Summary

Two bioassays were used to estimate the nutritional value and residual effect of compost made with fiber from empty fruit bunches of the oil palm. In a field experiment, sorghum (indicator plant) was planted in micro plots in an Inceptisol that was mixed with increasing amounts of the compost (treatments). A control plot received urea as a source of N. The results of this bioassay were compared with those from another bioassay conducted under laboratory conditions, where microbial growth was measured in mixtures of soil and similarly increasing amounts of compost.

Sorghum aerial dry matter accumulation 37 days after sowing, regressed significantly (P<0.05) with the increasing amounts of compost mixed with the soil, which indicates that the compost released nutrients in the short term, and that the availability of several nutrients increased with larger amounts of compost. This relationship tended to disappear during the following harvestings of the indicator plant. Still, the use of urea following the fourth cut was associated with the presence of statistical differences among treatments in the aerial contents of N,P,K and Zn during the fifth harvest.

The rather high contents of K, P and Zn in the sorghum tissue occurred despite the low soil contents of these elements, which indicated that the compost supplied part of these requirements in the short term (six and a half months). Dry matter accumulation and tissue N content in sorghum increased up to the addition of 5% compost to the soil in all cuts realized. Apparently, part of the N readily available in the compost may be lost through leaching or volatilization. In this respect, it behaves likes ureic N. The correlation between microbial biomass as determined in the laboratory, and dry weight of the sorghum in the field, was significant only for the data from the first harvest of the sorghum. We conclude that the microbial assay can only be used to estimate the nutritional value of the compost in the short term. Apparently, most of the N in the compost forms part of very complex organic molecules that are difficult to mineralize. However, the data of this assay indicates that the compost produced from empty oil palm bunches is of good quality.

Introduction

The process of extracting oil from the oil palm bunches generates large quantities of empty bunches, which may become a source of environmental pollution. Nevertheless, despite its high C/N ratio, this material is rich in some nutrients, and can be returned to the field as mulch. However, empty bunches are heavy and bulky, so the cost of transport per unit of nutrient is rather high and the distribution in the field is expensive and cumbersome, particularly during the rainy season (Huan 1989, Uexkull and Fairhurst 1991, Torres et al . 1999)

Each ton of fresh fruit bunches at the extraction mill generates approximately 0.22 t of empty bunches, and between 800 and 900 l of effluents and other by products. For instance, one of the mills located in Quepos, Costa Rica, processes approximately 100 000 t of fresh fruits each year, and the empty bunches contain large amounts of some nutrients: 90 t N, 43 t P and 143 t of K (Torres et al . 1999).

These residues can be used to make a compost that can supply part of the nutritional requirements of the oil palm, improve some of the physical, chemical and biological properties of the soil, and at the same time, solve a potential pollution problem.

To determine the amounts of compost to be supplied to the crop, it is first necessary to know the real nutritional value of these organic fertilizers (Vandevivere and Ramírez 1995a, Keeney 1985). For this purpose, several methodologies have been developed, based on quantitative measurements (Dick and McCoy 1993, Vandevivere and Ramírez 1995a). The "soil type" analysis of compost, which includes the use of an extracting agent, determines the interchangeable nutrients that are supposedly available to the plants. Such an analysis usually underestimates the real amounts of nutrients available in the compost, since it does not consider that a part of these nutrients is not available in the short term, and in order to be released they have to go through a mineralization process. On the other hand, a “tissue-type” analysis estimates the total nutrient content through a complete digestion of the sample with a strong acid. This type of analysis overestimates the nutrients present in the compost since, for example, not all nitrogen can be mineralized, but can be present in recalcitrant forms. Another difficulty is that these analyses do not provide information on the period when the nutrients would be available (Vandevivere and Ramírez 1995a).

