For high yields of fruit of good quality, you need to fill the surface soil with feeder roots of your trees.
To do this, the surface soil needs to be deepened, and mulched so that it is cool in summer, stable, soft, permeable, and stores enough water and nutrients for the roots, yet is well-drained so that oxygen can move to the roots, and toxic gases are removed.
The red duplex soils in Northern Victoria consist of shallow loams (100-150 mm deep) that overlie dense, hard, poorly drained clays.
When you hill-up the soil before you plant trees, and use weeds to add organic matter to the soil, you deepen the surface soil available for feeder roots.
However, unless you continue to manage your surface soil carefully, you will eventually be back where you started. The surface soil will slump when quickly wetted by rain or irrigation, and will set hard and dense when dry.
Such hard dense soils are hostile to feeder roots. Water from rain and irrigation will run off the bare surface into the inter-row, and be lost as evaporation and surface drainage, taking soil with it. That is, hilled-up soil under the trees will be eroded.
Winter weeds as mulch
In previous articles we suggested that you use winter weeds, especially grasses, to mulch the soil in the tree-line in summer (see Tree Fruit May, June, July, November 2014).
The mulch protects the soil from heavy rain, softens and cools the surface soil, adds organic matter, and improves the soil structure in the tree-line.
In this third article, we are suggesting how you can keep the mulched surface soil soft, permeable and well-aerated, and continue to encourage feeder roots to fill the surface soil.
Don’t add more organic matter than is needed
Your mulch of dead winter weeds should not add more organic matter than is needed to stabilise the soil, otherwise you will end up where you started— with hard soil and few feeder roots.
Under thick mulch, the soil remains wet for long periods after you have killed the weeds in spring. The soil remains stable, permeable and well-aerated, with plenty of large pores, but the wet soil coalesces. That is, stable aggregates merge so that the soil hardens and restricts feeder roots.
Add gypsum
To avoid this, add 2–3 t/ha gypsum every 2–3 years along with the weed mulch.
Gypsum is calcium sulphate, and works in two ways to prevent coalescence: Some calcium cations in the gypsum replace harmful exchangeable sodium and magnesium cations that are attached to the clay; and some of the calcium stays as soluble cations in the soil solution. This enables the clay particles (less than 2 micron in diameter) to coagulate, i.e. join together into small aggregates, but the soil will not coalesce.
Gypsum is moderately soluble, so eventually moves down the profile out of the root-zone and so needs to be replaced every few years.
Another big advantage of gypsum is that it strengthens the bonds between the organic molecules and the clay, increasing the stabilising effect of the organic matter.
Depth of soil affects tree performance
Your aim with high-density plantings on trellises is to start producing high yields per hectare when the trees are young.
Your trees’ feeder roots are dense and confined to small volumes of soil near the drippers or mini-sprinklers.
It is crucial that enough soil is soft, stable, and well-drained to enable, not only rain or irrigation water, but also oxygen, to reach all the feeder roots, and for toxic gases to be removed.
Ultimately, the depth of soil suitable for roots greatly affects the performance of fruit trees.
Complete soil management strategy
Together, our three articles discuss a strategy for soil management for orchards that:
removes limitations of hard-setting duplex soils to feeder roots
increases coagulation of clay particles, and stabilises aggregates to form deeper soft soil
does not waste water, so increases water-use efficiency
keeps the soil well-drained and well-aerated.
For more information, see Tree Fruit Dec 2014
