Post-harvest orchard managementResearch & Extension
After this year’s harvest it’s time to get tree nutrition, pruning and branch training right to ensure next season’s crop is a good one.
The period between harvest and leaf fall is very important to the tree for replenishing its nutrient reserves, as well as building carbohydrate reserves for the next growing season. This is also the time of the major root flush, which plays an important role in mining the soil for nutrients.
Because roots only grow in moist soil, adequate soil moisture is essential over the post-harvest period. Where soil is dry or autumn rain fall is inadequate several irrigations are necessary to insure satisfactory root growth.
With many of our modern varieties, eg Fuji, Cripps Pink and lower colour Gala strains, it is necessary to run the trees low in nitrogen towards harvest to enhance colour development so that there is adequate fruit colour for harvest before fruit maturity parameters exceed the optimum levels for picking and storage. This means that many orchard blocks have low nitrogen status and will come into next season with inadequate nitrogen levels to produce high quality spur and bourse shoot leaf and insufficient nitrogen for fruit set.
There is good evidence to show that while running low nitrogen levels is good for fruit colour, these low nitrogen fertiliser programmes increase biennial bearing and often result in poor fruit set as well. The trick is to run low nitrogen status going into harvest then, as soon as harvest is complete for each variety, apply a robust nitrogen fertiliser programme to build up the reserves for the next spring flush and fruit set requirements.
The post-harvest fertiliser programme needs to be predominantly foliar based because foliar uptake is much more efficient than soil so less is required and there is less chance of lifting soil nitrogen levels into the range where higher levels may adversely affect fruit colour. Several foliar urea sprays at 2-3% (2-3kg/100L) 10 to 14 days apart is a good starting point.
Where soil nitrogen reserves are known to be low there is need for some soil-applied nitrogen fertiliser as well. Signs of low nitrogen status include the following:
- 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.
Over the years we have done quite a lot of leaf sampling comparing areas of good fruit set with poor fruit set, collecting leaves two to four weeks after bloom. Normal leaf nitrogen levels at this timing need to be around 2.7% to 3% nitrogen. Where we have seen poor fruit set, leaf nitrogen levels have been less than 2.5% for samples taken at this stage of growth.
Incidentally, the low nitrogen signs listed above can also be associated with poor root health such as soil water logging or water stress – both of which interfere with nutrient uptake.
Post-harvest is also a good time to check for drainage and water stress problems that need to be fixed before the next growing season. It is also optimum timing for deep ripping because dry soils shatter more readily than damp soils.
Early harvested varieties such as the Gala group often have excess tree vigour. Where there are strongly growing shading branches in the upper tree taking these branches out immediately after harvest will improve light penetration into the lower canopy. This will strengthen bud development down there. Furthermore, removal of these vigorous branches at this time reduces their export of photosynthates to other parts of the trees, particularly the roots and this will have a calming effect on tree vigour.
Where there are canopy light penetration problems these are much more easily identified while the trees are in full foliate than in the dormant season.
In developing orchards, the late summer to autumn period prior to leaf fall is quite a good time for branch training. Although branch extension growth should have finished this is the main period for branch thickening. Branches trained at this time will set in their new positions quite quickly and fruit set in the following season will be considerably improved.
APAL’s Future Orchards® project is funded by Horticulture Innovation Australia Limited (HIA)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
John Wilton was a horticultural consultant with AgFirst, New Zealand.