Introduction

In Costa Rica, most commercial oil palm plantations are established in soils originating from alluvial deposits (alluvial inceptisols). More recently, however, this crop has been grown in soils derived from river deposits of volcanic material.

The andisols located in the south of the country have low bases contents and are also poor in contents of other elements (Alvarado et al. 2001). A previous study showed that these soils fixed more than 90% of the phosphorus applied (P availability was below 10 mg/l), and organic matter was high (12.5%) (Salas 1988).

A steady decrease in trunk diameter over time has been observed in a 5 year-old oil-palm commercial plantation in one of the andisols described. This tapering of the stem has been associated in the literature with phosphorus deficiency (ARAB 2001). Although P deficiency is not associated with any particular symptoms in the foliage of oil palm (Chan 1982), a gradual decrease in stem diameter over time may indicate a severe deficiency of this element. Aside from the reduced storage capacity of thinner stems, these limit the size and possibly the number of bunches that may be accommodated.

The possibility of a generalized P deficiency (despite apparently normal P contents in leaf 17) in one of the andisols now planted with oil palm, was confirmed by the results of an experiment with increasing amounts of P planted in this area. The phenomenon of a steady decrease in stem diameter over time has been observed, despite heavy applications of P up to 125 kg/ha/year, and normal or even high leaf-17 P-contents.

When P was applied to young oil palms (<3 years old), trunk diameter increased between 19 and 23% (Martin and Prioux 1972). Hawever, the presence of tapering stems may also occur in palms exposed to various stresses, such as a low availability of other nutrients and poor soil aeration (ARAB 2000).

This case study documents a situation where a marked reduction in stem diameter, in oil palms planted in an andisol, is associated with nutritional and soil factors, particularly low soil phosphorus contents, and an abnormal distribution of the element in the plant as a whole.

Age and expected changes in stem diameter

The oil palm maintains a steady increase in trunk diameter throughout the nursery stage and the first years in the field. However, the stem virtually ceases its growth in width before the internodes begin to elongate (Tomlinson, cited by Hartley (1997)). Fluctuations in nutrient uptake may produce changes in stem diameter (Turner and Gillbanks 1974).

Jacquemard (1979) followed the stem growth of a particular palm (Deli x La Mé origin), during a period when the palm was between four-and-a-half, and eight years old, approximately. Stem diameter decreased gradually with age, and at the end of the period (less than four years), the range in stem diameter was between 51 and 33 centimeters, which implied an average reduction of 18 cm.

Historical data from a group of more than 700 Deli-dura mother-palms growing in Coto, Costa Rica, indicated that stem diameter decreased in palms aged between five and ten years old. The range of the data at the end of the period was between 54 and 43 cm and the average reduction in stem diameter was 11 cm. 

The rate of stem growth in diameter with age is the result of an interaction between genotype and environment, and the study of cloned palms reduces genotypic effect to a minimum. An estimate of the environmental effect may be obtained from data of a group of 52 palms of a particular clone planted in 1990 in Coto. When these palms reached five years of age, stem diameter varied between 49 and 67 centimeters.

Experiment with increasing amounts of phosphorus

This experiment involved Deli x AVROS plants planted in 1996 in an andisol with low P soil contents ( Table 1 ). Treatments consisted of increasing amounts of P (0, 25, 50, 75, 100 y 125 kg/ha/year), which were applied in full during the year 2000, and 50% until the time when this document was written. The element is now being applied on the pile of leaves cut during bunch harvesting. During 1999 an attempt was made to level out the soil fertility of the different plots by applying different amounts of P and other elements, according to initial soil fertility. During this period, between 40 and 80 kg of P was applied to the different plots.

In some analysis, P concentration in leaf 17 was found to be adequate, or even relatively high ( Table 2 ). However, the stem diameter had decreased over the years. Trunk diameter, as related to the amounts of P applied during treatment, is shown in figure 1 .

Figure 1 shows a clear tendency toward a reduction in stem diameter. It is also evident that even the highest doses of P applications have failed to revert this tendency to date. On the other hand, leaf-17 phosphorus-contents are not associated with the increasing amounts of P applied. 

Stem growth in palms growing in two soils containing varying amounts of available phosphorus in the soil

The growth pattern of two varieties of oil palm grown in an andisol (with a low content of available P) was compared with that of palms grown in an inceptisol containing a fair amount of available P (around 20 ppm). In the second area, the symptom of pyramidal trunks was uncommon and less conspicuous (not so marked as in the andisol). These areas will be referred as the andisol and inceptisol respectively.

