Mulch and Antitranspirant application for Water Conservation in Oil Palm Plantations in Costa RicaE. Villalobos, R. A.Ortiz, C. Echandi and H. León ASD Oil Palm papers, Number 6,1992 ABSTRACTApplication of Vapor Gard antitranspirant (VG) and cover crop Pueraria phaseoloides (Roxb) Benth and Desmodium ovalifolium Wall mulch management were evaluated for their effectiveness in soil water conser- vation, in a two-year-old palm (Elaeis guineensis Jacq.) plantation. For each cover crop, an identical experiment was conducted on an Aquic Eutropept, in the Central Pacific coast of Costa Rica, during the dry season of 1992. A RCBD with four replications was used. The treatments were: 1. Total cover crop cutting and application as a mulch in the palm circle, 2. Same as 1, but cutting the cover crop in a 2 m. swath, outside the cleaned palm circle of the same radius, 3. VG applied to the palm, 4. VG applied to the cover crop, 5. VG applied to the palm and the cover crop, and 6. Untreated tester. None of the antitranspirant treatments improved the water status of the palms. Cuttings of cover crop used as a mulch around the palms at the beginning of the dry season improved soil moisture and palm growth, and induced high stomatal conductance. Thus, this practice appears to reduce competition for water between the cover crop and palm trees; furthermore, the mulch reduces evaporation in the palm circle. Application of total P. phaseoloides cuttings as mulch improved canopy growth of the young palms, whereas D. ovalifolium mulch did not affect palm growth. INTRODUCTIONBare soils in agricultural systems in the tropics are exposed to erosion, compaction and leaching of nutrients. Legume cover crops have been widely used in oil palm plantations to prevent these negative effects and to improve physical and nutritional characteristics of the soil, as well as to suppress weeds (Mendham, 1971; Bourke, 1975; Broughton,1976; Han and Chew, 1982; Yeow et al., 1982). Pueraria phaseoloides has normally been one of the best adapted legumes in several oil palm producing areas of the world (Mendham, 1971; Bourke, 1975; Yeow, et al., 1982) . This aggressive legume can fix large amounts of atmospheric nitrogen but requires labor to prevent it from climbing on young palms (Yost and Evans, 1988) . More recently, attention has also been focused on low-growing legumes, such as Desmodium ovalifolium , which has been more exploited in pasture mixtures, but has also shown potential as living mulch (Yost and Evans, 1988). Desmodium ovalifolium has a relative shade tolerance and, although it was not the best cover crop in several trials conducted in oil palm plantations in Malaysia, it performed well (Broughton, 1976; Han and Chew, 1982). Oil palms growing with legume ground covers usually show better growth, nutrition, and yield (Broughton, 1976), than monocropped palms. However, under some circumstances, cover crops can compete for water and nutrients with the economic crop (Jordan, 1982; Domínguez and de la Cruz, 1990). Literature concerning the competition for water of cover crops in oil palms is lacking, probably because some of the major oil palm producing areas of the world, such as Malaysia, are not exposed to prolonged periods of water deficit (Broughton, 1976). However, this is not the case for the Central Pacific region of Costa Rica, where the palms are exposed to prolonged periods of drought (Villalobos et al., 1991). Observations in young oil palm plantations made during the dry season in Quepos (H. León and E. Villalobos, unpublished results) indicated that P. phaseoloides, competes for water at the beginning of the dry season, at least. Then, the legume closes its stomata and defoliates almost completely, thus reducing its capacity to compete for water. The use of legume ground covers in green-manuring tree crops in the tropics is an old cultural practice (Salgado, 1936). A major advantage of using N2 fixing legume mulch is the input of nitrogen and the improvement in nutrient cycling which favors palm growth (Broughton, 1976). No information regarding the management of legume cover crop cuttings as a mulch, to reduce soil water evaporation was found. However, the use of oil palm wastes as mulch in oil palm plantations improves soil water conservation and promotes palm growth and yield (Chan et al.,1980; Hartley, 1981; Ortiz et al., 1992). For instance, Ortiz et al., (1992) found that the use of empty fruit bunch mulch in one-year-old palms in the Central Pacific region of Costa Rica, improved soil moisture, stomatal conductance