Problem Soils: Managing Deep Peat

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Gondang

There are 2.4 million hectares of peat in Malaysia, with 1.5 million hectares occurring in Sarawak alone. Oil palms are cultivated on peat on a large scale since the mid fifties. However, major problems were encountered especially on deep peat and it was not until the eighties that oil palms are successful grown on it.

The problems with deep peat lie in its physical and chemical characteristics. Peat, in its natural state, contains excessive amount of water due to its low physiography and water holding capacity of 20 to 30 times its own weight. Consequently, aeration is poor and bulk density is very low at less than 0.1 g cm^-3. Upon drainage, peat will undergo irreversible drying and extensive subsidence of 3.6 cm yr^-1. Apart from this, peat provides an imbalance nutritional medium for plant growth (Table 1). Although it has high total N content, it also has high C:N ratio, rendering a slow availability of N to the plant. Moreover, it has low K, Cu, Zn and B and high acidity of pH less than 4.0 (Gourmit et al., 1987).

Bearing this in mind, United Plantations Berhad (UPB) has developed various novel methods to alleviate the problems and allow successful cultivation of oil palms on deep peat. Therefore, this part of the lecture note is extensively drawn from a paper written by Gurmit et al. (1987).

The first problem confronting a developer is to remove the excessive water in the peat swamp before felling and clearing operation can be initiated. This is done by constructing a perimeter drain, the dimensions of which depend on the size of area to be cleared and distance from a river outlet, using an excavator. Due consideration should be given out to overdrain the area as this will result in rapid shrinkage of the peat and irreversible drying of the top layer, which adversely affects establishment and growth of oil palms.

Basically, the drainage system consists of a network of field, collection and main drains (Figure 1), the dimensions of which are:

The intensity of drains depends on the topography of the field and planting density but the primary objectives is to keep the water levels at 50 to 75 cm from the surface at most times. This is achieved through a series of stops, weirs and watergates. Periodic flushing of the acidic and excessive storm water during the rainy season is also carried out.

The low bulk density and subsidence earlier present obstacles to road construction and planting. Field and main roads are now created using spoils from roadside drains, levelled and compacted by bulldozer and then lined with laterite and mining ballasts. Before planting, the harvesting path and planting rows are mechanically consolidated by running an excavator 2 to 3 times over them. The completed operation leaves a 9.5 to 3 times over them. The completed operation leaves a 9.5 to 11.5 m wide area free of timber and compacted to a depth of 40 to 50 cm (Figure 2). Consolidation increases the bulk density from 0.11 to 0.20 g cm^-3, reduces the incidences of leaning and fallen palms by half and improves FFB yield by 25%. Planting density is also increased to 160 palms ha^-1 to attain optimum leaf area index of 6.0 for production by the 10^th year on this poor growing medium.

The irreversible drying of the top layer is prevented by maintaining satisfactory water-level of 50 to 70 cm from surface, and good ground vegetation of light grasses and low density of Nephrolepis biserrata . Moreover, blanket spraying may increase the risk of fire and affect the predator-pest balance.

Deep acid peat provides an interesting nutritional complexes to agronomists. While total N content can be high (1.3 to 1.5%), its availability is low due to high C:N ratio (Table 1). Upon drainage (Table 2), liming and decline in C:N ratio and higher N availability. Thus the priority is to provide high N rate (up to 1.2 kg urea palm^-1 yr^-1) in the initial immature phase and subsequently reduce it during the mature phase (0.5 to 1.25 kg urea palm^-1 yr^-1). This approach was supported by the work of Gurmit et al. (1987) which showed good FFB response to N in the first 4 years of harvests only (Table 3).

The mineralisation of peat also releases P to the system, which contains low Al and Fe for fixation. Therefore, only low P rates of 0.5 to 1.0 kg phosphate rock palm^-1 yr^-1 are generally provided. Excessive P application can leads to lower yield and Cu imbalance (Cheong and Ng, 1977). On the other hand, potassium is very deficient in peat and hence, high rate of muriate of potash up to 5.0 kg palm^-1 yr^-1 is recommended (Gurmit et al., 1987).

Although good response to liming has been obtained the effect is unlikely to be due to Ca. It is most probably a result of improved mineralisation rate, increased soil pH and a better cationic-anionic balance in the plant system (Cheong and Ng, 1980).

Peat is also deficient in Cu, Zn and B. Early dressings with these micronutrients are essential to avoid mid-crown chlorosis, peat yellow and stump leaves respectively. However, excessive B application must be avoided as it can be phytotoxic and adversely affect the uptake of Cu (Gurmit et al., 1987).

Draining the peat swamp increases acidity as shown in Table 2. This is alleviated by periodic flushing of the drain water, especially during rainstorms, and liming. Maintenance of correct water levels is also important since hyperacidity seems to occur only during prolonged dry spell.

Proper soil and water management of oil palms on deep peat has resulted in FFB production closely mirroring that on good mineral soils (Figure 3). However, we must caution that the problems with planting oil palms on deep peat escalated exponentially with the areas of peat, particularly in relation to the amount of good mineral soils in the plantation.