Problem Soils: Managing Podzols or Spodosols

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Podzols generally occur within BRIS (Beach Ridges Interspersed with Swales) soils although they have been found on moderate hills in East Malaysia. The total extent of these soils in Peninsular Malaysia alone has been estimated at 162,000 ha (Choo, 1991). Majority of these soils are used for tobacco, vegetables, cashew nut trees and star-fruit trees. Of late, some of these soils which occur in the plantations have been cultivated with oil palms.

The major constraints in Podzols are perched water table, low nutrient status and CEC and poor moisture retention capacity. Some podzols may not have perched water table and these soils resemble quartzipsamments, which are discussed later.

The obvious first priority is to remove the stagnant water on the soil surface. This is easily accomplished by digging scupper drains with lower depths breaking the hard spodic horizons. The intensity of drains is usually 1 in 8 palm rows although this varies with sites. The top width of the drain is 60 cm and the bottom width is 30 cm to allow for more gentle slope, therefore easier maintenance.

Upon surface drainage, the conditions reverted to the other extreme of likely severe moisture stress due to excessive drainage and low moisture retention capacity. Hence, water conservation practices similar to those described for lateritic soils earlier must be improved immediately. The EFB mulching rate should be increased to 60 to 80 t ha^-1 yr^-1 and this is continued for at least 5 years before a lower rate is adopted.

The poor nutrient status and retention capacity pose a dilemma of high total fertiliser input but low rate at each application. This is generally solved by using compound or mixture fertilisers supplemented with straight fertilisers. The total fertiliser applications may reach 7 to 9 rounds a year and this should minimise leaching losses. Despite the sandy soils with anticipated low P fixing capacity, high phosphate rock is still recommended to ensure good root development and activity. Very high rate of ground magnesium limestones (GML) is also necessary to build-up the soil Mg status and prevent Mg deficiency.

Good ground vegetation is also important in reducing the surface soil temperature, which helps to reduce soil water evaporation and improve microbiological activity. The leaf litter return also binds the soil particles for better structure and aggregation.

Our experiences with planting oil palms on Podzols with satisfactory rainfall or more than 2000 mm yr^-1 has been encouraging as shown in Figure 7.

Problem Soils: Managing Shallow Lateritic Soils

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Jitra

Shallow lateritic soils such as Malacca and Gajah Mati series, and their associated soils occupy 0.6 million hectares in Peninsular Malaysia (Law and Selvadurai, 1968). Early experiences indicated that oil palms grown on shallow lateritic soils came into bearing two years later and three times less compared to deep soils (Tan and Thong, 1975; Pang et al., 1977). Increasing the fertiliser rates only partially alleviate the constraint and improve yield by –% (Tan, 1973). Productivity also seems to improve with palm age (Tayeb et al., 1991).

These results show that the main problems with shallow lateritic soils are low effective soil volume, poor nutrient status and water holding capacity. These detriments further hinder root development, which aggravates oil palm growth and production. It has to be mentioned that the types and compactness of the laterites also play a major role on the degree of severity of limitations to oil palms. For example, the less compact and subangular laterites of Jitra series pose only moderate limitation to oil palms compared to very serious limitation in Malacca series despite both being shallow lateritic soils.

The main approaches to obtain satisfactory oil palms on shallow lateritic soils are to improve soil fertility and implement soil and water management adroitly. Improvement in soil fertility is necessary to increase nutrient uptake per unit soil volume. Since most lateritic soils are well weathered with low CEC and high P fixing capacity, it is necessary to maintain high and balanced rates of manuring as well as frequent applications to the palms. It is also essential to apply very large quantity of phosphate rocks to ensure sufficient P for good rooting activity.

The primary aims of soil and water management here are to reduce run-off and soil erosion, and build-up organic matter in the soil. These are achieved by:

• maintain desirable ground vegetation such as legumes during immaturity to early maturity phase and light grasses and Nephrolepis biserrata in later years,
• Spread the pruned fronds as broadly as possible. In flat areas, L-shaped frond stacking should be carried out while in terraced areas, they should be staked on the terraced lips and between the palms along the terraces,
• terraces must have sufficient back-slope and regular stops along the terraces to trap soil and water,
• mulching with empty fruit bunches (EFB) if available

Irrigation should only be conducted if it is economically viable, easy to maintain and a ready source of water during the dry season is available as mentioned in Part I of this lecture notes.

It is also advisable to increase the planting density to 148 palms ha^-1 and extend ablation by 3 to 6 months for maximum leaf area index and high better yields.

Proper implementation of above soil fertility, and soil and water managements had raised the oil palm. Yields on commercial scale as shown in Figure 6.

