A healthy avocado tree has a root system that can penetrate the soil to a depth of 1m. It is essential to determine in advance whether your soil is suitable for avocado production.
To do this, it is necessary to take soil samples. Dig profile pits at least 1,5m deep in different places on your land. Even if the soil on the surface seems to be much the same everywhere, dig at least one profile pit per hectare.
In hilly areas, dig the holes in different positions on the slope to get an indication of the drainage properties. (In a lower-lying area, you might have to dig furrows to prevent water from accumulating).
Note the following in each profile pit:
- Colour: Only reddish-brown, red and dark-brown soils are suitable.
- Texture: Avocados do best in soils with a clay content of 20% to 40%. If the clay content is lower, the soil has a limited water-retention capacity. Too much clay, on the other hand, makes irrigation difficult because over-irrigation and high rainfall lead to over-saturation of the soil.
- Soils that can be broken into hard clods when dry will restrict root development. Ideal avocado soils display only small, fine cracks when a dry profile wall is examined.
- Patches: If there is a light-coloured layer with many patches 1,8m or so below the soil surface, root problems might be experienced. This type of soil is not ideal for growing avocados.
You will also need a soil analysis. Take a representative sample, using a soil auger or a spade, from the proposed orchard at least nine months, but preferably 12 to 24 months, before planting. This will give you enough time to prepare the soil thoroughly.
Each soil sample should be homogeneous – that is, have no visible differences. If there are variations in colour and texture, provide a separate sample of each part.
Take samples up to 0,3m deep for topsoil and 0,3m to 0,5m for subsoil. A sample should be made up of at least 10 sub- samples, preferably more.
The area represented by the complete sample should not exceed 3ha.
Mix the sub-samples from a particular land in a clean container (not a fertiliser bag). Take a 2kg sample from this, place it in a suitable container and submit it for analysis.
The results will determine which fertilisers should be applied before planting.
If required, lime or phosphate should be thoroughly worked into the soil before planting.
Loosen the soil as deeply as possible before planting. If the soil is very acidic, heavy lime applications may be necessary.
Distribute about two-thirds of the recommended agricultural lime over the entire area 12 months before planting, mixing it into the topsoil by disking and ploughing it in as deeply as possible.
A cover crop can then be planted and ploughed in six months later to increase the organic-matter content of the soil.
Apply and lightly work in the remaining lime and the required phosphate simultaneously. The trees can be planted three months later. Do not fertilise recently planted trees too soon.
The trees must first become well established. In most cases, it is advisable to wait a year.
These applications should be very light, and the fertiliser must be applied evenly and not come into contact with the stem of the tree.
Irrigate immediately after applying the fertiliser.
Source: Department of agriculture in co-operation with the ARC-Institute for Tropical and Subtropical Crops.