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This year we saw an abnormally wet summer in Hawke's Bay and Gisborne. Nelson also had a wet spring, while Marlborough had a lot more rain than usual over the summer period.
Here in Hawke's Bay, AgFirst carried out soil moisture monitoring over the summer on a number of orchards. The technique we used was "Diviner" which enabled soil moisture to be tracked with soil depth, so it was possible to see how moisture levels were behaving at various depths through the soil profile. This proved to be very interesting, giving information on how moisture was travelling through the soil following rain or irrigation.
Because of the wet season, we did not have to recommend much irrigation until very late in the season, however by monitoring over the whole season we were able to identify periods of waterlogging which we believe had a negative impact on fruit sizing.
In one Braeburn orchard, we identified 46 days of waterlogging over the December/January period. As we were also measuring fruit size increment in the same block, we were able to detect a reduced rate of fruit sizing compared to other Braeburn orchards following this wet period.
On the same orchard, Fuji was also being monitored. Here there was only 23 days of waterlogging due to the soil being freer draining. With this reduced length of waterlogging compared to the Braeburn block, we did not see any marked reduction in fruit sizing rate for the Fuji.
Incidentally, on other orchards with Pacific Rose, we saw more severe necrotic leaf spot symptoms where there was indications of summer waterlogging than on deeper higher quality soils.
As a result of these observations, I believe we can make one or two generalizations about orchard performance this last season which may point to problems with either drainage or irrigation.
First Scenario
Orchard performed better than in a normal season, particularly in regard to growth in young trees and fruit size relative to crop load.
Here we can conclude that in a normal dryer growing season, water stress has been limiting orchard performance.
In this situation, the irrigation system may need upgrading, or if the orchard does not have irrigation it be installed provided there is a water supply to be tapped.
If it is not possible to irrigate, then soil management practices such as mulching to conserve soil moisture be investigated.
Second Scenario
Orchard under-performed relative to a normal season.
This is an indication of a problem with waterlogging in the root zone.
Here, further investigation of the soil profile and drainage is required.
This is best accomplished by getting out with a spade or post hole border and digging holes to determine root distribution, presence of signs of perched water tables such as zones of gleyed soil.
Once you have examined the soil profile, it is possible to decide on the best way to fix the problem.
Where shelterbelts are near drains, it is possible that their roots may have entered the drain and blocked it, so it no longer works. Casuarina, willows and poplars are particularly prone to entering drains and blocking them with their roots. Check that the drains are working.
Depending on the characteristics of the soil, the drainage may be improved by the following means:
- Extra tile drains - expensive, but highly effective.
- Pulling mole drains - very suitable for soils with high clay content.
- Pan busting - useful in stable soils with zones of impervious soil and free draining soil below.
Pan busting is best done when the soil is dry so that there is a good soil shattering effect. Also, with pan busting, the position of the tip of the pick relative to the soil pan is critical. The tip of the pick needs to be just under the pan so that the pan is forced to ride up over it. Set it too high and it will just ride over the top of the pan and do nothing.
Mole drains are best pulled in the spring/early summer as the soil is drying out. The soil is in optimum condition for moling when the surface is easily glazed by the mole plug to give a shiny surface to the inside of the mole drain. Moles pulled at this time will cure over the summer and become baked hard, so that when they start taking significant quantities of water, the surface is hard and resistant to erosion. Where moles are pulled across tile drains, the fill over the tile at that point needs to be very permeable to allow the water in the mole drain to seep readily into the tile below. In orchard crops, a mole plug pulled between each row is usually sufficient.
Planting trees on ridges is a good way to improve drainage in new orchards. Ridging of established orchards to improve surface drainage is generally not recommended because of scion rooting. I have also seen increased incidence of phytophthora root rots following ridging of established MM106 rootstock orchards.
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