and the palm water status during the dry season and produced a build-up in K and other nutrients in the soil. Antitranspirants reduce plant water losses and are sometimes used to prevent plant dehydration after transplanting (Gale and Hagan, 1966). This research was conducted in two-year-old oil palm plantations with D. ovalifolium or P. phaseoloides as cover crops. The objective was to evaluate the effectiveness of several treatments to increase water use efficiency or to improve plant water status during the dry season. The antitranspirant was also used to better understand the competition for water between the legumes and the palm trees. MATERIALS AND METHODSTwo experiments were conducted in an Aquic Eutropept in the Central Pacific region of Costa Rica, at 09° 31' N, 84°15' W and 6 meters of altitude. Rainfall records during the experimental period are shown in Fig. 1 . The palms were two-year-old Deli X AVROS. In one experiment, P. phaseoloides was the cover crop and D. ovalifolium was used in the other. Both legumes were in the second year of growth and were well established. Both experimental plots were set up under uniform soil conditions, as determined by a texture analysis. A RCBD with 4 replications was used. The treatments applied in each experiment were:
Each experimental unit consisted of 3 palms. Predawn leaf water potential (PDLWP) measurements were taken with a pressure chamber model PMS-600. The leaflets used to take these reading were those from the central part of the leaf 9. The leaflets were introduced in the chamber immediately after excision to obtain the reading, avoiding second cuttings of the petiole to reduce any eventual alteration of the measurement. One plant from each experimental unit was selected to take the PDLWP readings. The PDLWP data were taken from three replications on each sampling date, so that an analysis of variance could be conducted. First measure- ments were taken approximately one hour before sunrise and artificial lighting was used to assure precision in the reading. The abaxial water vapor conductance (Gab) at midday was determined directly, from the central part of the leaflets of similar position than those used to measure PDLWP, using a diffusion porometer (LI-700). Soil moisture was determined gravimetrically. Canopy growth measurements were taken at the beginning and at the end of the experiment, following the procedures of Corley and Breure (1981). A combined statistical analysis of variance and orthogonal contrasts was used to compare PDLWP in response to different treatments. A FPLSD test was used to compare canopy growth measurements in response to treatments in both experiments. The capacity of recovery of the legume crops after cutting was evaluated through monthly visual observations. RESULTS AND DISCUSSIONEffectiveness of the Antitranspirant Vapor GardSoil moisture, Gab and PDLWP values of the palms that received the three antitranspirant treatments were very similar. For this reason, and to avoid an overload figure, the average values of the three treatments for these parameters were drawn in one line in Fig. 1. The film-forming antitranspirant VG was applied at the highest recommended rate (2% V/V), but its effect in reducing Gab was negligible (Gab values similar to the untreated control treatment) 3 days after its first application (15 Jan) and 11 days after the second (28 Jan), in both experiments (Fig. 1). The percent reduction of transpiration caused by the antitranspirant is defined by the magnitude of the resistance ratio of the antitranspirant over the sum of the air layer and the stomatal resistances (Gale and Hagan, 1966). This means that under conditions of water deficit, the relative effect of the antitranspirant will be reduced concomitantly with the increase in stomatal resistance. However, Gab data recorded on 15 Jan., indicate that VG did not yield the expected results in reducing transpiration, since the stomata, particularly those in the Desmodium trial, were still open (Fig. 1). The antitranspirant was not effective in reducing Gab in the cover crop legumes either, as was confirmed a few days after the applications of the product. Apparently, the VG had a positive effect in reducing non-stomatal water losses from the palms and the cover crop, since the PDLWP values of the palms in the plots treated with VG were higher than those in the untreated control, when water deficit was more severe, in both experiments (Fig. 1). The palms treated with VG suffered from the infection caused by the fungus Curvularia sp. a