Vandevivere and Ramírez (1995b) developed a methodology to estimate the nutritional value of an organic fertilizer, which consists in stimulating the growth of the native microorganisms by adding glucose and a protozoan inhibitor. In this way, the available carbon does not become a limiting factor for microbial growth in a substrate, consisting of mixtures of soil with increasing amounts of compost. The increase of the microbial mass is determined by the amount of nutrients available in the mixture, and particularly in the compost. Microbial mass is measured through the method of induced respiration of a substrate (Anderson and Domsch 1978). Glucose is used to induce a maximum of respiration response of the microorganisms present in the soil, measured as CO₂ evolution, which is related to the biomass of the micro organisms (Anderson and Domsch 1978).

Microbial growth in the laboratory is also related with the uptake of nutrients and the growth of sorghum plants in the field (Salas 1997, Salas and Ramírez 2000). The amounts of nutrients that limit growth of the micro organisms apparently behave in a similar way to those that limit plant growth.

The objective of this research was to evaluate the utility of a microbial bioassay to predict the availability of nutrients in compost made from empty fruit bunches, to be used as a guide to estimate the amounts of such compost to be used in the field. Additionally, an indicator plant was used to estimate the ability of the compost to supply nutrients over a period of time.

The data of microbial biomass were correlated with those from the field bioassay with sorghum. In both bioassays, the soil was mixed with equivalent quantities of compost. To estimate the residually of the compost, successive cuts of the re-growth of the aerial part of the sorghum were performed to determine accumulation of dry matter and nutrient contents.

Materials and methods

The extraction mill in Quepos has a side process, whereby empty fruit bunches are shredded by rotating circular saws in order to obtain some extra oil after passing the fiber through a press. This fiber represents 12-15% of the fresh fruit bunches and was used as the raw material to make the compost (Torres et al . 1999).

Raw fiber was enriched with effluents (300 l/m 3 ), urea (10 kg/t), sludge (20 kg/t), calcium carbonate (6.6 kg/t) and phosphorus (0.86 kg/t), and arranged in composting beds of 7.5 m³ . Aeration was supplied manually by shoveling every week. During the first months, moisture was adjusted between 45 and 55%. After four months the compost was considered ripe. The physical and chemical characteristics of the raw material and the product are shown in table1 , table 2 and table 3 .

Field bioassay: compost as a nutrient supplier to the sorghum

The bioassay was conducted on land belonging to the Palma Tica Company in Cerros (Damas, Quepos, Costa Rica), in an area previously occupied by an old oil palm plantation that had been replanted a few months earlier. The soil was classified as a Fluventic Haplustepts. (Mata R. 2000; personal communication ). ( Table 4 ).

Sorghum ( Sorghum vulgare ) micro plots (1.25 m² : 1.25 m x 1 m) were spaced at 30 cm. Planting density was 18 cm between rows and seed was sowed in a continuous pattern. The superficial soil (first 20 cm) was mixed with increasing amounts of the compost: 0, 2.5, 5.0, 7.5, 10, 12.5 %, equivalent to 0, 58, 116, 174, 232 and 290 t of compost/ha. An additional treatment was included with 0.5 t of urea/ha (230 kg N/ha). Treatments were arranged in a complete randomized block design with four replications.

Fresh and dry weight of the sorghum was determined in successive cuts of the aerial part of the growth in 625 cm² within each plot. Tissue samples were taken for chemical analysis of nutrients. At the end of the experiment, the total dry weight accumulated was compared among treatments. With this data the residual effect of the compost was estimated in the short (1-3 months) and mid terms (seven months). The first cut of the aerial part of the sorghum was done 37 days after sowing. Successive cuts were done approximately every five weeks, except for the second one, which was done when plants were 52 days old.

Nitrogen was determined for all plots in the material collected in the first cut. A complete analysis (all elements) was also carried out for all the treatments, but combining the four replications. During the following harvest, an N determination was done for all treatments (replicates combined), and a complete analysis for the treatments one (control) and three (5% compost), combining all four blocks. The procedure was alternated until the fourth harvest. During the fifth cut, a complete nutrient analysis was conducted for all plots. Given that no differences between treatments were observed after the first cut, in terms of dry weight and nutrient content, it was decided to add urea to all treatments after the fourth cut.

