Soil Management: Soil Requirements of Plantation Tree Crops

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The first approach in good soil management is to attempt to match the crop requirements with soil properties, with or without amendments. Water and nutrients are probably the most limiting agronomic factors to growth and yield of plantation tree crops (Goh and Chew, 1994). This is because tropical environments usually provide sufficient and uniform sunshine for crop production. It is well known that the above limiting agronomic components are available to plants via the soils. However, to tap into water and nutrients efficiently, the plants need to have good rooting activity.

Rooting activity is influenced by many soil properties such as terrain, soil depths, stoniness, texture, structure, consistence, permeability, drainage and nutrients. These properties generally act in an interactive manner and an understanding of this dynamics is necessary to elevate the yield curve. Therefore correct identification of the types of soil limitations present is vital to soil management.

We also need to assess the severity of the identified soil properties which are limiting yield performances. Overcoming or alleviating them should upgrade the yield performances. It also enables proper formulation and implementation of specific soil management practices in each field in order to maximise return, avoid soil degradation and prevent environmental pollution.

The soil properties and their criteria for assessment of severity of their limitations for plantation tree crops are provided in Appendices 1 to 3 and briefly discussed below.

Terrain

Under Malaysian land law, it is unlawful to cultivate land with slopes more than 20 or 36%. These lands are classified as steepland in Peninsular Malaysia. Such landforms are commonly found in Sarawak and Sabah also and may occur within the plantations. They are usually left unplanted, planted with forestry trees or if the area is small, may be planted with plantation tree crops.

The main problems caused by steep topography are

• high risks on erosion, landslides and run-off losses of nutrients,
• poor water balance due to excessive run-off,
• the need to terrace implies the planting on less fertile sub-soil, which is commonly devoid of organic matter and generally firmer consistence,
• difficulty in harvesting or tapping and field maintenance operations with probably poorer crop recovery.

Therefore, poor tree performances are not uncommon on steep slopes. However, the effect of slope seems to be less detrimental if the soils are fertile, such as with Kobovan Family soil in Sabah which is derived from basic volcanic parent materials. Rubber and cocoa trees are also less influenced by slopes probably due to easier harvesting operations, tap root system and lower nutrient demand.

Steep terrain – terraced planting

Soil depth and stoniness

Adequate soil volume is a prime requirement for root development. Soil volume is a function of soil depth and stoniness. In plantation effective soil depth is measured with an Edelman auger to an impenetrable layer within 90 cm depth while increase degree of stoniness will correspond to decrease in rooting space.

The main effects of shallow effective soil volume are :

• limited root room for adequate amount of roots,
• weak anchorage,
• low available soil moisture,
• low exploitable soil nutrients

In Sabah, stoniness is a less important factor because most rock fragments are loosely formed, partly weathered and mainly only stone sizes and sparse. Therefore, it does not form a continuous barrier to root penetration, development and activity unlike lateritic soils found in West Malaysia.

Under permanent moisture stress conditions as induced by poor rooting volume, cocoa will suffer from drastic wilt and defoliation (Hutcheon et al ., 1973) whereas in oil palms, the inflorescence abortion rate will increase and inflorescence differentiation will tend towards maleness. Lately, work at AAR also confirms that rubber growth and yield will decrease with poorer soil depth.

Stoniness – shallow soils

Texture, structure and consistency

These three soil physical parameters are closely related. They determined soil aeration (porosity), water holding capacity, permeability (infiltration), root penetrability and nutrient retention capacity. Unfavourable soil physical conditions inhibit root growth and function, leading to poor plant growth even under liberal applications of fertilisers (Soong and Lau, 1977).

Soil texture can significantly influence our management decisions. For example, in loamy soil to sandy loam soils such as Lintang series, with high percolation and low nutrient and water retention capacity, will need more frequent manuring and mulching to improve crop productivity and reduce environmental pollution. On the other hand, tillage should be avoided on heavy soils such as Briah series to avoid soil compaction which degrades the soils and reduces crop productivity (Cheong and Ng, 1974).

Apart from soils with firm to very firm consistence such as Durian and Batu Anam series soils, soil compaction can be caused by logging activities or use of heavy machineries. The hard pan left behind is difficult to break and establishment of sensitive crops such as cocoa is often unsuccessful.

The permeability of soils to water depends more on the structure with pore spaces than texture itself (Table 1). The implementation of poor permeability on crop growth is discussed in the next section.

Structure also influence soil aerations, which if restricted can cause :

• inhibited root development
• impaired respiration of root system leading to reduced water and nutrient absorption
• inhibited beneficial microbial activities.

Total porosity may differ only slightly between soils of different structure and texture (Table 2) but air-filled porosity (non-capillary pores) is generally better for sandy soils or soils with good structure and friable consistence such as Munchong and Serdang series. Experience indicates that crop growth can be severely limited if air-filled porosity falls to 2% of the total porosity (Soong and Lau, 1977). Soils with good structure are also less erosive and less proned to landslides. The latter is common in Sabah where the soils are generally younger and have weak structures.

Permeability and drainage

Permeability is generally associated with internal soil drainage and it is closely related to soil physical properties as mentioned earlier. Poor permeability can cause

• perched water table such as in podzols
• imperfect drainage even in hilly soils such as Batu Anam series,
• poor rooting activity and its consequences as described earlier.

On flood plains and valley floors, high water table can be a common feature as in most Gleysols or Aquepts. Excessively high water table can give rise to :

• inadequate soil aeration which hampers root respiration and causes poor nutrient and water uptake,
• poor anchorage and lodging due to poor root development,
• canopies turn chlorotic resulting in poor photosynthesis

Generally, oil palm is less influenced by poor permeability and drainage compared to cocoa and rubber.

Poor permeablility

Poor drainage

Root anchorage

Nutrient status

Malaysian soils have highly heterogenous inherent soil nutrient status (Goh et al , 1993). For example, Selangor series soils and young soil derived from basic volcanic rocks tend to have high soil fertility compared to sandy or highly weathered soils such as Munchong series. Moreover, soils in Sabah generally have high Mg content compared to West Malaysian soils (Goh et al , 1993).

Nutrients, being one of the two most limiting factors to crop productivity, must be correctly assessed to afford proper fertiliser management practices. Over- or under-application of fertilisers can have disastrous effects such as

• poor crop productivity due to lack of fertiliser or imbalance,
• less in profit due to excess fertilisation,
• soil acidification and degradation,
• environmental pollution due to excessive leaching and run-off losses.

Apart from acid sulphate soils, serious to very serious limitations on soil pH do not occur for oil palms and rubber. However, cocoa trees are more sensitive to soil acidity and liming to raise pH above 4.5 or exchangeable Al saturation less than 30% is usually necessary for optimum production.

Other soil limitations

The other soil limitations are generally associated with particular type of soils. These limitations are high salinity or conductivity in saline soils (Bakau series), sulphidic layer in acid sulphate soil and peat. These will be discussed in detail in part two of this lecture note and in tomorrow lecture on problem soils.

Soil management requirements for plantation tree crops

After going through the soil properties and their limitations, we can broadly summarise the major soil management requirements into:

• soil and water conservation management,
• soil fertility management,
• soil acidity management,
• soil water management,

as described by Cheong and Goh (1988). Soil and water conservation management also includes the amelioration and improvement of soil physical properties.