few days after the second application of the product. Apparently, the antitranspirant creates a favorable microenvironment for the establishment of that disease. These results were unexpected since many antitranspirants have shown a mechanical barrier against the incidence of some pathogens (Gale and Hagan, 1966). Cover Crop Mulch EffectThe use of cover crop cuttings (total or partial) as a mulch around the young palms improved soil moisture and induced higher values of PDLWP and Gab as compared to the untreated palms, throughout the dry season (Fig. 1, Table 1). It is worth mentioning that PDLWP was a very useful water stress indicator, as was previously reported (Villalobos, et al., 1991). The rationale of using the cover crop mulch treatments was effective, since it reduced the eventual competition for water, when the cover crop was cut at the beginning of the dry season. On the other hand, evaporation of water from the palm circle was also reduced with the legume mulch, similar to what has been observed with the application of industrial oil palm wastes around the palms (Khoo and Chew, 1979; Chan et al., 1980; Hartley, 1981; Ortiz et al., 1992). The partial cutting of the cover crop was intended to enhance a fast recovery of the legume upon termination of the dry season. No difference in PDLWP was found between the partial and the total mulch treatments (Table 1). However, the greater amount of P. phaseoloides mulch caused an increase in palm canopy growth (Table 2), whereas it did not occur in the D. ovalifolium experiment (Table 3). This is a good indication that P. phaseoloides is more competitive for water and probably for nutrients than D. ovalifolium. Additional observations such as the higher Gab, the greater leaf area and the faster recovery of P. phaseoloides in response to the sporadic rains that occured during the dry season, also suggest this behavior. These observations, however, cannot be proved with the statistical model used. Nevertheless, the competition for water during the dry season per se is not the most important factor to discriminate between these two legumes, since the level of competition detected is still far from that observed between Arachis pintoi and the heart palm (Domínguez and de la Cruz, 1990). Pueraria phaseoloides showed greater recovery after cutting as compared to D. ovalifolium after the end of the dry season in May (Table 4). Pueraria phaseoloides reached 100% recovery 45 days after the end of the dry season. In contrast, D. ovalifolium required approximately 135 days to reach a percent recovery over 80% (Table 4). A greater invasion of both, broadleaf and grass weeds was observed in the D. ovalifolium plots, as compared to the P. phaseoloides plots. In general, D. ovalifolium showed a slow recovery upon rewatering, even in those plots where it was partially cut. The proliferation of weeds in the Desmodium mulch plots due to the low aggressiveness of this species upon rewatering, precludes the cutting of this cover crop for use as a mulch during the dry season. No treatment effect was observed on the average number of visible spear (unfolded) leaves (Table 5). The number of spears usually correlates well with the intensity of water stress that the palms are suffering (Villalobos, et al, 1992). However, these results have shown that PDLWP and Cab are more sensitive to changes in soil moisture than the number of spear leaves accumulated (Fig. 1). CONCLUSIONSThe use of P. phaseoloides ground cover as a mulch on the palm circle at the beginning of the dry season is a recommendable cultural practice for young oil palm plantations exposed to prolonged periods of water deficit. This cultural practice reduces the eventual competition of the cover crop for water, protects the palm circle from evaporation and probably concentrates organic matter and nutrients around the palms. This recommendation, however, is not totally suited for D. ovalifolium or less aggressive legumes that may have difficulties in recovering upon the termination of the dry season, since this practice jeopardizes the reestablishment and even the survival of the legume. A reduction in irrigation frequency with the use of cover crop mulching can be expected. However, experimental confirmation is needed. ACKNOWLEDGEMENTSThe authors wish to acknowledge Mr. Luis García, Palma Tica, Quepos Division for his support and cooperation provided to conduct this research. We also thank Dr. D.L. Richardson for his advice and the manuscript review and Mr. Guido Monge for the data processing. REFERENCES
.
|