Microbial bioassay

The bioassay was conducted at the Biotechnology Laboratory of the Agronomy Faculty of the University of Costa Rica, using the methodology of Vandevivere and Ramírez (1995b). The compost was mixed with soil in the same proportions (%w/w) that were used in the field bioassay with sorghum. A treatment using only urea (12.6 mg N/50g of soil) was also included.

Both the compost and the soil were passed through a 2 mm mesh and then air-dried. Treatments (increasing amounts of compost mixed with soil) were arranged in a complete randomized block design. Each of three blocks was a group of experimental units that received the treatments during the same incubation period, under the same environmental conditions (i.e. relative humidity, temperature), and handling (respiration measurement). The whole experiment was repeated three times. Each experimental unit was a one-liter Erlenmeyer with 50 g of the mixture soil/compost with moisture adjusted to field capacity.

Microbial biomass determination 

Protozoan activity was inhibited with triton. Glucose (1 % of the mixture soil/compost: 0.5 g/flask), was added and then the mixture was incubated in the dark for 48 hours. Microbial biomass was estimated with the "substrate-induced respiration method" (RIS) of Anderson and Domsch (1978), with the modifications described by Cheng and Coleman (1989). After the incubation period, 0.5 g of glucose were again added to each flask and mixed with the substrate that was let standing for 30 minutes. Flasks were then connected to an air flux system ( Fig 1 .) to eliminate the CO₂ that had accumulated in the tubing and flasks. The air flux was passed through a NaOH (0.2N) trap to be cleaned of CO₂ and was then pumped in a 2.5 cm tube (internal diameter), which has several terminals where the flasks are connected.

After fluxing the system from CO₂ , traps (60 ml assay tubes with 40 ml of NaOH 0.05M) were connected and air was allowed to bubble for an hour to trap the CO₂ produced by the microbial activity. Air flux was at least 3 l/h. Empty flasks were also connected to the system (controls) with their respective CO₂ traps.

The product of the reaction (NaOH + CO₂ = HCO + H₂ O), was quantitatively transferred to 250 ml Erlenmeyer flasks and 6 ml of BaCI₂, (0.2 N) was added to each to form barium carbonate, which was titrated with HCI 0.2M after adding three drops of phenolphthalein as an indicator.

Results and Discussion

Biomass production and nutrient content determination in the aerial portion of the sorghum. Sorghum dry weight (mainly in first, second and third cuts) tended to increase with larger proportions of compost added to the soil ( Fig. 2 ). This response was also evident in the nitrogen content of the aerial part of the plant in all cuts ( Fig. 3 ). However, no statistical differences were found between treatments after the first cut for the dry weight variable. For N content, statistical differences were observed during the third and fifth cuts.

The microbial biomass in the laboratory correlated with the dry weight of the sorghum, only during the first harvest of the aerial portion of the plants. This indicates that the compost released nutrients in the short term (37 days), and the availability of some elements (mainly P, K and Zn) increased with the amounts of the compost mixed with the soil ( Table 5 ).

Aerial nutrient contents were similar in the urea treatment and in the control without compost, except for nitrogen, which was the highest during the first cut in the treatment with urea. ( Table 5 ). This is a somewhat expected result, since urea only supplies N, but compost also supplies important quantities of other nutrients. This is one of the reasons for the rather low dry matter production observed in the treatment with just urea ( Fig. 2 ).

Accumulated dry weight ( Fig. 4 ) and N contents of the aerial portion of the sorghum ( Fig. 5 ) may indicate that the plant's uptake was saturated with 5% compost in the soil, since beyond this value there was no increase in the values of these variables.

The total amount of N accumulated (all cuts) in the aerial portion of sorghum is only a small fraction of the total N estimated to be present in the different amounts of compost added to the soil ( Fig. 6 ). Besides this, the increment in N contents in the sorghum, with increasing amounts of compost added, was not proportional with the nitrogen added. The treatment with 5% of compost was the highest with just 12% of N supplied by the compost. On the other hand, the treatment with 7.5% of compost supplied an estimated 8.3% of N. This may indicate that the organic matter in the compost formed highly stable N compounds that behaved as a slow release N source, which was not available to the plant on the short term (Nommik 1965).

