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Difference between well-drained and poorly drained soils in macadamia production

Difference between wel-drained and poory drained soils in macadamia production – By Gareth Glasspool Soil Scientist

Soil characteristics impact crop growth and development
Soil classification plays a huge role in the development and eventual success of a macadamia orchard. Soil classification refers to the examination of the soil’s physical properties and the classification thereof according to a taxonomic system. Soils are grouped into categories based on various characteristics including texture, structure, pH, nutrient content, drainage, etc. In South Africa, soil is classified according to the South African Soil Taxonomy system (Soil Classification Working Group 1991).

The physical characteristics of soil can significantly impact the growth and development of macadamia trees. Therefore, by knowing and understanding the characteristics of the soils within the desired planting areas, informed decisions can be made regarding soil preparation and cultivation prior to planting.

The macadamia industry in South Africa is a rapidly growing market within the agricultural sector. Due to the significant costs required to establish a productive macadamia orchard, it is very important to follow the correct steps right from the beginning and to do the basics well.

Soils and macadamia production
Soil is regarded a critical component of the various key aspects that a macadamia producer should be focused on. Soil types differ in their inherent fertility and suitability for macadamia production. Put simply, different soils need to be managed differently. These varying managementpractices may include land preparation and the choice of drainage or irrigation systems and -scheduling required to ensure effective drainage placement.

Macadamia trees require well-drained soil to achieve optimal production. Poorly drained soils often result in the development of root diseases (Phytophthora etc.) and other problems such as preventing the optimal uptake of nutrients from the soil. Thus, when selecting a site for orchard establishment, appropriate drainage should be a key consideration.

The importance of drainage is highlighted in the following figures of different soil profiles.

Soil Classification

Figure 1: A well-drained apedal soil, classified as a Clovelly (Cv) as defined by the South African Soil Taxonomy system

Soil Classification

Figure 2: A poorly drained soil showing underlying wetness in the subsoil. The soil type in the picture is a good example of a Pinedene (Pn) that displays a well- drained topsoil with underlying wetness in the clay dominant subsoil.

Figure 3: A close-up look at the clay layer visible in Figure 2, indicating anaerobic conditions

Contrasts are clearly visible when comparing the two soil profiles (Fig 1 and Fig 2), the most striking being the difference in colour of the subsoil of the Clovelly and Pinedene soils (brown vs grey).

What are well-drained soils?
‘Well-drained soils’ refers to soil profiles in which excess water drains away relatively quickly after a rainfall or irrigation event. This is due to the soil being limiting layer free, thus having no layer/feature that impedes water from percolating out of the root zone. The make-up of these soils also facilitates an effective infiltration and drainage rate, which rate is determined by the soil texture. This concept is also illustrated in Figure 1 with reference to the brown and red soil colours, indicating well- oxygenated soil profiles.

When comparing the soil profiles depicted in Figures 1 and 2 respectively, a key difference is the presence of a dense clay layer in the Pinedene subsoil. Due to the slower drainage taking place in the clay layer (Figure 2), the profile has become waterlogged, leading to the development of anaerobic conditions. These conditions increase the risk for the development of root diseases (Phytophthora, etc.), prevents effective nutrient uptake and limits root growth.

The challenge of poorly drained soils
Figure 2 clearly highlights a poorly drained subsoil of Pinedene. The grey colouring of the clay visible is a clear indication that anaerobic conditions exist. This is highlighted further in Figure 3 where grey and yellow mottling is clearly visible.

Anaerobic conditions occur in soils when there is a lack of oxygen, usually as a result of waterlogged conditions. The grey and yellow colouring highlight the fact that the subsoil is naturally saturated for long periods and that high quantities of leaching have taken place. As the soil pores are almost constantly filled with water, the oxygen is forced out of the soil profile. This results in either a low oxygen or oxygen-free environment, resulting in a shift towards the occurrence of reducing reactions. The lack of oxygen causes anaerobic-type microorganisms to flourish and dominate the nutrient cycle process. The anaerobic-type microorganisms use nitrogen as primary food source instead of oxygen.

These organisms also excrete sulphur, causing a very poorly drained soil, often with a smell similar to that of rotten eggs. Anaerobic soil conditions therefore inhibit the growth and reproduction of beneficial microorganisms in the soil, reducing the soil’s ability to support healthy plant growth.

A solution for poorly drained soils does exist
It is important to note that anaerobic conditions in the soil could be a temporary challenge, as drainage can be installed as the solution to remedy the problem. To ensure that effective drainage takes place, drainage needs to be installed at the correct depth and the correct type of drainage system (herringbone or parallel) for the type of soil, contour and landscape in question must be used. Spacing will also vary depending on the layers present in the soil. In addition to drainage, the construction of growth beds can also aid to increase the potential rooting depth by lifting roots above the poorly drained zone. Irrigation management is equally crucial to prevent over-irrigation which will exacerbate the already waterlogged conditions.

Know and understand your soil type
From the above it is clear that a firm understanding of the type of soil one is working with when establishing a macadamia orchard, is absolutely critical. Macadamia trees are very sensitive to anaerobic conditions and prefer well-drained well-aerated sandy soils. It is however possible to produce good yields on other soil types, on condition that the basics have been executed correctly and a management strategy is implemented to effectively manage and counteract soil limitations.

Through the MYSOIL CLASSIFICATION™ service provided by Agri Technovation, a blueprint of the soil on your farm is constructed and soil profile maps are produced, identifying potential problem areas and challenges that may prevent optimal production. With reference to the soil profile maps, a set of practical step-by-step solutions are provided, indicating how these challenges can be effectively managed and improved. This would however only be possible once the soil’s physical characteristics have been determined through a soil classification.