Drought strategies and fertilisers in Orchards

Henry Schneider, Fruitcheque Officer, DPI: Cobram

Nitrogen fertiliser still needs to be applied to fruit trees after harvest – especially taking into account the difficult season the trees have been through since the spring of 2006. Nitrogen after harvest helps with flowering and fruit set after winter dormancy and fruit size development in the first 4 weeks after flowering. Some post-harvest irrigation water will still be needed to apply nitrogen in one or two applications once the crop is off. Where possible, decide on the best time to stretch the final irrigations and apply the fertiliser then, rather than applying an irrigation immediately after harvest. An example here would be to apply the post harvest fertilisers about a week to 10 days after the harvest of a variety such as Tatura 204 peach or Williams pears - because the weather is still hot and the trees will need water as well as the nitrogen fertiliser. Later maturing varieties may need the nitrogen to be applied after harvest, but with only enough water to either just water the nitrogen into the topsoil, or put it through the irrigation lines and then flush the lines clean. This may take only about an hour or so as there is no need to actually irrigate as the trees have been well irrigated for the last 8 weeks or so to size the fruit.

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Early leaf fall can be expected on most orchards, but tress will flower and set fruit given some post harvest irrigation and nitrogen fertiliser

There is also a need for the trees to have enough time to use the fertiliser and store it in the buds, main branches and root system before leaf fall.

If it is not possible to irrigate to put the nitrogen into the topsoil or to use fertigation, foliar applications of low biuret (foliar grade) urea should be applied as soon as possible after harvest. It is important to apply the foliar sprays while there is still some moisture for the leaves to remain active and use the urea.

It is important to remember that the trees have enough time to use the fertiliser and store it in the buds, main branches and root system before leaf fall. Especially this year as leaf fall could be quite early this season given the low rainfall and restricted water availability.

Priorities for Irrigation:
During this very difficult season it is worth keeping in mind that there is still a need to keep fruit tree leaves green and active as long as possible during late summer and early autumn.

With stone fruit, if there is a severe limitation on water, one last irrigation in the first week of March will see them through the season – even if leaf fall occurs within 4 weeks or so after that (early April).

Apples will generally need irrigation until then anyway (except Gala) and the final irrigation leading up to harvest can have nitrogen applied either via ground application and watered in, or preferably applied via fertigation.

The main difficulty is with Williams pears – they will need irrigations as long as possible so that a cool spell followed by rain does not induce them into an autumn flowering.

Phosphorus and Potassium application this season:
Phosphorus and potassium are both very necessary elements fruit trees need for productive growth. If needed, these can be applied at any time during autumn or early spring and do not need immediate watering into the topsoil as they are not volatile and can wait for rain.

However, in the Goulburn-Murray Valleys, around Shepparton and Cobram, potassium is usually adequately supplied from all but the sandiest soils in these districts. Potassium in leaf tests of stone fruit orchards in these areas have in fact been higher than needed in the past. In these situations, extra potassium in an NPK fertiliser mixture may even be detrimental to fruit quality - as well as being a costly expense to the total fertiliser bill. The excess potassium certainly does not improve the fruit colour – as many orchardists have already found out at some expense! If potassium is actually needed, as indicated by a leaf test, it is best spread in the tree line in early spring (September).

Phosphorus fertiliser is also another cost that may not be needed each season. The applications of phosphorus on many orchards has been high. If the soil pH is in the adequate range of 6.0 to 7.0, then leaf tests have often indicated adequate phosphorus for many years.

However when leaf phosphorus levels do slide into the marginal range, the most efficient way to apply it is in a narrow ‘band’ down each side of the tree. This is not usually done when an NPK mixture is spread over much of the orchard with a fertiliser spreader. Phosphorus fertiliser should be spread on its own in a band along the tree line at any stage during the season. This is usually more convenient in spring or after harvest.

In Summary:

• Keep trees actively growing as late into autumn as available irrigations will allow – especially Williams pears.
• Post harvest nitrogen is needed every season in stone and pome fruit.
• Phosphorus and potassium fertilisers are not applied in the same way as nitrogen. They should be applied on the basis of need from a leaf test and in different ways for best uptake by the trees.