Appropriate Soil Maintenance Methods

Maintenance of the soil after establishing the crop includes:

• Protecting the surface against erosion.
• Minimizing compaction.
• Guarding against salinisation.
• Limiting acidification.

Protection Against Erosion

The best method to protect an orchard floor against erosion is to use a natural grass cover or cover crop.

The orchard floor should not be kept completely free of weeds. The area underneath and between the trees should be weed-free, forming a strip one metre wider than the diameter of the tree canopies. On the rest of the orchard floor, including the paths between the rows, a natural grass cover must be established as quickly as possible. This is not always possible in low rainfall areas because the paths between the rows are not irrigated.

The width of the grass strip will decrease as the trees grow and must never reach underneath the canopy. The grass must be mowed frequently, and the cuttings are blown underneath the canopy. These strips of grass also harbour beneficial insects and play a major role in pest control.

As explained in the previous section, by using surface mulch, water infiltration into the soil is improved and it also reduces runoff. If mulch is not or cannot be used, erosion can still be controlled by making sure the soil surface has a crumb structure rather than a smooth structure.

Minimising Compaction and the formation of Surface Crusts

Compaction of the soil after soil preparation is inevitable but must be minimised as much as possible. Practices that accelerate compaction are vehicle traffic, high precipitation rates and sanitation.

Vehicle traffic has the severest impact. All vehicles should be restricted to the 2.5m middle sections in the inter-row paths and must never drive closer to the trees than that. In vegetable and field crops, vehicle traffic should be restricted to fixed pathways (called tram lines) in the field. This will still lead to compaction, but only in certain parts of the field and not the whole crop area.

When water is applied at a rate exceeding the infiltration rate of the soil, the soil particles float and, on drying out, will settle in a more compacted state. This reduces the infiltration rate even more, with the consequences of more surface compaction. This is also referred to as crusting.

Salinity, and especially an increase in the SAR of the soil, accelerates crusting. Soil analyses will help detect the development of crusting, but the first symptom is run off halfway through the irrigation cycle.

Guarding Against Salinisation

Apart from crusting, which is the first symptom of salinisation, accumulation of sodium or reduction of calcium deeper in the profile should be monitored continuously. For this purpose, regular soil analyses are required. Although the ratio of the cations is a good indicator of developing salinity, the subsoil should also be sampled from time to time. The intervals will be determined by the conditions prior to planting when the profile was analysed.

As for indicators of developing salinity, the calcium to total cation ratio should be 70% to 75% and the sodium <3.00%.

Limiting Acidification

Acidification is more active in the sub- than the topsoil. Subsoil sampling is, therefore, more important to monitor the pH.

The ammonium form of nitrogen is the cheapest nitrogen source but also one of the greatest sources of acidification. Even in fertilizers like LAN, acidification can be a potential hazard for subsoil pH levels. Liming the subsoil alone is not possible and the pH of the sub-soils must be monitored granularly. The frequency of sampling can only be determined by historic data.

Record Keeping

During a soil survey, many soil properties are evaluated, analysed, described and recorded. Many of these properties are stable and will not change. These records are therefore valuable for replanting and for future developments, and should not have to be redone. The stable properties include:

• Slope
• Aspect
• Soil depth
• Clay content
• Structure
• Water-holding capacity
• Cation exchange capacity

The variable properties of soil are:

• pH
• Nutrient content
• Resistance

It is in this regard that soil and leaf analyses, especially historical data from routine leaf and soil analyses, have their value. This data indicates the direction in which the nutritional status of the soil and trees are moving. Records of the variable data should be kept in an easily accessible format for at least three years.

The format may vary but should enable the reader to compare year-on-year figures with ease.