A research report by Judy Tisdall, soil scientist at La Trobe University, Melbourne, and Bas van den Ende, a tree fruit consultant in Australia’s Goulburn Valley.
Establishing a high-density orchard is costly. It’s important to do it the right way, because you only have one chance. Once the orchard is established, it is difficult and costly to correct soil problems in later years, yet properties in the soil affect the growth of roots.
To produce high yields of good-quality fruit, trees need lots of feeder roots in the surface soil so they can take up plenty of water and nutrients. To enable this, the surface soil should be deep, soft, stable, well-structured, well-drained, fertile, and cool in summer. The pH level should be between 5.8 and 6.5.
So, you need to improve the fertility and structure of the surface soil and increase the depth of surface soil if it is shallow in your orchard.
Here are six steps to preparing the soil:
1) Have your soils tested
Whether it is new land or old orchard land, have the surface soil tested to see if you need to add lime, gypsum, and/or phosphorus, and to what depths.
Methods for collecting, preparing and submitting soil samples vary in different regions, states and countries. These methods are described on various websites, so follow the methods appropriate for you.
As you sample the soil, you will also see how deep the surface soil is and whether there are any hard layers that restrict water, air, and roots from penetrating deeper layers.
Lime will be needed if the soil pH is acidic (5.7 or less). Gypsum will be needed if the soil is hard due to dispersion. Phosphorus will be needed unless superphosphate has previously been applied each year to the soil and a soil test shows that there is an adequate amount of soluble phosphorus available to the young trees.
2) Grade your block (if necessary)
Before applying the lime, gypsum and/or phosphorus (if soil test results indicate these are needed), laser-grade the block to make sure that it has a slight slope (e.g. 1 in 80) so that excess water will drain from the surface of the soil. This will help avoid waterlogging of the surface soil.
3) Apply amendments and rip and cultivate the soil
Some surface soils are naturally hard and dense, while others have a plow sole or shallow hard pan due to excessive cultivation and traffic. All these hard layers need to be broken up. After spreading lime, gypsum, and/or phosphorus (if soil tests indicated these were needed), rip the soil both ways to a depth of about 300 to 400mm. Be careful not to mix heavy subsoil with the surface soil.
After ripping the soil, slightly cultivate the moist, but well-drained, surface soil to form small clods. Do not pulverize the soil, which would happen if the soil was cultivated when too dry. This is also the time to put in the mains and sub-mains of a new irrigation system.
Lime and phosphorus are not very soluble and move very slowly in soil, so they need to be cultivated into the surface soil. Apply agricultural limestone (calcium carbonate) over the whole block, but apply superphosphate along the future planting lines about 1 to 2 meters (3 to 6 feet) wide and rototill it in. Phosphorus is important for root growth, and young trees will benefit from phosphorus if it is nearby, i.e. if mixed into the surface soil.
Gypsum is moderately soluble, so, if applied to the soil surface, it might eventually be washed down the profile to the subsoil.
Why do some soils need lime?
The feeder roots in the surface soil need soft, stable, well-drained soil, with a pH of between 5.8 and 6.5. In acidic soils (with a pH below 5.8) excess aluminum and possibly manganese become available and are directly toxic to roots. The roots become stunted and unable to take up sufficient water and nutrients.
Other nutrients such as calcium and magnesium may be present in acidic soil but become unavailable to roots. Also phosphorus and sulfur may be present in acidic soil, but combine with aluminium to form aluminium phosphate and aluminium sulphate compounds, which cannot be taken up by roots.
Why do some soils need gypsum?
Gypsum (calcium sulfate) is sometimes needed to soften surface soils and to improve their structure.
With gypsum, the soluble calcium swaps with some of the exchangeable cations, such as sodium and magnesium. Gypsum does this better than lime does, because gypsum is more soluble than lime. Gypsum does not affect soil pH, but lime does.
Cations (positive ions) such as sodium and calcium exist in soil as either exchangeable cations (loosely bound to clay particles) or soluble cations (dissolved in soil water). The soluble cations often swap with exchangeable cations in soil. When exchangeable sodium makes up more than 5 percent of the total exchangeable cations, and there are low concentrations of soluble cations, the soil is sodic and unstable.
Sodic surface soils are very dense and hard, so it is very difficult for feeder roots to grow through them.
When sodic soils are wetted, the clay particles push each other apart. First the aggregates swell and decrease the size of the pores. On further swelling, small groups of clay particles separate from the larger aggregates and become suspended in the water until the clay particles block the small pores. This is called soil dispersion.
4) Hill up the surface soil
Most feeder roots grow in the surface soil, so when the surface soil is shallow, these roots are severely restricted. Few roots grow in the compacted surface soil in the traffic lanes between rows of trees. If the land is also flat, the soil can easily be waterlogged in wet conditions.
To solve these problems, use a road grader to take the wasted surface soil from the traffic lanes, and hill-up the surface soil before you plant the trees. This increases the volume of surface soil for the feeder roots to explore, and the sloping beds also allow excess rain water to run off. Surface drainage is as important as irrigation in wet climates.
5) Sow ryegrass onto the beds or let voluntary weeds develop
This step must be carried out in early autumn to ensure that the ryegrass or weeds become established before the winter sets in. Use irrigation water to germinate and establish ryegrass or weeds.
6) Spray out ryegrass or weeds before you plant trees
Ryegrass or weeds are needed to keep the soil covered to avoid impact from heavy rain, avoid impermeable crusts from forming, and to stabilize the soil. Kill the ryegrass or weeds in spring, because they compete with trees for water and nutrients during the growing season. The dead roots of ryegrass or weeds have done their job in improving soil structure.