Evaluating the data from the microbial bioassay in the laboratory, it was calculated that only 17.7% of the N estimated to be present in the treatment with 5.5% compost, was in an easily available form. However, the field bioassay had determined that only 12% of that N was actually incorporated into the aerial part of the sorghum, up to the fourth cut ( Fig. 6 ). The conclusion, then, is that in the short term, the sorghum was not very efficient in using up the N available in the compost. In addition, the data indicates that there was not much slow release N available in the compost after the second cut.

The N present in a "slow release" condition in the compost could not be estimated with precision in the field assay, since the evaluation period was rather short. The presence of a slow release form of N in the compost has the advantage of supplying small amounts of the nutrient over a period of time, and also reduces losses. Both conditions are desirable in a perennial crop like oil palm. This factor must be considered when adjusting the amount of compost to be used and the time for its application in the field.

Residual effect of the compost

Yields of dry biomass and N contents decreased in successive cuts of the aerial portion of the sorghum plants, falling to a minimum during the fourth cut ( Fig. 2 and Fig. 3 ). This is a clear indication of the reduction of the residual effects of the compost, or the presence of an adverse factor affecting uptake of nutrients. Nonetheless, during the fifth harvest, significant differences appeared between treatments with respect to N, P,K and Zn contents in the tissue, as a consequence of the application of urea after the fourth cut. This response can be explained only in terms of a residual effect of the compost, which supplied those nutrients in direct proportion to the quantities added to the soil. The response also indicates that N was the limiting element. The ion NH4+ from the urea can be retained in the multiple interchangeable sites available in the organic matter, and this will increase the availability of the nutrient (Mohammed et at . 1991).

The amounts of N, P, K and Zn in the aerial part of the sorghum were related to dry biomass production. During the first, third and fifth cut, these variables tended to increase with larger amounts of compost added to the soil. Soil contents of K, P and Zn were originally low in the area ( Table 4 ). However, the sorghum plants had adequate contents of these elements, in accordance with reference levels (Veroni 1997; Jones et al. 1991). It can be assumed then, that the compost supplied most of those nutrients.

Tissue manganese tended to decrease with increasing amounts of compost added to the soil, and these differences were significant during the fifth cut. Manganese deficiency is associated with high organic contents in the soil (Cresser et al . 1993, Tan 1994; Schatz et al . 1964; Dick and McCoy 1993; Bolle -Jones 1976).

Microbial bioassay as an indicator of the nutritional value of the compost

The correlation between microbial biomass as determined in the laboratory and dry weight of the sorghum plants during the first cut (37 days after planting) was statistically significant (r= 0.83; P=0.007, Fig.7 ). This correlation, however, was not significant for the other cuts of the sorghum plants. This indicates that the microbial bioassay can be a very useful tool to estimate the nutritional value of the compost only in the short term.

The microbial growth obtained in the laboratory bioassay also indicates that the compost made from oil palm empty fruit bunches is of very good quality: values of 3 mg of microbial carbon were obtained when the compost was mixed in a proportion equivalent to 10% of the top soil ( Fig. 8 ). These data are similar to those obtained with other high quality composts (Salas and Ramírez 2000). Microbial growth showed values of over 2 mg of microbial carbon/0,1g in mixtures with more than 5% compost. These values are considered intermediate, but high enough to supply the nutritional requirement of the sorghum during the first month and a half (Salas 1997; Salas and Ramírez 2000).

Bibliography

Anderson, J.P.E., Domsch, K.H. 1978. A physiological method for the quantitative measurement of microbial biomass in soils. Soil Bio. Biochem.10:215-221.

BOLLE -JONES, E.W. 1976. Organic matter in relation to soil fertility, p 212- 219. In FAO (ed). Organic recycling in Asia. Papers presented at the FAOISIDA workshop on the use of organic materials as fertilizers in Asia. Rome, Italy.

