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Last spring we took a few leaf samples from apple orchards in which there were pale foliage and generally heavy fruit drop.
The general trend we found were low levels of nitrogen, manganese, zinc and sometimes magnesium.
Because these nutrients were low and limiting growth potential, there tended to be a build up of other nutrients, particularly phosphorus and sometimes potassium.
Where nitrogen was really short, fruitset tended to be poor. Other signs of low nitrogen status which incidentally show up at this time of the year are advanced and high fruit colour, early or premature autumn tints, and early leaf fall. Where deficiency is severe, leaf will be small and pale, extension growth weak and fruit size small. At better but still subnormal nitrogen levels, tree vigour may appear normal or even excessive if pruning was severe, so vigour cannot be considered a good guide to nitrogen status in apples.
Conversely, excess nitrogen will show up as poor fruit colour development, particularly on varieties sensitive to high nitrogen such as Fuji. Leaf colour is deep green and leaf drop delayed.
A lot of New Zealand orchard soils are rather variable, may have been badly eroded, or been smoothed or leveled prior to the orchard being planted. This leads to large differences in fertile top soil depth across the orchard which in turn leads to variation in tree behaviour in a block, so it is possible to have both nitrogen excess and deficiency occurring within the same orchard block. Trees growing where the topsoil is deep will be vigourous and show all the symptoms of excess nitrogen while those trees where topsoil is thin show low vigour, have high fruit colour and often suffer with fruit size problems.
In these blocks soil testing and leaf analysis needs to be targeted and remedial fertiliser treatments also targeted to zones showing similar behaviour.
The low nitrogen levels this spring may have been due to the previous year's higher than normal rainfall leaching more nitrogen out of the soil than normal. Some people are claiming the cold spring interfered with uptake. For permanent crops, most of the spring growth flush requirement for nitrogen comes from the mobilization of stored reserves, so I do not think a strong argument in favour of poor spring uptake for these crops can be made. A more likely reason for low levels is a lack of uptake the previous autumn.
With this year's drier summer there should be better soil nitrogen reserves due to lower leaching losses; even so it would appear prudent to apply post harvest nitrogen dressings to orchards which look as if they require it. If summer leaf nitrogen levels are below 2.4%, it is probable that there will be response to autumn nitrogen fertiliser applications.
Most information on nitrogen requirements that has been published for apples suggests that where a response has been measured to nitrogen the requirement tends to be in the range of 50 to 100 kg N/ha/year.
Half to a third of this amount should be applied immediately post harvest with the remainder in the spring. It is a good strategy to apply the post harvest dressing on a regular basis then make the spring dressings discretionary based on the behaviour of the trees. Where vigour is low, several spring dressings may be needed.
Soil tests give good information on pH, phosphorus, potassium, calcium and magnesium. Dressings of these nutrients and lime should be based on soil analysis.
On established orchards, levels of phosphorus and potassium tend to be good, or even on the high side, so it is questionable whether or not regular applications of these nutrients is necessary. Even so, there are examples where application of small amounts of readily available phosphorus, potassium and magnesium may lead to better crop performance, so unless soil test levels are particularly high, it may be prudent to regularly apply these nutrients.
The DSIR Appleby fertiliser trials, for instance, gave their best responses when annual dressings of phosphorus and potassium as well as nitrogen were made, even though the greatest response to a single nutrient was from nitrogen. In these trials cropping was also much less variable from year to year when a full fertiliser programme was applied.
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