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Problem Soils: Managing Saline Soils

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Kranji

Saline soils occur by the sea or around river mouths and are constantly inundated by sea or brackish water. Consequently, they have a young A/C profile with conductivity commonly above 10,000 µmhos cm^-1. In potential acid sulfate soils such as Bakau series, may contain high water soluble sulfate exceeding 0.35%. Saline soils generally occur in low rainfall region in Malaysia.

Our plantation tree crops are not salt tolerant and hence cannot be grown on saline soils before ameliorations. Despite this, a number of large plantation companies in Malaysia, such as K.L. Kepong Bhd., Sime Darby Bhd. and Golden Hope Plantation Bhd., have successfully grown oil palms on it. However, before reclamation work proceeds, we have to ensure that at least the following conditions prevail at the site.

• materials for bunding are available,
• if (a) is unavailable, then the “n” value of the soils should be less than 0.7
• most of the land boundary should not be erosional surface,
• the land should preferably be higher than the sea or river level at low tides,
• rainfalls should be sufficient (> 1700 mm yr^-1) to allow flushing and leaching of salts,
• land area must be sufficiently large to dilute the cost of reclamation and maintenance to economic level

Preventing further intrusion of sea or brackish water of more than 1000 µmhos cm^-1 into the land is central to reclamation of saline soils. This is accomplished by construction a band around the periphery of the land. The bund should be at least 3 feet above the highest tide level.

Consideration must be given to the river and its tributaries in the land in deciding the course of the bund.

Upon completion of bund construction, a drainage network comprising main and collection drains must be laid down to reduce the water table and allow for subsequent flushing of the drains. There must be sufficient watergates and water pumps to remove the water trapped in the land. The periodic flushings usually continue for two to four years before the conductivity drops below 2000 µmhos cm^-1 within the top 45 cm to allow successful planting of oil palms.

Once the above is achieved, field drains are then constructed to lower the water table to between 50 to 70 cm from the soil surface. Planting of oil palms and other cultural practices resemble those of coastal soils. However, boron application is generally unnecessary.

Bund maintenance to prevent seepage and leakage and sound water management are necessary to ensure successful reclamation of saline soils for oil palms. An example of yield profile of oil palms on saline soils with mean annual rainfalls of 18.22 mm is shown in Figure 5. The mean FFB yields were low due to two periods of distinct dry season per year although occasionally they may exceed 24 t ha^-1 yr^-1.

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Problem Soils: Managing Shallow Acid Sulfate Soils

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Sedu

Acid sulfate soils are estimated to cover an area of about 110,000 ha in Peninsular Malaysia with at least 20,000 ha under oil palms (Poon and Bloomfield, 1977). These soils are characterised by very low pH values (< 3.5) and the presence of yellowish jarosite (Kfe₃ (So4)₂ (OH)₆) mottles (Shamshuddin and Auxtero, 1991).

The problems with acid sulfate soils are:-

• they tend to be waterlogged in their natural state and must be drained before cultivation, and
• draining beyond the pyrite layer will generate excessive acidity which is detrimental to palm growth.

The latter is due to the oxidation of pysite to form sulphuric acid as shown below:

This oxidation also causes breakdown of clay minerals which releases Al, Mn and K into the soil solutions (Shamshuddin and Auxtero, 1991). The drop in pH to below 3.0 is not uncommon and the oil palms will suffer hyperacidity symptoms and poor yields Toh and Poon (1982) further, classified acid sulfate soils into 3 categories based on oil palm performances. Their severe category has acid layer at 0 to 60 cm while current soil classification in Malaysia tag it at 0 to 50 cm for shallow acid sulfate soils, such as Linau and Sedu series.

Hew and Khoo (1970) found that liming was generally ineffective to control acidity in acid sulfate soils. Poon and Bloomfield (1977) then showed that by creating anaerobic conditions, the reaction in equation (1) will not proceed and thus, preventing the generation of acidity. Since inadequate drainage will give rise to flooded conditions which also adversely affect palm performance, a balance has to be struck between over and under drainage.

This balance is achieved through a network of field, collection and main drains similar to those found in peat swamp as described earlier but their objective differs. The prime requirement in the management of acid sulfate soils is that the water-table should be maintained above the pepsitic layer for as long as possible. This is again carried out using stops, weirs and watergates, their numbers are largely determined by the depth to pysiritic layer and slope of the land. Normally, the water-table is maintained between 45 to 60 cm from the soil surface, hence, the depth of field drains should not exceed 75 cm. Otherwise, there is a risk of accelerated oxidation of the pyritic layer during dry weather conditions (Poon, 1983).

