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Nutrient cycling within the developing oil palm-legume ecosystem
Agriculture, Ecosystems and Environment, 13 (1985) 111--123 Elsevier Science Publishers
B.V., Amsterdam
--
111
Printed in The Netherlands
NUTRIENT CYCLING WITHIN THE DEVELOPING LEGUME ECOSYSTEM
OIL PALM--
P. AGAMUTHU and W.J. BROUGHTON*
Department of Genetics and CellularBiology, University of Malaya, Kuala Lumpur (Malaysia) (Accepted for publication 13 December 1984)
ABSTRACT Agamuthu, P. and Broughton, W.J., 1985. Nutrient cycling within the developing oil palm--legume ecosystem. Agric. Ecosystems Environ., 13: 111--123. Oil palms (Elaeis guineensis Jacq.) are normally planted in fields stripped of all other vegetation. Leguminous cover crops are planted simultaneously to protect the soil surface and to provide other, less tangible benefits. Nutrient cycling, especially of nitrogen, was followed in a commercial oil palm plantation and showed that: (a) legumes contribute about 150 kg nitrogen ha-' year-' to the system through nitrogen fixation; (b) during the early stages of oil palm growth the legumes absorb 149 kg nitrogen ha-' year-' from the soil; (c) with a loss of nitrogen through litter fall of 123 kg nitrogen ha-' year-', legumes accumulate a net amount of 176 kg nitrogen ha-' year-' in their foliage; (d) in comparison with "natural" covers, leguminous covers reduce leaching losses by 63 kg nitrogen ha-' year-' so that the total benefits of leguminous covers amount to 239 kg nitrogen ha-' year-'; (e) the combined inputs from fertilization and oil palm debris in plots without legumes is 208 kg nitrogen ha-' y e a r - ' ; and (f) during the initial growth phase, oil palms need only 175 kg nitrogen ha-' year-'. Since legumes fix nitrogen and thoroughly scavenge the soil for minerals they eventually provide more nitrogen to the oil palms than is needed for growth. This surplus nitrogen is " b a n k e d " in the legume foliage. Then, when the rooting system of the oil palms has grown under the inter-rows, competition for nutrients causes a gradual decline in the cover~crop. Nutrients "deposited" in the legumes are thus slowly re-released stimulating root-growth and general development of the oil palm.
INTRODUCTION Tropical soils are generally deficient in nitrogen (Date, 1973). In young oil p a l m s (Elaeis guineensis J a c q . ) , i n s u f f i c i e n t n i t r o g e n c a u s e s p a l i n g o f t h e seedlings, yellowing of the leaves, and eventually necrosis (Bull, 1957). N i t r o g e n d e f i c i e n c i e s in m a t u r e p a l m s s i g n i f i c a n t l y i n c r e a s e f l o r a l a b o r t i o n a n d t h e r e b y r e d u c e y i e l d s ( B r o e s h a r t e t al., 1 9 5 7 ) . A c c o r d i n g l y , n i t r o g e n h a s *Present address: Max-Planck-Institut fiir Ziichtungsforschung, D-5000 KSln 30, Federal Republic of Germany.
0167-8809/85/$03.30
© 1985 Elsevier Science Publishers B.V.
112
to be carefully husbanded in commercial plantations. Obviously, nitrogen deficiencies can be cured by application of any one of a number of fertilizers (Pushparajah et al., 1974; Hartley, 1977). Sowing leguminous crops simultaneously with oil palms is another, and potentially cheaper, way of boosting nitrogen levels in the palm fronds (see Broughton, 1976). Growth and yield of fresh-fruit bunches (fib) are thereby stimulated, often to levels greater than that achievable with fertilizers. It has been observed that the effect of enhanced nitrogen levels persists long after the legumes have died out (due to competition for light or other causes). These benefits do not seem to stem directly from nitrogen fixation by the legumes, but rather as a long-lasting effect that pre-conditions the palms to higher yields (Broughton, 1976). One possible explanation concerns nutrient cycling within the oil palm--legume ecosystem. Measurements were made of various agronomic and physiological parameters in an oil palm-legume plantation especially with regard to nitrogen. This paper summarizes the findings. MATERIALS AND METHODS
Experimental site The clearing of 92 ha of old rubber land at the Damansara Estate, Batu Tiga, Selangor began in September 1975. Of this area, 6.5 ha was set aside for special attention, while the rest of the field was planted with legumes and oil palms in the normal estate manner. One-year-old oil palm seedlings raised in large polyethylene bags in the nursery were planted in the experimental block in October 1975. Each was set 8.9 m apart on an equilateral triangular pattern. Then the entire experimental area was divided into 32 plots, the boundaries marked, and treatments randomly allocated. Table I gives details of the soil (sandy clay loam) in the experimental site. The mean maximum daily temperature was 33.8 + 0.2°C, whereas the minimum was 21.4 -+ TABLEI Some properties of the soil in the experimental plots I Stone (%) Gravel (%) Coarse sand (%) Fine sand (%) Silt (%) Clay (%) pH Organic carbon (%) Total nitrogen (%) Ca (mEq%)
0 0 22.3 31.6 7.7 38.1 4.1 1.4 0.16 1.0
I Full details of the analytical methods employed are given in Agamuthu (1979).
113
0.2 ° C. The mean maximum daily relative h u m i d i t y was 95.7 + 0.3% while the m i n i m u m was 59.2 + 0.5%. Further information is given in Agamuthu (1979).
Experimental design Randomized blocks of eight treatments, replicated four times, and with fully guarded plots were laid out in the experimental area in October 1975. Each plot covered an area of 0.17 ha and contained 25 palms. Only the central nine palms (or the area they occupied), were used for measurements. Five of the eight treatments (all eight are described in Agamuthu et al., 1980) were relevant to this paper, and these were: Ca) " N a t u r a l " regeneration consisted mostly of the grass Paspalm conjugarum, but later the creeper Mikania cordata and the fern Nephrolepis biserrata invaded the plots. Pueraria phaseoloides also occurred sporadically. Woody shrubs and noxious weeds (e.g., M. cordata) were regularly removed by hand. (b) "Bare". This treatment was used mainly to quantify nutrient cycling in the system. Except for the oil palms, the land was maintained free of vegetation by a combination of hand-weeding, and Paraquat application (see below). (c) " C o n t r o l " . After clean-weeding each legume plot, three parallel drills, 2.1 m apart, were hoed between the rows of palms. Scarified Centrosema pubescens and P. phaseoloides seeds (2:3 by weight) were mixed with the appropriate a m o u n t of Rhizobium (UMKL44 -- see Broughton and John, 1979) and planted into the drills Crate of seed application -- 8.0 kg ha-'). Loose soil was then pressed back over the seeds. Plots were maintained free of weeds by repeated hand-weeding (at roughly m o n t h l y intervals). (d) "Fertilizer". Plots were prepared and maintained exactly as described for " C o n t r o l " [(c) above] except that palms received twice the normal fertilizer dosage. (e) " O x y f l u o r f e n 0.25". Oxyfluorfen or " G o a l " or RH-2915 = 2-chloro-1(3~thoxy-4-nitrophenoxy)-4-(trifluoromethyl)benzene (Rohm and Haas Company), was applied to legumes sown as described in (c) the day after planting at a rate of 0.25 kg a.i. ha-'. The herbicide was applied in water (560 1 ha-') with a 9-1 pump operating at about l 0 s Pa pressure. A series of strips 2-m wide were sprayed over the entire area using a 1.6 mm internal diameter nozzle. Clean-weeded circles (2 m diameter) were established around each palm, and maintained in this condition throughout the experiment by a combination of hand-weeding and prophylactic herbicide application (Paraquat at 90 ml 15 1-' water). Fertilizers were applied to palms in all the plots on the following basis.
