitrate or potassium sulfate can be used to correct in-season deficiency, but soil application is the best long-term solution.

Calcium

Calcium deficiency is implicated in bitter pit, but the relationship is not clear or well understood.  Nonetheless, foliar applications are prudent when bitter pit is of concern.  Bear in mind that calcium sprays to ‘McIntosh’ and ‘Honeycrisp’ have been demonstrated to advance maturity in Ontario.  Watch closely for early ripening, especially when using higher rates later in the season.  A good discussion of rates and timing can be found in the OMAFRA FactSheet “Bitter Pit Control in Apples”.

Magnesium

Magnesium deficiency becomes more common when higher rates of potassium are applied.  Mg deficiency is known to induce premature fruit-drop in ‘McIntosh’.  Foliar sprays can correct a deficiency in-season, whereas soil application is the better long-term solution.

Boron, Iron, and Manganese

These micronutrients should be applied to address deficiencies uncovered by visual symptoms or leaf analysis.  Deficiency tends to occur more on higher pH soils.  Foliar applications are best used to correct deficiencies because the soil pH that causes the problem renders soil applications unavailable for uptake.  Boron deficiency is likely the most common in Ontario and is more likely in dry years without irrigation.  Judicious use of micronutrients is important because excess boron or manganese can result in reductions in yield and/or quality.

Notes:

• Optimum N ranges will be approximately 0.2% higher if the planting is not yet bearing
• Increase optimum N ranges by 0.2% for all apple cultivars on size controlling rootstocks.

Related Link

• OMAFRA Factsheet "Bitter Pit Control in Apples"