Another important aspect in the management of shallow acid sulfate soils is to provide for periodic flushing of the drains to remove the accumulated toxic polyvalent ions such as Al³⁺ and the extremely acidic water (Poon, 1983). Therefore, during the wet season, all the water retention blocks and watergates are opened to allow flushing. One to two flushing during the wet season are usually adequate. Before the end of the wet season, the blocks and watergates are again closed to allow fresh water to build up to the required level.

The other aspects of management of acid sulfate soils are similar to those of coastal non-acid sulfate soils. The success in using water control to manage oil palms on shallow acid sulfate soils is best illustrated by Figure 4.

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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.

Soil Management: Problem Soils

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Introduction

The first part of this paper elucidates general the principles of soil management, soil requirements and proper soil management practices for plantation tree crops. We shall now discuss how we can combine them interactively to manage problem soils for tree crops.

The term “problem soils” appeared many times in literature but it has not been well-defined yet. Longman dictionary describe the word “problem” as “a difficulty that needs attention and thought”. Therefore, problem soils may be defined as soils which require special or specific attention, though and methods to successfully managed them. Within the plantation industry, the conceptional idea of problem soils is probably “unsuitable soils for cultivation in their natural states but upon proper soil management and amendments, they can be converted for plantation tree crops with yield performances, at times, matching those on suitable soils”.

Based on this concept, there are probably six groups of problem soils, namely,

• deep peat
• shallow acid sulfate soils
• saline soils
• shallow lateritic soils
• podzols or spodosols, and
• sandy soils (quartzipsamments)

Each group of soils requires its own specific soil management practices and crop species. With the present lack of labour, cost of management and price of produce, oil palm is the primary tree crop grown on these soils. Therefore, this paper will be confined to practices pertaining to oil palm only while another lecture will present their characteristics and management for other crops.

There is a growing and discernible pressure from some quarters to utilise problem soils for oil palms despite the much more effort, time, difficulty and cost to do so which reduces competitiveness. This might stem from the reports of high yields on these soils but more so, from the lack of large scale experience to manage them for oil palms or for political gains. Our own experiences generally indicate that it is probably inadvisable to have more than a quarter of problem soils in any one plantations for long-term viability. Nevertheless, it is still critical to manage these soils correctly from economic standpoint, environmental consideration and to maintain competitiveness.

Reference 
Goh, K.J. and Chew, P.S. (1995). Managing soils for plantation tree crops. II. Managing Problem Soils in Malaysia. In: Course on Soil Survey and Managing Tropical Soils (ed. Paramanathan, S.). MSSS and PASS, Kuala Lumpur: 246-256.

Note: The full list of references quoted in this article is available from the above paper.

Soil Management: Conclusions

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Cultivation of plantation tree crops is still expanding rapidly in south-east Asia, particularly in Malaysia and Indonesia. A diversified range of soils is used with increasing proportion of marginal soils. It is vital that good soil management is implemented to ensure high sustainable production for economic viability and maintain or improve soil fertility. There is also a growing concern on soil degradation and environmental pollution with high inputs agriculture but these can be avoided with good soil management.

The first approach in soil management is to identify the soil constraints to crop production and assess their degree of severity. In the humid tropics, these detriments are closely related to nutrients and water which are the most limiting factors to crop productivity. Both are available via the soil to the plants, particularly those with good rooting activity.

The rooting activity of plants are influenced by many soil properties such as terrain, texture, structure, consistency, permeability, drainage and inherent nutrients. They require interactive management approach to achieve the basic objectives of crop productivity and maintenance of soil fertility and to do so in an environmentally acceptable way. These soil management approaches encompass soil and water conservation management, soil fertility management, soil acidity management and soil water management.

The concepts of good soil management is nothing new and best exemplified by the following quotation from Sanskrit, the classical, literary language developed from about 1500 B.C. by the Hindus in Northern India (Johnson, 1995).

“Upon this handful of soil our survival depends. Husband it and it will grow our food, our fuel and our shelter and surround us with beauty. Abuse it and the soil will collapse and die taking man with it”.

This adage which has evolved through time, still poignantly encapsulates a major concern today; so when will man learn to ensure plentiful food and beauty for the generations to come, if it is not now.

Soil Management: Soil Water Management

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Water management practices include drainage and irrigation of the land, and soil moisture conservation practices. Adequate soil moisture is required for good growth and yield of plantation tree crops. Hence, good responses to irrigation have been reported (Hutcheon et al ., 1973; Kee and Chew 1993, Lim et al ., 1994). However, excessive water such as high water table can reduce crop productivity substantially (Lim et al ., 1994).