114 Date
Fertilizer
R a t e ( 1 5 0 p a l m s h a -I )
March 1976 J u n e 1976 A u g u s t 1976 S e p t e m b e r 1976
CIRP 1 CIRP 1 CCM-112 CIRP
100 100 125 100
kg kg kg kg
h a -~ h a -~ h a -~ h a -~
C h r i s t m a s Island rock p h o s p h a t e (15% p h o s p h o r u s ) . 2 A c o m m e r c i a l fertilizer c o n t a i n i n g 11% n i t r o g e n a n d 8% p h o s p h o r u s . Palms in " F e r t i l i z e r " p l o t s received twice t h e fertilizer dosages listed here.
Analytical Assessment of herbicide effects, rate of legume cover growth and elemental analysis were made by sampling the vegetation periodically. To do this, 10 quadrats (each 0.25 m 2) were randomly selected from each plot and all the vegetation was removed and divided into shoots, litter and roots. These were then cut into small pieces {1--2 cm) and mixed thoroughly at the site. Then a portion was oven
115
12 MONTHS P K CaMgMn
26 MONTHS N P K Ca MgMn
(a) Oxyfluorfen 0.25 vs naturals i
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(c) Oxyfluorfen 0.25 vs Control
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-25 N
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P K Ca Mg Mn
116
Soil analysis On each sampling occasion, 10 soil samples were collected from each plot (to 15 cm only) using a hollow boring device (1.25 cm internal diameter, 30 cm long). Then the soil was dried at 70°C for 6 days, powdered, sieved through a 250 p sieve and analyzed for N, P, K, Mg, Ca and Mn (Bremner, 1965). Soil pH (of a 1:2.5 w/v suspension), soil carbon (Piper, 1942) soluble phosphate (Arnold, 1947), exchangeable K, Mg, Ca and Mn (Department of Agriculture, 1975) as well as exchangeable ammonium-nitrogen in the soil (and by difference, nitrate) were estimated by methods described in Bremner (1965). Bulk density was determined using a constant volume cylindrical container (5.1 cm X 5 cm int. diameter) and surface soil (0--10 cm) was used for the estimation (the bulk density is the ratio of the weight of oven dry soil over the volume). Further details of all these methods are given in Agamuthu (1979). RESULTS
Figure 1 summarizes the differences in mineral contents between the fastest-growing cover (legumes established using the pre-emergent herbicide " O x y f l u o r f e n " at 0.25 kg active ingredient ha -1) and the other types of (or other means of establishing) cover crops. The contents of minerals (especially nitrogen) in the legume vegetation (shoots plus litter) were significantly higher than in any of the other covers. Furthermore, these differences increased with time. Similar differences, though smaller in magnitude, also existed among legumes established by pre-emergent herbicide application, and those maintained weed-free b y repeated hand-weeding. Only when "Oxyfluorfen"-established legumes were compared with those manually kept free of weeds, but receiving extra fertilizers, did the advantages of herbicide treatment disappear. In this case " O x y f l u o r f e n " established legumes contained rather more nitrogen, but were relatively deficient in some minerals, especially calcium. As the pH of the soils in the experimental site averaged about 4.3, the extra 300 kg of Christmas Island rock phosphate applied to the "Fertilizer" plots easily accounted for this difference. Nutrient composition in the soil tended to be the mirror image of that in the cover-crops/oil palms (Table II). In the "Bare" and " O x y f l u o r f e n " treatments, those minerals absorbed in large quantity (or readily leached) generally decreased with time (e.g., P, Ca and Mg), while changes in nitrogen content depended upon treatment, remaining relatively static in those plots containing legumes, and decreasing in "Bare" plots. The exception to this was in soil under "Oxyfluorfen"-treated legumes and "Natural" in which there was a steady rise of approx. 0.01% nitrogen during the course of the experiment.
N (%)
0.13+0.01
"Bare" 1.27_+0.19 1.28-+0.18 0 . 1 4 + 0 . 0 2
i M e a n s of f o u r replicates -+ S.E.
