Several points need consideration in deciding how to lime an orchard before planting.
1 Acid soils in an orchard have to be avoided because of chemical changes that affect nutrient uptake by tree roots.
Trouble in the root zone starts when soil acidity, measured as soil pH in water or calcium chloride solution, drops below a value of 5.8 or 5.0 respectively.
The situation will get progressively worse as pH falls further. Low soil pH is a widespread problem in orchards.
2 Spreading lime once the trees have been planted does very little good.
The lime cannot penetrate quickly enough below more than 2 to 3 cm, and will not make much difference in the root zone where it is really needed.
Goulburn Valley orchards are particularly difficult in that way. Past research projects at the Tatura Research Institute have tried several alternatives to agricultural lime, but none of them significantly improved the result.
Any new method that may be developed also has to make economic sense.
3 Soil acidity accumulates all the time as a result of microbes working on urea and ammonium based fertiliser and organic matter.
Rapid acidification, say a drop in pH of 1.5 in less than 10 years, is mostly caused by applying nitrogen fertiliser the wrong way, and by excessive or wrongly timed irrigation.
Research projects at Tatura developed new management methods that minimise acidification and save nitrogen fertiliser. However, while that is very good it is unlikely that a grower can prevent a slow drop in soil pH under commercial conditions.
A minimal target would be to keep the pH (in water solution) of the root zone above 5.7 over 20 years after planting.
4 The rate at which soils acidify depends not only on nitrogen fertiliser practices, but also on the soil type, its lime content and how large the root zone is.
Potentially, microjet irrigated orchards are a little more at risk than sprinklers, but acidification happens in all irrigation systems.
A higher lime content provides a bigger safety margin.
Practical solution
There is only one compelling and practical answer to this problem and that is: (cont next issue)
See this article in Tree Fruit June 2022