Cheng, W., Coleman, D.C. 1989. A simple method for measuring CO₂ in a continuous airflow system: modifications to the substrate-induced respiration technique. Soil Bio. Biochem. 21 (3): 385-388.

CRESSER, M; KILLHAM, K; EDWARDS, T. 1993. Soil chemistry and its applications. New York, USA. Cambridge University Press.192 p.

DICK, W.A; McCOY, E.L. 1993. Enhancing soil fertility by addition of compost, p. 622 -644 In H.A Hoitink and H.M Keener (ed) Science and engineering of composting. Renaissance Publications. Worthington, Ohio, USA.

HUAN, L.K. 1989. Trial on Composting EFB of Oil Palm with and without Prior Shredding and Liquid Extraction, p 217-224. In International Palm Oil Development Conference. PORIM. Malaysia.

JONES, J.B; WOLF, B; AND MILLS, H.A. 1991. Plant Analysis Handbook. Methods of Plant Analysis and interpretation. Athens, Georgia, USA. Micro -Macro Publishing, Inc. 213 p.

Keeney, D.R. 1985.Nitrogen management for maximum efficiency and minimum pollution. p. 605-649. In. Stevenson, F.J. (ed). Nitrogen in agricultural soils. ASA, Inc. Publisher, Wisconsin.

MOHAMMED, A.T; ZAKARIA, Z.Z; DOLMAT, M.T; FOSTER, H.L; BAKAR, H.A; HARON, K. 1991. Relative Efficiency of Urea to Sulphate of Ammonia in Oil Palm: Yield Response and Environmental Factors, p 340 -348. In Inti. Palm Oil conference, Agriculture. PORIM. Malaysia.

NOMMIK, H. 1965. Soil Nitrogen: Ammonium fixation and other reactions involving a non-enzymatic immobilization of mineral nitrogen in soil. Ed. By Bartholomew, w. V. Clark, F. E. Madison, Wisconsin, USA; American Society of Agronomy. p: 240 -248.

RUSSELL, R.S. 1977. Plant root systems: Their function and interaction with the soil. Great Britain, England. McGraw- Hill Book Company (UK) Limited. 298 p

SALAS, E. 1997. Bioensayo microbiano para estimar los nutrimentos disponibles en los abonos orgánicos: Calibración en el campo. Thesis Master of Science. University of Costa Rica. Faculty Agronomy, Escuela de Fitotecnia. 85 p.

SALAS, E; RAMIREZ, C. 2000. Bioensayo microbiano para estimar los nutrimentos disponibles en los abonos orgánicos: calibraci6n en el campo. To be published in Agronomía Costarricense.

SCHATZ, A; SCHATZ, V; SCHALSCHA, E.B; MARTIN, J.J. 1964. Soil organic matter as a natural chelating material. Part I: The chemistry of chelation .Compost Science, p 25- 28.

TAN, K.H. 1994. Environmental Soil Science. New York, Marcel Dekker, Inc. 255 p.

TORRES, R; CHINCHILLA, C; RAMIREZ, C. 1999. Compostaje de los desechos agroindustriales de la palma aceitera, p. 411 -418. In F. Bertsch; J. Garcia; G.Rivera; F. Mojica; W. Badilla (ed). XI Congreso Nacional Agronómico y de Recursos Naturales. UNED, San Jose, Costa Rica.

Uexkull, H.R., Fairthurst, T.H. 1991. The oil palm. Fertilizing for high yield and quality. IPI. Bulletin #12. Bern/Switzerland. 79p.

VANDEVIVERE, P; RAMIREZ, C. 1995a. Control de calidad de los abonos orgánicos por medio de bioensayos. In Simposio Centro Americano Sobre Agricultura Organica J. A. Garcia, J.M. Najera (ed) UNED, San Jose, Costa Rica. p 121 -140.

VANDEVIVERE, P; RAMIREZ, C. 1995b. Bioensayo microbiano para determinar los nutrimentos disponibles en abonos organicos. BOL TEC 28 (2): 90 -96.

VERONI, M. 1997. Plant analysis: an interpretation manual. Second edition. Australia.CSIRO. 572