" O x y f l u o r f e n " ( 0 . 2 5 kg A I h a -1) 1 . 3 6 + 0 . 3 6 1.71-+0.05 0 . 1 6 + 0 . 0 2 0 . 1 7 - + 0 . 0 1
25+2
0.14-+0.01
"Natural" 1.23_+0.11 1.43_+0.06 0 . 1 3 + 0 . 0 1
48+14
29 +9
27+5
"Control" 1 , 1 3 - + 0 . 2 5 1.50-+0.12 0 . 1 4 - + 0 . 0 2 0 . 1 4 - + 0 . 0 1
13
27-+9
28
p (ppm)
1 . 4 5 - + 0 . 1 0 1.54-+0.19 0 . 1 5 - + 0 . 0 1 0 . 1 4 - + 0 . 0 1
"Fertilizer"
Treatment (cover)
Months after sowing cover 13 28 13
C (%)
30-+9
18-+4
108+2
44-+13
35+9
28
28
13
Mg ( m e q %)
13
0.15-+0.06 0 . 4 5 + 0 . 0 9
0.17-+0.04 0.66+0.17
28
Ca (meq %)
28
4.6-+0.34.2+0.1
13
1.03-+0.34 4.6-+0.34.2-+0.2
0.99+0.14
28
pH
0.15-+0.02 0.14+0.01
4.2-+0.1 3 . 9 + 0 . 1
0.21-+0.06 0.10-+0.02 0.76 +0.14 0.56-+0.08 4.2+0.14.0-+0.1
0.10-+0.03 0.11-+0.02 0.13-+0.06 0.08-+0.02 0.56-+0.31 0 . 3 6 + 0 . 0 9
0.09-+0.02 0.12-+0.02 0.12-+0.04 0.15-+0.04 0.47-+0.12 0.62-+0.13 4.4-+0.14.1-+0.1
0.09-+0.02 0.13-+0.03 0 . 1 1 + 0 - 0 3
0.13-+0.03 0.15-+0.01 0 . 1 7 + 0 . 0 4
13
K ( m e q %)
N u t r i e n t c o m p o s i t i o n a n d p H o f soil at 13 a n d 28 m o n t h s a f t e r s o w i n g c o v e r s 1
T A B L E II
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Total N (kg ha -1 )
I iql
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NH&*-N(kg ha-l) I
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NO3- -N(kg ha -1 ) I
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Monthly
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rainfall {ram ) O0
119
Levels of C and K remained relatively constant throughout the experiment. A disturbing feature of these experiments was the continual decline in soil pH. Since the pH was initially higher in "Fertilizer" plots, application of Christmas Island rock phosphate obviously raised the soil pH at the beginning. Furthermore, since the fall in pH was greatest in the "Bare" plots, it is tempting to assume that the gradual increase in soil acidity was because of the constant loss of soil nutrients. Although maximum and minimum temperatures and relative humidity remained relatively constant throughout the year, precipitation followed a more seasonal trend (Fig. 2). Maximal rates of rainfall occurred from September to November in each of the four seasons studied, with a second, smaller peak in April--May. Total amounts of nitrogen leached from the soil followed this pattern (Fig. 2). The heaviest loss of nitrogen through the soil occurred in March 1978 when the rainfall for that m o n t h was 404 mm. At all times, legumes established with the help of " O x y f l u o r f e n " induced the greatest reduction of leaching losses. Soils under hand-weeded "Control" lysimeters had the next lowest leaching loss. As expected, heaviest leaching occurred in "Bare" soils (127 kg nitrogen • ha -1 year-' ), although soils under "Natural" plots were nearly comparable. In the latter stages, however, leaching from lysimeters with "Natural" covers decreased to levels comparable to those in legume treatments. Generally, more nitrate-nitrogen was lost than ammonia-nitrogen (Fig. 2). This was probably a reflection of soil pH and seemed to vary according to the treatment applied. Legumes established using " O x y f l u o r f e n " retained more ammonia and nitrate than other treatments. Another way of ascertaining leaching losses is to measure the amounts remaining in the soil at different times. One and a half years after the experiment began, soils with legume covers established using " O x y f l u o r f e n " contained 295 kg total nitrogen ha -1 (205 kg ha-' ammonia-nitrogen, 90 kg ha-' nitrate-nitrogen) soils under "Natural" covers contained 201 kg ha -~ and "Bare" soils 102 kg ha-', respectively (Table III). Thus the effect of wellmanaged legumes represented a difference of almost 200 kg ha-' total nitrogen saved over that leached from "Bare" soils. This amounts to a saving of 130 kg ha -~ year-' total nitrogen over "Bare" plots, and 63 kg ha-' year-' over "Natural" plots. Furthermore, the more pronounced leaching of nitratenitrogen is reflected in the higher soil levels of ammonia in the lysimeters (Table III). Of course, nitrogen-fixing vegetation within the lysimeters contributes nitrogen: (a) to the soil (Table II); (b) to the vegetation (Table IV} which Fig. 2. Total nitrogen, nitrate-nitrogen and ammonia-nitrogen leached through lysimeters having different covers (or maintained free of vegetation) at different times and the mean monthly rainfall at the experimented area. (e---e)"Control"; (D--~) "Oxyfluorfen 0.25"; ( o - ~ ) "Bare", and (~--~) "Natural" (see material~ and methods and the legend to Fig. 1).
120 TABLE III A m o u n t of exchangeable nitrogen in the soils within the lysimeters 19 months after planting covers (kg N ha-' -+ S.E.) l Treatment Control (hand-weeded) "Natural" "Bare" "Oxyfluorfen" (0.25 kg A1 ha-')
NH4+-N
NO3--N
Total N
193 ± 60 171 ± 10 77 -+ 8
43 -+ 4 30 ± 2 25 ± 0
236 201 102
205 ± 21
90 ± 17
295
i Means of four replicates. TABLE IV Total nitrogen in vegetation of the lysimeters (means of four replicates -+ S.E.) Treatment
Control (hand-weeded) "Natural" "Oxyfluorfen" (0.25 k g A I h a -1)
Dry wt. (kg m -2)
Nitrogen (% of dry wt.)
Change in total nitrogen (kg ha -l year -I)
1.33 ± 0.14 0.83 -+ 0.24
2.58 -+ 0.09 1.76 ± 0.22
217 92
2.26 + 0.19
2.58 _+ 0.08
368
TABLE V Bulk density of the soils under the various treatments (g ml -I ± S.E.) Treatment
Bulk density I
Control (hand-weeded) "Fertilizer" "Natural" "Bare" "Oxyfluorfen" (0.25 kg AI ha-')
1.29 1.27 1.19 1.25
+ 0.06 ± 0.08 ± 0.06 ± 0.10
1.27 -+ 0.09
i Means of four determinations made 24 months after planting the legumes. e v e n t u a l l y r e t u r n s m u c h o f t h e f i x e d n i t r o g e n t o t h e soil; a n d (c) t o t h e n u t r i e n t s l e a c h e d f r o m t h e soil ( T a b l e I I I ) . A s s u m i n g t h a t d e - n i t r i f i c a t i o n is i n s i g n i f i c a n t , it is t h e o r e t i c a l l y p o s s i b l e t o c a l c u l a t e t h e r a t e o f n i t r o g e n fixation from these data, but the sampling errors involved in determining n i t r o g e n c o n t e n t o f t h e soil m a k e t h i s i m p r a c t i c a l i n l y s i m e t e r s (cf. T a b l e II). I n s p i t e o f t h e e x t r a o r g a n i c m a t t e r t h a t a c c u m u l a t e d i n soils u n d e r l e g u m e s
121
there was no significant difference in bulk density of the soils under any of the treatments (Table V). DISCUSSION
All known inputs and losses of nitrogen from the legume--oil palm ecosystem are shown in Fig. 3. Major omissions from the data include nitrogen consumed and returned to the soil by animals as well as losses from denitrification and ammonia volatilisation. Denitrification was unlikely to be substantial, however, as the experimental soils were acidic (Tables I and II). On the other hand, contributions from atmospheric precipitation were particularly high (21 kg h a -1 year-' ) as the experimental site is in the approaches to a major airport and it is surrounded b y chemical industry. Despite the uncertainties concerning nitrogen loss to the atmosphere, it seems clear that legumes contributed about 150 kg nitrogen ha-' year-' to
ATMOSPHERIC-N2 ,/[
~ I'
Denitrification |
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Fig. 3. Major nitrogen transformations in the developing legume--oil palm ecosystem. All values are given in kg nitrogen ha-' year-' except that for atmospheric nitrogen, which is given in kg ha-' (Stevenson, 1965). Data marked with the superscripta are from Corley et al. (1976). The nitrogen Contribution from legumes (176 kg ha-' year-') is the sum of N fixed by legumes (150) and the amount absorbed via roots (149) less the quantity lost in the litter (123). N=-fixation was estimated as the mean difference in nitrogen content of the vegetation of legume and "Naturals" covers.