Two plots were chosen in the andisol, where 161 Deli x AVROS and 219 Deli x Ekona palms were randomly selected to obtain a frequency distribution of the measurements of the stem diameter. The data were compared with those obtained from one hundred palms of each variety grown in the inceptisol.

Stem diameter was measured at the height of leaf bases numbers 41 and 81, and the results were expressed as the ratio (S) between those measurements. Values of S smaller than one, would indicate a stem in which the diameter had decreased over time. In order to simplify the comparisons, it was assumed that a value of S smaller than 0.8 indicated a stem with a clear pyramidal tendency.

The percentage of palms with a pyramidal tendency in AVROS growing in the inceptisol was 28%, and 8% in Ekona. In the andisol, the percentage of pyramidal palms in AVROS was 53% and 56% in Ekona ( Fig.2a , Fig.2b , Fig.2c and Fig.2d ).

A routine (commercial) foliar analysis is done during the dry season. Such analysis indicates that the tendency toward a gradual reduction in stem diameter in the andisol has been maintained despite normal or even rather high foliar contents of P (0.20-0.21%) in the whole area (both groups of palms considered: normal and with pyramidal stems). By contrast, in the inceptisol, where the symptom of pyramidal stems was rather uncommon and less conspicuous, leaf levels of P were lower (0.18-0.19%).

Vegetative growth and nutrient content in palms with normal and pyramidal stems

Growth

Vegetative growth of Ekona palms (7 years old) with normal stems grown in the inceptisol was compared with another group of palms (5 years old) with pyramidal stems grown in the andisol. There were no palms of the same age available to make comparisons. To complement the information, tissue samples of leaf 17, and stem (at leaf bases 41 and 81) were also taken. Stem samples were extracted using a borer. Wounds were sealed with a solid wood stick and treated with insecticide and a fungicide. Sampling was done in August, during a period of rather heavy rains.

In the andisol, a total of 15 palms were sampled in each of two categories (normal and pyramidal stems). In the inceptisol, 15 normal palms were sampled, but only seven pyramidal palms were found in the sampling area. Results where compared using a Student T- test.

The phenomenon of pyramidal stems was quite common in the andisol, but not so common in the inceptisol. In fact, the group classified as "normal" in the andisol (S >0.84) contained palms that might well be considered pyramidal in the inceptisol where to be considered normal, a palm needed to have an S value of 0.93 or more. This clearly indicates that the condition of reduced stem diameter with age was more or less generalized in the whole area of the andisol. A t-test mean-comparison between the palms considered "normal" in the inceptisol and the same category in the andisol, showed highly significant differences ( Table 3 ).

Within the inceptisol site, leaf area and the PxS value of leaf 17 of normal palms were statistically higher than in palms with pyramidal stems. However, no significant differences were found when normal and pyramidal palms where compared within the andisol site.

Stem growth is definitively better in all palms grown in the andisol, where even the pyramidal palms have a thicker stem than the normal palms of the inceptisol. An explanation for this cannot be obtained from the present data, as additional information is still required, such as dry matter content in the trunk. However, we may assume that the andisol site provides a better environment for more vigorous growth of the oil palm when, compared with the inceptisol. The better physical characteristics of the soil in the andisol could account for these differences. Soil aeration is excellent in these soils since they facilitate water infiltration. Some previous studies in the inceptisol had shown that drainage was a limiting factor for oil palm growth in this site. Thus, the factor associated with the reduction in stem diameter in the andisol seems to be purely nutritional.

Stem nutrient contents

In August, leaf-17 P-contents were only slightly higher in palms growing in the andisol compared with the inceptisol. No differences between normal and pyramidal palms were observed ( Table 4 ). In all cases, leaf nutrient contents may be considered rather low. This was partly due to the time of sampling, which coincided with a period when the soils were saturated with water, something that is known to adversely affect nutrient uptake.

P content in the upper part of the stem (at the level of leaf base 41) was higher than in the lower portion (at the level of leaf base 81). In the inceptisol, P content in the upper part of the stem of normal palms was the highest of the four categories of palms sampled (normal and pyramidal in the two soils). It was already noted that the palms classified as normal in the andisol had a pyramidal tendency when compared with the "true" normal palms of the inceptisol.

Potassium leaf content may also be considered low in the four categories of palms studied; part of the reason being the sampling season. K contents in the stem were higher in the inceptisol in both categories of palms, which may indicate that K is another element involved in the stem reduction phenomenon, as mentioned by Chan (1982a). Stem K gradient seems to be inverse to that of P in the inceptisol (K contents were higher in the base of the stem). However, in the andisol, K content was similar to those of P along the stem, with higher contents of both elements in the upper portion of the stem. 

