To produce early and high yields of good quality fruit, 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. Here are steps to achieve that.
Preparing the soil before planting (cont. from last month)
Establishing a high-density orchard is costly. It is important to do it the right way, because you get only one chance.
Once the orchard is established, it is difficult and costly to correct soil problems in later years.
To produce early and high yields of good quality fruit, 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. Here are steps to achieve that.
The following steps will help you to plan any new planting of fruit trees.
1. Have your soils tested.
Whether it is new 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 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 fruit trees.
2 Rip the surface soil lightly to break up any hard parts and grade your block (if necessary).
Before applying lime, gypsum and/or phosphorus (if soil test results indicate these are needed), laser-grade the block to make sure that it has a slope of at least 1 : 80 along the traffic lanes so that excess water will drain from the surface of the soil.
This will help avoid waterlogging of the surface soil. The aim is to keep the subsoil not only drainable but also impermeable to discourage roots growing into this layer.
3 Apply lime, gypsum and phosphorus (if necessary), and rip and lightly till the soil
Some surface soils are naturally hard and dense, while others have a plow sole or shallow hard pan due to excessive tillage and traffic.
All these hard layers need to be broken up. After spreading lime, gypsum and/or phosphorus (if soil test indicated these are needed), rip the soil both ways to a depth of about 300 to 400 mm.
Be careful not to mix heavy subsoil with the surface soil.
After ripping the soil, slightly till the moist, but well-drained, surface soil to form small clods. Do not pulverise the soil, which would happen if the soil was tilled 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 tilled 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 m wide and rototill it in.
Phosphorus is important for root growth, and young fruit trees will benefit from phosphorus if it is nearby, i.e. if mixed into the surface oil.
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 between 5.8 and 6.5.
In acidic soils (pH below 5.8) excess aluminium and possibly manganese become available and are 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 sulphur may be present in an 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 sulphate) is sometimes needed to soften surface soils and to improve their structure. Soil stability is improved and pores can be created chemically by sticking together (flocculation) of clay particles by the addition of gypsum.
With gypsum, the soluble calcium swaps with some of the exchangeable cations, such as sodium and magnesium. Gypsum does this better than lime because gypsum is more soluble than lime. Gypsum will not neutralize acidic soils or effectively raise pH does, 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 about 5 per cent of the total exchangeable cations, and there are low concentrations of soluble cations, the soil is sodic and unstable. Sodic 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.
Adding calcium in the form of gypsum causes flocculation (sticking together into clumps) to form a building block for improved soil structure.
4 Hill-up the surface soil (cont. next month)
See this article in Tree Fruit Dec 2017