122
the system through nitrogen fixation (Agamuthu et al., 1980). During the early stages of palm growth the legumes also absorb 149 kg nitrogen ha -1 year -1 from the soil. With a loss through litter fall of 123 kg nitrogen ha -~ year -1 this means that the legume foliage accumulates 176 kg nitrogen ha -~ year -~. Coupled with the ability of legumes to reduce leaching by 63 kg nitrogen ha -~ year -1 as compared to "Natural" plots, the aggregate benefits of legume covers amount to 239 kg nitrogen ha -1 year -~ above that obtained using "Natural" covers. This is more than the combined inputs from fertilization and oil palm debris in plots without legumes (208 kg nitrogen ha -~ year -~), and is considerably more than the oil palm requires for growth (175 kg nitrogen ha -~ year -1). Growth of the legumes declines as the oil palms mature (Broughton, 1976; Agamuthu et al., 1980). Thus the minerals they have accumulated are rereleased to a soil ramified with an extensive oil palm root system. In fact the slow decline in legumes (presumably due to competition with the developing oil palms for light in the above-ground portions and nutrients, especially water in the underground system) means that the minerals they have previously " b a n k e d " are slowly released (in contrast to fertilizer application) and further stimulate oil palm root development (Agamuthu and Broughton, 1985). Not only are legumes better " b a n k s " than other covers (they grow more rapidly and are more effective at reducing leaching), but the extra nitrogen they accumulate through nitrogen fixation is a form of "interest". Eventually this total nutrient "capital" is repaid in full to the oil palm which is then, through its expanded root system, better able to utilize it. A betterdeveloped root system leads to enhanced vegetative growth and eventually to higher yields that are maintained throughout the productive life of the palm (Broughton, 1976). Later, when legumes have virtually died out, the nitrogen requirements of the oil palms can be met through fertilizers and from rain. In both cases oil palms previously raised together with legumes are better able to utilize exogenous fertilizers because of their more extensive root systems (Broughton, 1976). ACKNOWLEDGEMENTS
The authors thank Khoo Khee Ming, Han Siew King, Chan Yik Kuan, Rolf Jesinger, E. Eakin, Parvathy, Jodhy, Kokilam and Ramadevi for their assistance with this work and to the laboratory staff of the Soil and Analytical Service Division of the Department of Agriculture for their assistance with some of the analyses. Financial assistance was provided b y Sime Darby Plantations Ltd., R o h m and Haas Asia Inc. and the University of Malaya.
123 REFERENCES Agamuthu, P., 1979. Factors affecting the development of oil palm (Elaeis guineensis) seedlings. M.Sc. Thesis, University of Malaya, Kuala Lumpur, 202 pp. Agamuthu, P. and Broughton, W.J., 1985. Factors affecting the development of oil palm rooting systems. Agric. Ecosystems Environ., in preparation. Agamuthu, P., Chan, Y.K., Jesinger, R., Khoo, K.M. and Broughton, W.J., 1980. Effects of diphenyl ether pre-emergence herbicides on legume cover establishment under oil palm (Elaeis guineensis Jacq.). Agric. Ecosystems, 6: 193--208. Arnold, C.Y., 1947. Analysis of vegetable fertiliser plots with a soil test which measures acid soluble and adsorbed phosphorus. Soil Sci., 6 4 : 1 0 1 - - 1 0 9 . Bremner, J.M., 1965. Inorganic forms of nitrogen. In: C.A. Black, D.D. Evans, J.L. White, L.E. Ensminger and F.E. Clark (Editors), Methods of Soil Analysis (Part 2): Chemical and Micro-Biological Properties. American Society of Agronomy, Wisconsin, U.S.A. Agron. Monogr., 9: 1179--1237. Broeshart, H., Ferwerda, J.D. and Kovachich, W.G., 1957. Mineral deficiency symptoms of the oil palm. Plant Soil, 8: 289--300. Broughton, W.J., 1976. Effect of various covers on the performance of Elaeis guineensis (Jacq.) on different soils. In: D.A. Earp and W. Newall (Editors), International Oil Palm Developments. Proceedings of the Int. Oil Palm Conf., Kuala Lumpur. Incorp. Soc. Planters, Kuala Lumpur, pp. 501--525. Broughton, W.J. and John, C.K., 1979. Rhizobia in tropical legumes. III. Experimentation and supply in Malaysia, 1927--1976. In: W.J. Broughton, C.K. John, J.C. Rajarao and B. Lim (Editors), Soil Microbiology and Plant Nutrition. University of Malaya Press, Kuala Lumpur, pp. 113--136. Bull, R.A., 1957. Technique for visual diagnosis of mineral disorders and their application to the oil palm. Proceedings of the Anglo-French Conference on the oil palm, Jan. 1956. Bull. Agron. Minist. Fr. Outre Mer, 14, pp. 137--149. Corley, R.H.V., Hardon, J.J. and Wood, B.J., 1976. Inflorescence, absorption and sex differentiation. In: Developments in Crop Science. I: Oil Palm Research, Elsevier, Amsterdam, 532 pp. Date, R.A., 1973. Nitrogen, a major limitation in the productivity of natural communities, crops and pastures in the Pacific area. Soil Biol. Biochem., 5: 5--18. Department of Agriculture, 1975. Determination of (total) soil nitrogen. In: H.K. Lira (Editor), Analytical services, Manual No. 6. Soils and Analysis Services, Kuala Lumpur. Hartley, C.W.S., 1977. The Oil Palm. 2nd edn., Longman, London, 806 pp. Piper, C.S., 1942. Soil and plant analysis: a laboratory manual of methods for the examination of soils and the determination of the inorganic constituents of plants. University of Adelaide, Adelaide, 368 pp. Pushparajah, E., Sivanadyan, K. and Yew, F.K., 1974. Efficient use of fertilisers. Rubber Research Institute of Malaysia Planter's Conference. Preprint No. 8, pp. 1--12. Rubber Research Institute of Malaysia, 1970. Manual of Laboratory Methods of Plant Analysis. Rubber Research Institute of Malaysia, Kuala Lumpur, 73 pp. Stevenson, F.J., 1965. Origin and distribution of nitrogen in soil. In: W.V. Bartholomew and F.E. Clark (Editors), Soil Nitrogen. Agronomy Ser. No. 10, Am. Society Agron., U.S.A., pp. 1--42.