Soil moisture conservation measures or irrigation will be beneficial in the following areas :

• inadequate rainfall of less than 1700 mm per year,
• poor rainfall distribution pattern,
• somewhat excessively to excessively drained soils,
• very shallow soil or soil causing restricted rooting.

As irrigation is frequently impossible due to inavailability of water and high capital costs of installation, water conservation measures should be aimed primarily at maintaining maximum use of rainfalls on the plantations. This includes minimising run-off and erosion, and maintaining or improving infiltration of water into the soil. Therefore, soil and water conservation practices are complementary to a large extent.

Irrigation, despite giving good yield responses, should only be implemented if the following conditions can be met :

• regular severe moisture stress is limiting growth and yield,
• adequate water with salinity less than 1000 mhos cm^-1 can be ensured during dry season,
• the irrigation system is easy to maintain,
• an economic system of irrigation is possible.

The other extreme of water management is when excess water occurs in the area. Proper drainage systems are essential to prevent prolonged flooding which is detrimental to crop production.

The primary aim of drainage for plantation tree crops is to maintain the water table at 75 cm and not less than 50 cm from ground surface at most times. To achieve this, a good outlet with sufficient capacity for the water discharge requirements is vital. Otherwise the excess water could still be contained within the planted area. The direction of field and main drains should be in the line with the flow direction of the water. The intensity and dimension of drains depend largely on the expected amount of water to remove during the wet months. Cheong and Ng (1974) proposed higher intensity of field drains for clayey soils compared to sandy soils. The water level in the drains may be controlled using water gates, weirs and stops.

In sandy podzols, perched water table may occur due to poor percolation of water. Scupper drains which break through the hard-pan (spodic horizon) are required to remove the stagnant water before planting. Similarly, in compacted soil with poor infiltration rate, aeration drains have been found to be beneficial.

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Soil Management: Soil Acidity Management

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This aspect involves two major issues : one, soils with low pH and two, soils acidified by our management practices such as manuring and terracing. It also involves the crop species since cocoa is sensitive to low pH and high Al saturation while oil palm and rubber are not. In fact, high soil pH which generally goes with high exchangeable Ca and Mg may be detrimental to rubber yield since the named nutrients can caused unstable latex and pre-coagulations.

Excluding acid sulphate soils which will be discussed in part two of this lecture, soils of pH less than 4.5 or Al saturation more than 30% should be limed if cocoa is grown on them. This is because good responses to liming were obtained by Lam and Lim (1991) and Lim and Ho (1994) for such soils cultivated with cocoa. Oil palm and rubber can tolerate low acidity, and pH 3.5 does not seem to influence crop productivity.

Soil acidification is generally a natural process of soil formation such as leaching of nutrients, nutrient uptake by plants and pollutants. However, soil acidification which is generally regarded as soil degradation can occur through fertilisation with acidifying fertilisers such as ammonium sulphate as shown.

(NH₄) 2SO₄ + 8 0 —> 2 NO₃ + H 2 SO₄ + 2H₂0

Kee et al . (1993) showed that this process occurs in oil palm agroecosystem where soil pH decreased from 4.2 to 3.8 after 7 years of NK applications (Table 8). Further reduction occurred a month after fresh application of NK fertilisers. However, K uptake did not seem to be influenced by such low pH (Kee et al ., 1993). Although further trial is necessary to ascertain this, some plantation sector has taken the precaution to avoid soil acidification by applying NK fertilisers in the avenues of fully mature oil palms. EFB mulching and liming are known to increase soil pH and might be used if the degree of acidification is found to be detrimental.

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Soil Management: Soil Fertility Management

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Law and Tan (1973) and Goh et al . (1993) had clearly shown that large variations in soil fertility occurred within and between soil series in Malaysia. These variations also occurred spatially at macro and micro scales (Goh et al ., 1995) thus demanding site-specific management approach to maximise efficiency. This is in agreement with the varied responses of plantation tree crops to fertilisations, where yield responses for oil palms ranged from 0 to 250%, for cocoa from 0 to 47% and for rubber from 0 to 39%. This further indicates that soil fertility management is not only site-specific but also crop-specific.

The realisation of above has caused scientists to develop schemes or methods to measure or assess soil fertility quantitatively or qualitatively. One of the schemes called fertility capability classification system (FCC) has been discussed in earlier lecture. In plantation tree crops, the assessment of soil fertility generally takes the format of single nutrients approach as shown in Appendix 4. The soil physical and biological properties are not included because they are generally handled separately.

This part of the lecture will cover only the fertiliser management of soil fertility since the other aspects have been dealt with.

