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Post-harvest nutrient management

Research & Extension

As we head toward the end of harvest, it’s time to think about your fertiliser plan to boost your trees’ nutrition reserves in preparation for next season.

Post-harvest nutrition is a key event in the orchardist’s calendar. Once the fruit is off the tree it’s time to replenish the tree’s reserves for next season.

The window between fruit removal and leaf fall is narrow but it is a critical period to get right. It’s a golden opportunity to top-up vital nutrients to maximum effect by working with what the tree is naturally doing; translocating nutrients and carbohydrates from the leaves back into the buds and woody tissue in preparation for winter dormancy.

Early season growth potential is determined by the nutritional concentration and carbohydrate reserves that have been built up in the buds at leaf fall. So once each variety is harvested you need to implement your post-harvest nutrient plan.

Timing

The best time to apply post-harvest nutrients is as soon as practically possible after each variety has been harvested.

To maximise nutrient uptake the leaves need to be in good condition and fully functioning. Don’t wait until the last variety has been harvested before implementing your program since leaf condition on the early maturing varieties have already deteriorated significantly.

Root activity is also largest immediately post-harvest so maximise this by methodically applying post-harvest fertiliser to each variety as soon as possible following the last pick.

Which nutrients are most important post-harvest?

We advocate a nice simple message with regard to nutrition and that is:

“You can’t manage what you don’t measure.”

Make sure you’re taking sufficient soil, leaf and fruit tests to know what nutrients you need to address. The most likely ones you can do something about post-harvest are nitrogen, boron, zinc and magnesium.

Nitrogen (N)

Ensuring the floral buds have adequate nitrogen is vital to their viability, which is their ability to set fruit. It also lowers biennial bearing tendencies. Autumn is the best time to apply nitrogen to ensure adequate nitrogen is supplied to the floral bud. It has minimal effect on spring shoot growth but has the highest impact on final fruit set compared to applying nitrogen in spring or summer.

Foliar nitrogen applications are the most effective way to directly raise nitrogen concentrations in the floral bud. Nitrogen is needed in large quantities, so a couple of foliar sprays are normally necessary. Urea applied post-harvest at 2-3 per cent solution (2-3kg/100L) 10-14 days apart is a good start.

On soils where soil nitrogen reserves are known to be low, a soil-applied solid nitrogen will also be needed. Try to target 40 per cent of the total trees’ nitrogen requirements post-harvest.

Leaf samples taken in February showing nitrogen levels less than 2.5 per cent, indicate nitrogen applied in autumn would be beneficial.

Signs of low nitrogen status include:

  • Poor fruit set
  • Small fruit size relative to crop load • Enhanced fruit colour development
  • Small, pale green foliage
  • Early onset of autumn foliage colour development and premature leaf drop.

Boron (B)

Boron is most effectively applied immediately post-harvest. Boron is needed for pollen tube formation and therefore fruit set. Good boron levels mean more seeds are set; the higher the seed numbers, the higher the calcium levels in the fruit.

Zinc (Zn)

Zinc is an often misdiagnosed, under-rated or ignored trace element, yet a zinc-deficient orchard will be responsible for significant production losses if not addressed. Zinc is important for the formation and activity of chlorophyll and in the functioning of several enzymes and the growth hormone auxin.

Low levels of auxin, as a result of zinc deficiency, cause severe stunting of leaves and shoots, commonly known as ‘little leaf syndrome’. It can also be responsible for blind buds. Symptoms can be erratic whereby some branches show symptoms and others don’t.

Unlike boron, zinc does not have any direct effect on pollination and fruit set, however, with low levels of zinc, bourse leaves never become fully expanded resulting in less carbohydrates for blooms and developing fruit. Research has shown that a positive relationship exists between bourse plus spur leaf area and calcium accumulation in the fruit; the larger the bourse plus spur leaf area, the higher the fruit calcium levels.

Soil applications are ineffectual because zinc reacts with the organic component and other minerals in the soil and is quickly rendered unavailable to the tree. Therefore foliar sprays must be used. However, zinc sprays can russet the fruit and therefore great care is required if treating trees that still have fruit on them.

Post-harvest sprays are therefore the best opportunity to get zinc into the tree, allowing this trace element to be translocated back into the tree and buds as the leaf senesces. Mid-dormancy winter sprays of higher rates of zinc sulphate can also be used but these must be applied well before any bud movement to avoid bud damage. Apply zinc before any oils go on.

Please note, that a zinc based fungicide program does not address an underlying zinc deficiency. The fungicide formulation means the zinc molecules are too large and won’t get absorbed into the leaf. When the leaf test shows excessively high zinc levels it’s usually sample contamination from the zinc based fungicide. Make sure you request a light acid wash when sending leaf samples to the lab which should remove any contaminants from the outside of the leaf.

Magnesium (Mg)

Magnesium is the central molecule of chlorophyll, the green colour of the leaf. Without it, photosynthesis cannot occur. Other equally important but less known roles of magnesium include: energy transfer; cell wall formation; protein, starch and oil synthesis; and it assists with nutrient uptake from the soil to the roots, especially phosphorus. Magnesium is very mobile in the plant and so a deficiency is first seen in the older leaves.

While a post-harvest magnesium foliar spray is an opportunity to top up the buds, magnesium is needed in much larger quantities than trace elements like zinc and boron. Therefore, there must be follow up applications of both soil and foliar applications the following season to address an inherently magnesium-deficient orchard over the long-term.

Summary

Post-harvest to leaf drop period is a relatively narrow but important window to apply nutrients. The key nutrients to focus on are nitrogen, boron, zinc and magnesium. Foliar sprays are very effective post-harvest as nutrients applied to the leaf will be translocated back into the tree and to the buds as the leaves naturally senesce.

Post-harvest nutrition is an important strategy for orchardists to sustain high yields of quality fruit. It can lower biennial bearing risk, maximize early season growth potential, improve fruit set and is the best time to apply some key nutrients. Don’t miss this opportunity!

Acknowledgement

About the author:

APAL’s Future Orchards® program is funded by Horticulture Innovation Australia Ltd using the apple and pear industry levy funds from growers and funds from the Australian Government. AgFirst is a key Future Orchards partner

About the author

Dean Rainham, Horticultural Consultant, AgFirst e: dean.rainham@agfirst.co.nz

 

Tagged:
Future Orchards nutrition and irrigation

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