B.V., Amsterdam
--
111
Printed in The Netherlands
NUTRIENT CYCLING WITHIN THE DEVELOPING LEGUME ECOSYSTEM
OIL PALM--
P. AGAMUTHU and W.J. BROUGHTON*
Department of Genetics and CellularBiology, University of Malaya, Kuala Lumpur (Malaysia) (Accepted for publication 13 December 1984)
ABSTRACT Agamuthu, P. and Broughton, W.J., 1985. Nutrient cycling within the developing oil palm--legume ecosystem. Agric. Ecosystems Environ., 13: 111--123. Oil palms (Elaeis guineensis Jacq.) are normally planted in fields stripped of all other vegetation. Leguminous cover crops are planted simultaneously to protect the soil surface and to provide other, less tangible benefits. Nutrient cycling, especially of nitrogen, was followed in a commercial oil palm plantation and showed that: (a) legumes contribute about 150 kg nitrogen ha-' year-' to the system through nitrogen fixation; (b) during the early stages of oil palm growth the legumes absorb 149 kg nitrogen ha-' year-' from the soil; (c) with a loss of nitrogen through litter fall of 123 kg nitrogen ha-' year-', legumes accumulate a net amount of 176 kg nitrogen ha-' year-' in their foliage; (d) in comparison with "natural" covers, leguminous covers reduce leaching losses by 63 kg nitrogen ha-' year-' so that the total benefits of leguminous covers amount to 239 kg nitrogen ha-' year-'; (e) the combined inputs from fertilization and oil palm debris in plots without legumes is 208 kg nitrogen ha-' y e a r - ' ; and (f) during the initial growth phase, oil palms need only 175 kg nitrogen ha-' year-'. Since legumes fix nitrogen and thoroughly scavenge the soil for minerals they eventually provide more nitrogen to the oil palms than is needed for growth. This surplus nitrogen is " b a n k e d " in the legume foliage. Then, when the rooting system of the oil palms has grown under the inter-rows, competition for nutrients causes a gradual decline in the cover~crop. Nutrients "deposited" in the legumes are thus slowly re-released stimulating root-growth and general development of the oil palm.
INTRODUCTION Tropical soils are generally deficient in nitrogen (Date, 1973). In young oil p a l m s (Elaeis guineensis J a c q . ) , i n s u f f i c i e n t n i t r o g e n c a u s e s p a l i n g o f t h e seedlings, yellowing of the leaves, and eventually necrosis (Bull, 1957). N i t r o g e n d e f i c i e n c i e s in m a t u r e p a l m s s i g n i f i c a n t l y i n c r e a s e f l o r a l a b o r t i o n a n d t h e r e b y r e d u c e y i e l d s ( B r o e s h a r t e t al., 1 9 5 7 ) . A c c o r d i n g l y , n i t r o g e n h a s *Present address: Max-Planck-Institut fiir Ziichtungsforschung, D-5000 KSln 30, Federal Republic of Germany.
0167-8809/85/$03.30
© 1985 Elsevier Science Publishers B.V.
112
to be carefully husbanded in commercial plantations. Obviously, nitrogen deficiencies can be cured by application of any one of a number of fertilizers (Pushparajah et al., 1974; Hartley, 1977). Sowing leguminous crops simultaneously with oil palms is another, and potentially cheaper, way of boosting nitrogen levels in the palm fronds (see Broughton, 1976). Growth and yield of fresh-fruit bunches (fib) are thereby stimulated, often to levels greater than that achievable with fertilizers. It has been observed that the effect of enhanced nitrogen levels persists long after the legumes have died out (due to competition for light or other causes). These benefits do not seem to stem directly from nitrogen fixation by the legumes, but rather as a long-lasting effect that pre-conditions the palms to higher yields (Broughton, 1976). One possible explanation concerns nutrient cycling within the oil palm--legume ecosystem. Measurements were made of various agronomic and physiological parameters in an oil palm-legume plantation especially with regard to nitrogen. This paper summarizes the findings. MATERIALS AND METHODS
Experimental site The clearing of 92 ha of old rubber land at the Damansara Estate, Batu Tiga, Selangor began in September 1975. Of this area, 6.5 ha was set aside for special attention, while the rest of the field was planted with legumes and oil palms in the normal estate manner. One-year-old oil palm seedlings raised in large polyethylene bags in the nursery were planted in the experimental block in October 1975. Each was set 8.9 m apart on an equilateral triangular pattern. Then the entire experimental area was divided into 32 plots, the boundaries marked, and treatments randomly allocated. Table I gives details of the soil (sandy clay loam) in the experimental site. The mean maximum daily temperature was 33.8 + 0.2°C, whereas the minimum was 21.4 -+ TABLEI Some properties of the soil in the experimental plots I Stone (%) Gravel (%) Coarse sand (%) Fine sand (%) Silt (%) Clay (%) pH Organic carbon (%) Total nitrogen (%) Ca (mEq%)
0 0 22.3 31.6 7.7 38.1 4.1 1.4 0.16 1.0
I Full details of the analytical methods employed are given in Agamuthu (1979).
113
0.2 ° C. The mean maximum daily relative h u m i d i t y was 95.7 + 0.3% while the m i n i m u m was 59.2 + 0.5%. Further information is given in Agamuthu (1979).
Experimental design Randomized blocks of eight treatments, replicated four times, and with fully guarded plots were laid out in the experimental area in October 1975. Each plot covered an area of 0.17 ha and contained 25 palms. Only the central nine palms (or the area they occupied), were used for measurements. Five of the eight treatments (all eight are described in Agamuthu et al., 1980) were relevant to this paper, and these were: Ca) " N a t u r a l " regeneration consisted mostly of the grass Paspalm conjugarum, but later the creeper Mikania cordata and the fern Nephrolepis biserrata invaded the plots. Pueraria phaseoloides also occurred sporadically. Woody shrubs and noxious weeds (e.g., M. cordata) were regularly removed by hand. (b) "Bare". This treatment was used mainly to quantify nutrient cycling in the system. Except for the oil palms, the land was maintained free of vegetation by a combination of hand-weeding, and Paraquat application (see below). (c) " C o n t r o l " . After clean-weeding each legume plot, three parallel drills, 2.1 m apart, were hoed between the rows of palms. Scarified Centrosema pubescens and P. phaseoloides seeds (2:3 by weight) were mixed with the appropriate a m o u n t of Rhizobium (UMKL44 -- see Broughton and John, 1979) and planted into the drills Crate of seed application -- 8.0 kg ha-'). Loose soil was then pressed back over the seeds. Plots were maintained free of weeds by repeated hand-weeding (at roughly m o n t h l y intervals). (d) "Fertilizer". Plots were prepared and maintained exactly as described for " C o n t r o l " [(c) above] except that palms received twice the normal fertilizer dosage. (e) " O x y f l u o r f e n 0.25". Oxyfluorfen or " G o a l " or RH-2915 = 2-chloro-1(3~thoxy-4-nitrophenoxy)-4-(trifluoromethyl)benzene (Rohm and Haas Company), was applied to legumes sown as described in (c) the day after planting at a rate of 0.25 kg a.i. ha-'. The herbicide was applied in water (560 1 ha-') with a 9-1 pump operating at about l 0 s Pa pressure. A series of strips 2-m wide were sprayed over the entire area using a 1.6 mm internal diameter nozzle. Clean-weeded circles (2 m diameter) were established around each palm, and maintained in this condition throughout the experiment by a combination of hand-weeding and prophylactic herbicide application (Paraquat at 90 ml 15 1-' water). Fertilizers were applied to palms in all the plots on the following basis.