Nutrient requirements

The nutrient requirements of plantation tree crops are usually calculated based on the nutrient balance concept (Chew et al ., 1994b; Kee et al ., 1994). This involves the equating of factors of nutrient removal against those of nutrient supply (Figure 1). Therefore, the fertiliser requirements will depend, apart from the crop removal, also on the inherent soil nutrient status. Foliar analysis is also used as a supplementary tools for the diagnosis of nutrient requirements.

Therefore, the key steps of an effective fertiliser management programme are :

• determination of growth and yield targets,
• assessment of the action required;
  1. What nutrients are needed?
  2. What rates of nutrients are needed?
  3. How best to achieve the most efficient and cost effective application of fertilisers to meet nutrient requirements ?
  4. What types of fertiliser to apply?
• Assessment of the results and further action required,
• Computation of the economics of the results.

Detailed description of the above is provided by Chew et al . (1994b) and interested readers should refer to the paper. We shall instead discuss the fertiliser application technique in plantation tree crops which is one of the key factors in determining an efficient and environment friendly approach to soil fertility management.

Fertiliser application techniques

The higher the fertiliser efficiency the lower is the risk of manuring on the environment. This simple relationship demands that we maximise or attain satisfactory efficiency of the fertiliser applied. Proper application methods are essential to achieve this, especially in areas where the soils are proned to high run-off and leaching losses and to combat these, we generally rely on frequency, timing and placement of applied fertilisers.

Frequency of application

Foong (1993) using field lysimeter reported that after the first four years, low leaching losses in Munchong series soil were recorded for all nutrients except Mg (Table 6). However, Chang and Zakaria (1986) working on the sandier Serdang series recorded leaching losses of 10.4% for N and 5.1% for K with 2352 mm of rain per year. This apparent correlation between nutrient loss via leaching with soil texture was also illustrated by Pushparajah et al . (1993) in laboratory trials.

Time of application

Although we are in the humid tropics, the rainfall patterns differ considerably between locations. On-going studies (Chew et al ., 1994b) show that high rainfalls prior to fertiliser application resulted in substantial nutrient loss, especially in high fertiliser concentration areas (Table 7).

The general guideline is to avoid fertiliser applications during:

• period with high rainfall months of more than 250 mm month^-1,
• months with high rainfall days of more than 16 days month^-1,
• months with high rainfall intensity of more than 25 mm day^-1.

Placement of fertiliser

Fertilisers should be applied in areas with anticipated active root development and maximum feeder root distribution, which vary according to plant age and species. Therefore, fertilisers are applied close to the tree base in the initial years and gradually extended to the tree avenues when the canopy has overlapped and good root development is found there. In hilly terraced areas with mature trees, the fertilisers should be applied broadcast in the terrace itself and between the trees. In areas with platforms, the fertilisers should logically be placed around them.

Application of palm oil mill by-products

The palm oil mill produces substantial amounts of by-products such as EFB and anaerobic sludge. The applications of these by-products are encouraged because they return the organic matter and nutrients to the soil and hence, help to maintain soil fertility without causing environmental pollution.

In rubber and cocoa systems, the by-products are seldom returned to the fields due to their low nutrient values, logistic problems and maintenance problems. It has to be noted that pod husks in cocoa plantations are thrown back to the field during harvesting.

EFB

Gurmit et al . (1982) reported that 1 tonne of EFB contains 15.3 kg of ammonium sulphate, 2.5 kg of Christmas Island rock phosphate (CRIP), 18.8 kg of muriate of potash and 4.7 kg of kieserite. Hence, in mature oil palms, 40 t ha^-1 of EFB are generally applied in the interrows to supply sufficient nutrients for a year. Supplementary fertiliser applications such as CIRP may be required to balance the nutrient requirements of oil palms.

Apart from being a source of nutrients, EFB also improves the soil physical properties and reduces soil water evaporation (Lim and Messchalck, 1979). Therefore, preference for EFB application should be given to problem soil areas such as the sandy podzols and shallow Malacca series.

Anaerobic sludge

The application of anaerobic sludge would also help to partly relieve moisture stress in soils susceptible to moisture deficits, in view of the considerable amount of water in the sludge. The usual recommended rate of application for mature oil palms is 450 l palm^-1 yr^-1. The fertiliser equivalents according to the nutrient composition of 3.6g N, 2.4g K, 1.2g P and 1.5g Mg per litre (Lim, 1984) are 7.6 kg palm^-1 of ammonium sulphate, 1.6 kg palm^-1 of CIRP, 2.1 kg palm^-1 of muriate of potash and 2.6 kg palm^-1 of kieserite.

Supplementary fertiliser applications may again be required to ensure balance nutrition. The application areas of anaerobic sludge should also be in the palm avenues.