Nitrogen contents in the stem at the level of leaf base 41 were higher in the andisol. In the stem, and particularly at the base, there was a tendency to exist more N with respect to K in the andisol.

The P/Zn ratio in leaf 17 (41), and in the upper portion of the stem (43) were similar in normal palms grown in the inceptisol. In palms of the other three categories (with respect to stem diameter) there seemed to be too much P in relation to Zn in leaf 17, when this ratio is compared with that of the upper part of the stem. This situation may indicate a problem in P (and possibly other elements) movement within the different organs of the plant.

If we consider the normal palms in the inceptisol as the standard for comparison, then we should expect a similar P/Zn ratio in leaf 17 and in the upper portion of the stem. This ratio would be altered in favor of P in leaf 17 in those palms where there are conditions leading to the development of pyramidal palms. Of course, more information is needed to conclude that the ratio P/Zn in leaf 17 is a better indicator of the real status of these two elements in the plant.

The condition of the different nutrients in palms growing in the andisol may indicate the presence of unbalances between two or more elements, particularly P, K, N and Zn, which is associated with a gradual stem reduction as the palms grow.

Conclusions

P content in leaf 7 may not indicate the real availability of this element for the oil palm as a whole in some soils.

The information gathered on the vegetative growth of palms planted in an andisol with very low content of available P, indicate that content of this element in leaf 17 -which may be considered adequate, or even high for normal growth of the oil palm-, could be associated (at least in part) with a concentration effect in the tissue due to a sub- optimum development of the aerial part of the plant. However, even palms with a clear pyramidal tendency in the andisol had a trunk volume higher than the normal palms of the inceptisol. This apparent inconsistency may have its origin in the very good physical condition of the andisol, which promotes a vigorous initial growth of the palms. However, such growth can only be sustained through a continuous and adequate nutrition, given the low natural fertility of these soils. 

As a commercial practice, 750 g of DAP is placed at the bottom of the planting hole, providing a rich localized source of available P to help the plant's initial establishment. Several more doses of fertilizations (including P and other elements) are applied very near to the base of the plant during the first years of growth. However, as the plant grows, the fertilization band is widened, and part of the fertilizer is eventually placed on the pile of leaves, which starts to accumulate as harvesting proceeds. This practice increases the amount of element that is fixed to the soil, making a lower portion available to the plant. The end result would be a stem whose diameter decreases over time, and less vigorous foliage growth. A possible (but costly) way to tackle this situation would be to maintain a localized zone for fertilizer placement.

In the inceptisol, physical characteristics are not so favorable and act as a limiting factor for plant growth. However, the natural fertility of these soils is better, and there are no serious problems with respect to fixing P. Palm stems, even though thinner, maintain a cylindrical growth pattern over time.

This case study may indicate that an inadequate P supply during the palm's life may indeed be related to the presence of stems with a pyramidal tendency in oil palm. However, other nutritional unbalances, involving phosphorus, potassium, nitrogen and zinc may also be related to this condition. The P/Zn ratio may provide a better indication of the status of both of these elements in the plant as a whole. 

References

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ARAB. 2001. Pocket guide: Identifying and treating nutrient deficiencies and other disorders in the oil palm ( Elaies guineensis ). Agricultural research and advisory bureau. www.arabis.org

Chan, K.W. 1982. Phosphorus requirement of oil palm in Malaysia: fifty years of experimental results. In:E.Pushparajah; Sharifuddin H.A. Hamid (Eds). Phosphorus and Potassium in the Tropics. Malaysian Society of Soil Science, Kuala Lumpur. p 395-423.

Chan, K.W. 1982a. Potassium requirement of oil palm in Malaysia: fifty years of experimental results. In:E.Pushparajah; Sharifuddin H.A. Hamid (Eds). Phosphorus and Potassium in the Tropics. Malaysian Society of Soil Science, Kuala Lumpur. p 323-423.

Hartley, C.W.S. 1977. La Palma de aceite. Compañía editorial continental, S.A. de C.V., México. 958 p.

Martin, G.; Prioux, G. 1972. Les effets de la fumure phosphatée sur le palmier a huile au Brésil. Oleagineux, 27(7): 351-354.

Salas, R. 1998. Curvas de adsorción de P en suelos andisoles de la compañía Palma Tica. Informe de consultoría. Departamento de Agronomía, PIPA, Coto 47. 9p.

Von Uexkull, H.R.; Fairhurst, T.H. 1991. Fertilising for high yield and quality- The oil palm. IPI Bulletin No.12. Int. Potash Inst., Switzerland pp.79.