114 Date
Fertilizer
R a t e ( 1 5 0 p a l m s h a -I )
March 1976 J u n e 1976 A u g u s t 1976 S e p t e m b e r 1976
CIRP 1 CIRP 1 CCM-112 CIRP
100 100 125 100
kg kg kg kg
h a -~ h a -~ h a -~ h a -~
C h r i s t m a s Island rock p h o s p h a t e (15% p h o s p h o r u s ) . 2 A c o m m e r c i a l fertilizer c o n t a i n i n g 11% n i t r o g e n a n d 8% p h o s p h o r u s . Palms in " F e r t i l i z e r " p l o t s received twice t h e fertilizer dosages listed here.
Analytical Assessment of herbicide effects, rate of legume cover growth and elemental analysis were made by sampling the vegetation periodically. To do this, 10 quadrats (each 0.25 m 2) were randomly selected from each plot and all the vegetation was removed and divided into shoots, litter and roots. These were then cut into small pieces {1--2 cm) and mixed thoroughly at the site. Then a portion was oven
115
12 MONTHS P K CaMgMn
26 MONTHS N P K Ca MgMn
(a) Oxyfluorfen 0.25 vs naturals i
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P K Ca Mg Mn
116
Soil analysis On each sampling occasion, 10 soil samples were collected from each plot (to 15 cm only) using a hollow boring device (1.25 cm internal diameter, 30 cm long). Then the soil was dried at 70°C for 6 days, powdered, sieved through a 250 p sieve and analyzed for N, P, K, Mg, Ca and Mn (Bremner, 1965). Soil pH (of a 1:2.5 w/v suspension), soil carbon (Piper, 1942) soluble phosphate (Arnold, 1947), exchangeable K, Mg, Ca and Mn (Department of Agriculture, 1975) as well as exchangeable ammonium-nitrogen in the soil (and by difference, nitrate) were estimated by methods described in Bremner (1965). Bulk density was determined using a constant volume cylindrical container (5.1 cm X 5 cm int. diameter) and surface soil (0--10 cm) was used for the estimation (the bulk density is the ratio of the weight of oven dry soil over the volume). Further details of all these methods are given in Agamuthu (1979). RESULTS
Figure 1 summarizes the differences in mineral contents between the fastest-growing cover (legumes established using the pre-emergent herbicide " O x y f l u o r f e n " at 0.25 kg active ingredient ha -1) and the other types of (or other means of establishing) cover crops. The contents of minerals (especially nitrogen) in the legume vegetation (shoots plus litter) were significantly higher than in any of the other covers. Furthermore, these differences increased with time. Similar differences, though smaller in magnitude, also existed among legumes established by pre-emergent herbicide application, and those maintained weed-free b y repeated hand-weeding. Only when "Oxyfluorfen"-established legumes were compared with those manually kept free of weeds, but receiving extra fertilizers, did the advantages of herbicide treatment disappear. In this case " O x y f l u o r f e n " established legumes contained rather more nitrogen, but were relatively deficient in some minerals, especially calcium. As the pH of the soils in the experimental site averaged about 4.3, the extra 300 kg of Christmas Island rock phosphate applied to the "Fertilizer" plots easily accounted for this difference. Nutrient composition in the soil tended to be the mirror image of that in the cover-crops/oil palms (Table II). In the "Bare" and " O x y f l u o r f e n " treatments, those minerals absorbed in large quantity (or readily leached) generally decreased with time (e.g., P, Ca and Mg), while changes in nitrogen content depended upon treatment, remaining relatively static in those plots containing legumes, and decreasing in "Bare" plots. The exception to this was in soil under "Oxyfluorfen"-treated legumes and "Natural" in which there was a steady rise of approx. 0.01% nitrogen during the course of the experiment.
N (%)
0.13+0.01
"Bare" 1.27_+0.19 1.28-+0.18 0 . 1 4 + 0 . 0 2
i M e a n s of f o u r replicates -+ S.E.
" O x y f l u o r f e n " ( 0 . 2 5 kg A I h a -1) 1 . 3 6 + 0 . 3 6 1.71-+0.05 0 . 1 6 + 0 . 0 2 0 . 1 7 - + 0 . 0 1
25+2
0.14-+0.01
"Natural" 1.23_+0.11 1.43_+0.06 0 . 1 3 + 0 . 0 1
48+14
29 +9
27+5
"Control" 1 , 1 3 - + 0 . 2 5 1.50-+0.12 0 . 1 4 - + 0 . 0 2 0 . 1 4 - + 0 . 0 1
13
27-+9
28
p (ppm)
1 . 4 5 - + 0 . 1 0 1.54-+0.19 0 . 1 5 - + 0 . 0 1 0 . 1 4 - + 0 . 0 1
"Fertilizer"
Treatment (cover)
Months after sowing cover 13 28 13
C (%)
30-+9
18-+4
108+2
44-+13
35+9
28
28
13
Mg ( m e q %)
13
0.15-+0.06 0 . 4 5 + 0 . 0 9
0.17-+0.04 0.66+0.17
28
Ca (meq %)
28
4.6-+0.34.2+0.1
13
1.03-+0.34 4.6-+0.34.2-+0.2
0.99+0.14
28
pH
0.15-+0.02 0.14+0.01
4.2-+0.1 3 . 9 + 0 . 1
0.21-+0.06 0.10-+0.02 0.76 +0.14 0.56-+0.08 4.2+0.14.0-+0.1
0.10-+0.03 0.11-+0.02 0.13-+0.06 0.08-+0.02 0.56-+0.31 0 . 3 6 + 0 . 0 9
0.09-+0.02 0.12-+0.02 0.12-+0.04 0.15-+0.04 0.47-+0.12 0.62-+0.13 4.4-+0.14.1-+0.1
0.09-+0.02 0.13-+0.03 0 . 1 1 + 0 - 0 3
0.13-+0.03 0.15-+0.01 0 . 1 7 + 0 . 0 4
13
K ( m e q %)
N u t r i e n t c o m p o s i t i o n a n d p H o f soil at 13 a n d 28 m o n t h s a f t e r s o w i n g c o v e r s 1
T A B L E II
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119
Levels of C and K remained relatively constant throughout the experiment. A disturbing feature of these experiments was the continual decline in soil pH. Since the pH was initially higher in "Fertilizer" plots, application of Christmas Island rock phosphate obviously raised the soil pH at the beginning. Furthermore, since the fall in pH was greatest in the "Bare" plots, it is tempting to assume that the gradual increase in soil acidity was because of the constant loss of soil nutrients. Although maximum and minimum temperatures and relative humidity remained relatively constant throughout the year, precipitation followed a more seasonal trend (Fig. 2). Maximal rates of rainfall occurred from September to November in each of the four seasons studied, with a second, smaller peak in April--May. Total amounts of nitrogen leached from the soil followed this pattern (Fig. 2). The heaviest loss of nitrogen through the soil occurred in March 1978 when the rainfall for that m o n t h was 404 mm. At all times, legumes established with the help of " O x y f l u o r f e n " induced the greatest reduction of leaching losses. Soils under hand-weeded "Control" lysimeters had the next lowest leaching loss. As expected, heaviest leaching occurred in "Bare" soils (127 kg nitrogen • ha -1 year-' ), although soils under "Natural" plots were nearly comparable. In the latter stages, however, leaching from lysimeters with "Natural" covers decreased to levels comparable to those in legume treatments. Generally, more nitrate-nitrogen was lost than ammonia-nitrogen (Fig. 2). This was probably a reflection of soil pH and seemed to vary according to the treatment applied. Legumes established using " O x y f l u o r f e n " retained more ammonia and nitrate than other treatments. Another way of ascertaining leaching losses is to measure the amounts remaining in the soil at different times. One and a half years after the experiment began, soils with legume covers established using " O x y f l u o r f e n " contained 295 kg total nitrogen ha -1 (205 kg ha-' ammonia-nitrogen, 90 kg ha-' nitrate-nitrogen) soils under "Natural" covers contained 201 kg ha -~ and "Bare" soils 102 kg ha-', respectively (Table III). Thus the effect of wellmanaged legumes represented a difference of almost 200 kg ha-' total nitrogen saved over that leached from "Bare" soils. This amounts to a saving of 130 kg ha -~ year-' total nitrogen over "Bare" plots, and 63 kg ha-' year-' over "Natural" plots. Furthermore, the more pronounced leaching of nitratenitrogen is reflected in the higher soil levels of ammonia in the lysimeters (Table III). Of course, nitrogen-fixing vegetation within the lysimeters contributes nitrogen: (a) to the soil (Table II); (b) to the vegetation (Table IV} which Fig. 2. Total nitrogen, nitrate-nitrogen and ammonia-nitrogen leached through lysimeters having different covers (or maintained free of vegetation) at different times and the mean monthly rainfall at the experimented area. (e---e)"Control"; (D--~) "Oxyfluorfen 0.25"; ( o - ~ ) "Bare", and (~--~) "Natural" (see material~ and methods and the legend to Fig. 1).
120 TABLE III A m o u n t of exchangeable nitrogen in the soils within the lysimeters 19 months after planting covers (kg N ha-' -+ S.E.) l Treatment Control (hand-weeded) "Natural" "Bare" "Oxyfluorfen" (0.25 kg A1 ha-')
NH4+-N
NO3--N
Total N
193 ± 60 171 ± 10 77 -+ 8
43 -+ 4 30 ± 2 25 ± 0
236 201 102
205 ± 21
90 ± 17
295
i Means of four replicates. TABLE IV Total nitrogen in vegetation of the lysimeters (means of four replicates -+ S.E.) Treatment
Control (hand-weeded) "Natural" "Oxyfluorfen" (0.25 k g A I h a -1)
Dry wt. (kg m -2)
Nitrogen (% of dry wt.)
Change in total nitrogen (kg ha -l year -I)
1.33 ± 0.14 0.83 -+ 0.24
2.58 -+ 0.09 1.76 ± 0.22
217 92
2.26 + 0.19
2.58 _+ 0.08
368
TABLE V Bulk density of the soils under the various treatments (g ml -I ± S.E.) Treatment
Bulk density I
Control (hand-weeded) "Fertilizer" "Natural" "Bare" "Oxyfluorfen" (0.25 kg AI ha-')
1.29 1.27 1.19 1.25
+ 0.06 ± 0.08 ± 0.06 ± 0.10
1.27 -+ 0.09
i Means of four determinations made 24 months after planting the legumes. e v e n t u a l l y r e t u r n s m u c h o f t h e f i x e d n i t r o g e n t o t h e soil; a n d (c) t o t h e n u t r i e n t s l e a c h e d f r o m t h e soil ( T a b l e I I I ) . A s s u m i n g t h a t d e - n i t r i f i c a t i o n is i n s i g n i f i c a n t , it is t h e o r e t i c a l l y p o s s i b l e t o c a l c u l a t e t h e r a t e o f n i t r o g e n fixation from these data, but the sampling errors involved in determining n i t r o g e n c o n t e n t o f t h e soil m a k e t h i s i m p r a c t i c a l i n l y s i m e t e r s (cf. T a b l e II). I n s p i t e o f t h e e x t r a o r g a n i c m a t t e r t h a t a c c u m u l a t e d i n soils u n d e r l e g u m e s
121
there was no significant difference in bulk density of the soils under any of the treatments (Table V). DISCUSSION
All known inputs and losses of nitrogen from the legume--oil palm ecosystem are shown in Fig. 3. Major omissions from the data include nitrogen consumed and returned to the soil by animals as well as losses from denitrification and ammonia volatilisation. Denitrification was unlikely to be substantial, however, as the experimental soils were acidic (Tables I and II). On the other hand, contributions from atmospheric precipitation were particularly high (21 kg h a -1 year-' ) as the experimental site is in the approaches to a major airport and it is surrounded b y chemical industry. Despite the uncertainties concerning nitrogen loss to the atmosphere, it seems clear that legumes contributed about 150 kg nitrogen ha-' year-' to
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Fig. 3. Major nitrogen transformations in the developing legume--oil palm ecosystem. All values are given in kg nitrogen ha-' year-' except that for atmospheric nitrogen, which is given in kg ha-' (Stevenson, 1965). Data marked with the superscripta are from Corley et al. (1976). The nitrogen Contribution from legumes (176 kg ha-' year-') is the sum of N fixed by legumes (150) and the amount absorbed via roots (149) less the quantity lost in the litter (123). N=-fixation was estimated as the mean difference in nitrogen content of the vegetation of legume and "Naturals" covers.
122
the system through nitrogen fixation (Agamuthu et al., 1980). During the early stages of palm growth the legumes also absorb 149 kg nitrogen ha -1 year -1 from the soil. With a loss through litter fall of 123 kg nitrogen ha -~ year -1 this means that the legume foliage accumulates 176 kg nitrogen ha -~ year -~. Coupled with the ability of legumes to reduce leaching by 63 kg nitrogen ha -~ year -1 as compared to "Natural" plots, the aggregate benefits of legume covers amount to 239 kg nitrogen ha -1 year -~ above that obtained using "Natural" covers. This is more than the combined inputs from fertilization and oil palm debris in plots without legumes (208 kg nitrogen ha -~ year -~), and is considerably more than the oil palm requires for growth (175 kg nitrogen ha -~ year -1). Growth of the legumes declines as the oil palms mature (Broughton, 1976; Agamuthu et al., 1980). Thus the minerals they have accumulated are rereleased to a soil ramified with an extensive oil palm root system. In fact the slow decline in legumes (presumably due to competition with the developing oil palms for light in the above-ground portions and nutrients, especially water in the underground system) means that the minerals they have previously " b a n k e d " are slowly released (in contrast to fertilizer application) and further stimulate oil palm root development (Agamuthu and Broughton, 1985). Not only are legumes better " b a n k s " than other covers (they grow more rapidly and are more effective at reducing leaching), but the extra nitrogen they accumulate through nitrogen fixation is a form of "interest". Eventually this total nutrient "capital" is repaid in full to the oil palm which is then, through its expanded root system, better able to utilize it. A betterdeveloped root system leads to enhanced vegetative growth and eventually to higher yields that are maintained throughout the productive life of the palm (Broughton, 1976). Later, when legumes have virtually died out, the nitrogen requirements of the oil palms can be met through fertilizers and from rain. In both cases oil palms previously raised together with legumes are better able to utilize exogenous fertilizers because of their more extensive root systems (Broughton, 1976). ACKNOWLEDGEMENTS
The authors thank Khoo Khee Ming, Han Siew King, Chan Yik Kuan, Rolf Jesinger, E. Eakin, Parvathy, Jodhy, Kokilam and Ramadevi for their assistance with this work and to the laboratory staff of the Soil and Analytical Service Division of the Department of Agriculture for their assistance with some of the analyses. Financial assistance was provided b y Sime Darby Plantations Ltd., R o h m and Haas Asia Inc. and the University of Malaya.
123 REFERENCES Agamuthu, P., 1979. Factors affecting the development of oil palm (Elaeis guineensis) seedlings. M.Sc. Thesis, University of Malaya, Kuala Lumpur, 202 pp. Agamuthu, P. and Broughton, W.J., 1985. Factors affecting the development of oil palm rooting systems. Agric. Ecosystems Environ., in preparation. Agamuthu, P., Chan, Y.K., Jesinger, R., Khoo, K.M. and Broughton, W.J., 1980. Effects of diphenyl ether pre-emergence herbicides on legume cover establishment under oil palm (Elaeis guineensis Jacq.). Agric. Ecosystems, 6: 193--208. Arnold, C.Y., 1947. Analysis of vegetable fertiliser plots with a soil test which measures acid soluble and adsorbed phosphorus. Soil Sci., 6 4 : 1 0 1 - - 1 0 9 . Bremner, J.M., 1965. Inorganic forms of nitrogen. In: C.A. Black, D.D. Evans, J.L. White, L.E. Ensminger and F.E. Clark (Editors), Methods of Soil Analysis (Part 2): Chemical and Micro-Biological Properties. American Society of Agronomy, Wisconsin, U.S.A. Agron. Monogr., 9: 1179--1237. Broeshart, H., Ferwerda, J.D. and Kovachich, W.G., 1957. Mineral deficiency symptoms of the oil palm. Plant Soil, 8: 289--300. Broughton, W.J., 1976. Effect of various covers on the performance of Elaeis guineensis (Jacq.) on different soils. In: D.A. Earp and W. Newall (Editors), International Oil Palm Developments. Proceedings of the Int. Oil Palm Conf., Kuala Lumpur. Incorp. Soc. Planters, Kuala Lumpur, pp. 501--525. Broughton, W.J. and John, C.K., 1979. Rhizobia in tropical legumes. III. Experimentation and supply in Malaysia, 1927--1976. In: W.J. Broughton, C.K. John, J.C. Rajarao and B. Lim (Editors), Soil Microbiology and Plant Nutrition. University of Malaya Press, Kuala Lumpur, pp. 113--136. Bull, R.A., 1957. Technique for visual diagnosis of mineral disorders and their application to the oil palm. Proceedings of the Anglo-French Conference on the oil palm, Jan. 1956. Bull. Agron. Minist. Fr. Outre Mer, 14, pp. 137--149. Corley, R.H.V., Hardon, J.J. and Wood, B.J., 1976. Inflorescence, absorption and sex differentiation. In: Developments in Crop Science. I: Oil Palm Research, Elsevier, Amsterdam, 532 pp. Date, R.A., 1973. Nitrogen, a major limitation in the productivity of natural communities, crops and pastures in the Pacific area. Soil Biol. Biochem., 5: 5--18. Department of Agriculture, 1975. Determination of (total) soil nitrogen. In: H.K. Lira (Editor), Analytical services, Manual No. 6. Soils and Analysis Services, Kuala Lumpur. Hartley, C.W.S., 1977. The Oil Palm. 2nd edn., Longman, London, 806 pp. Piper, C.S., 1942. Soil and plant analysis: a laboratory manual of methods for the examination of soils and the determination of the inorganic constituents of plants. University of Adelaide, Adelaide, 368 pp. Pushparajah, E., Sivanadyan, K. and Yew, F.K., 1974. Efficient use of fertilisers. Rubber Research Institute of Malaysia Planter's Conference. Preprint No. 8, pp. 1--12. Rubber Research Institute of Malaysia, 1970. Manual of Laboratory Methods of Plant Analysis. Rubber Research Institute of Malaysia, Kuala Lumpur, 73 pp. Stevenson, F.J., 1965. Origin and distribution of nitrogen in soil. In: W.V. Bartholomew and F.E. Clark (Editors), Soil Nitrogen. Agronomy Ser. No. 10, Am. Society Agron., U.S.A., pp. 1--42.