COCOS (1990), 8, 1 - 1 2 
Printed in Sri Lanka 

i>2>f .GIG", 4 3 L i ' ;• 5 ^ 

Studies On Potassium Magnesium Interaction 
In Coconut (Cocos nucifera) 

M. JEGANATHAN 

Coconut Research Institute, Lunuwila, Sri Lanka 

Key words: Coconut, Cocos nucifera L., copra, leaf analysis, nut water analysis, ultisol 

ABSTRACT 

The widespread deficiency of magnesium in coconut acts as a limiting factor to increased 
production. Two ongoing experiments on potassium-magnesium (K-Mg) interaction in 
coconut conducted in lateritic gravels (ultisols), in the wet and intermediate agroclimatic zones 
of Sri Lanka showed significant yield responses (P=0.01) to differential K treatments, in the 
wet zone, but not in the intermediate zone. Differential Mg treatments, however, did not give 
rise to yield responses. Leaf and nut water analysis showed significant changes in the 
concentrations of Na, K and CI (P=0.001), with a distinct inverse relationship between Na and 
K when K was applied. Differential Mg applications showed a significant effect only for leaf 
Mg (P=0.001), in the fourth year of the experiment, in the wet zone. Results indicate the 
usefulness of nut water analysis as an additional diagnostic tool, for Na, K and CI. 

INTRODUCTION 

In the plantation agriculture in Sri Lanka, coconut (Cocos nucifera L.) occupies the 
highest area of nearly 420,000 ha. The estate sector (8 ha or more) comprises 104,000 ha 
(24.7%) of the area while the balance, 316,000 ha (75.3%) is categorized as small holdings. 

Coconut is a very important commodity in Sri Lanka. It is used traditionally as a 
component in food, and about 70% of the production is consumed locally. About 9% of the 
total expenditure on food of an average household is spent on coconut, the principal source of 
edible oil and fat. Coconut provides about 22% of the total intake of calories of an average 
consumer in Sri Lanka second to rice. These data amply demonstrate the importance of 
coconut. 

In order to meet the high domestic consumption and also maintain the export market, 
increased production can be achieved with regular and increased fertilization. 

Reprinted from Plant Nutrition - Physiology and Applications (1990) with kind permission of 
Kluwer Academic Publishers. 



M. JEGANATHAN 

Potassium is the most important nutrient for the production of nuts and copra, the latter 
used in oil extraction. Although the importance of K nutrition has been experimentally shown 
(Loganathan and Balakrishnamurthi, 1979; Salgado. 1950) widespread occurrence of Mg 
deficiency in the lateritic and the sandy soils has been a limiting factor in coconut production. 
It is necessary therefore to study the limitations in the use of high levels of K obviously due to 
K-Mg interactions and their antagonistic effects. This paper describes results of two 
experiments conducted to elucidate this aspect in coconut nutrition with a view to determine 
levels of Mg in the soil and leaf for more effective K usage. 

MATERIALS AND METHODS 

Two experiments were conducted on adult tall coconut (variety typica) of age about 55 
years, in 7 and 14 hectare blocks, at Sirikandura Estate, Dodanduwa (Southern Province) and 
at Heemmeliyagara Estate, Hiruwalpola (North Western Province), from October 1984. 

Sirikandura Estate (SE) falls in the wet zone receiving an annual rainfall of 1875 to 2500 
mm. while Heemmeliyagara Estate (HE) is in the intermediate zone receiving an annual 
rainfall of 1000 to 1875 mm. The rainfall is bimodal, with peaks in April/June and 
October/November. The soils in both experimental sites are Red Yellow Podzolic with soft 
and hard laterite (Ultisols). 

The chemical characteristics of the soils are given in Table 1. 

Design and treatments 

The experimental design consisted of a 4 X 4 factorial arrangement for the two nutrients 
K and Mg in three blocks, each consisting of 16 plots of eight effective palms separated by a 
single row of guard trees. The planting density is 158 palms/ha. 

Fertilizer applications were made annually starting in 1984, as follows: 

Muriate of Potash 
(60% K20) 
kg/pal m/yr 

K1 0.0 
K2 1.2 
K3 2.4 
K4 3.6 

Kieserite 
(24% MgO) 
kg/palm/yr 

Mg1 0.0 
Mg2 0.6 
Mg3 1.2 
Mg4 1.8 

Basal applications of urea and rock phosphate, each at the rate of 0.7 kg/palm/yr, 
equivalent to 0.32 kg N and 0.19 kg P2O5, were made in all treatments. 

At SE, only the first three levels of treatment were imposed. The fertilizer was applied on 
the surface of a circular area with a radius of 1.75 m, around the base of the palm, and 
incorporated into the soil using a fork. 

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Studies on Potassium Magnesium Interaction 

Soil and leaf sampling 

3 

Four palms were chosen randomly from each plot. In these palms, the 14th leaf from the 
top, the 1 s t being the fully opened leaf with its leaflets separated, was selected and six leaflets 
were taken from the mid region and composited to form a sample, oven-dried at 85 °C and 
ground for analysis. 

Soil samples were collected from the manure circle. Two borings were made at 1 m from 
the base on either side of the palms and samples were collected from two depths, 0-25 cm, and 
25-50 cm and composited separately. 

Nut water analysis 

Nut water or 'coconut water' is the colourless liquid found within the hard white kernel 
(the solid endosperm), both of which are enclosed by the hard shell. From each plot, eight 
nuts representing each of the palms, selected at random, were sampled and the nut water from 
them was mixed well, filtered and aliquots were taken for analysis. 

Chemical analysis 

For soil analysis a 1 M C H 3 C O O N H 4 extractant was used for the extraction of 
exchangeable Na, K, Ca and Mg and determined by atomic absorption, Na and K in the 
emission and Ca and Mg in the absorption modes. CI was read off the Chlor-O-Counter after 
extraction in 1:1 soil water ratio. 

For leaves, N was determined in 0.1-g samples digested in Se/H 2 S 0 4 mixture, and P, Na, 
K, Ca and Mg, in 0.5 g samples digested in a HNO3/HCLO4 mixture. N and P were estimated 

colorimetrically on a Technicon Auto Analyzer and Na, K, Ca and Mg, as for soils, by atomic 
absorption. CI was determined in 2.0-g samples, ashed in a muffle furnace at 450° C, 
extracted with water and read off the Chlor-O-Counter. 

Nut water was analysed by diluting 20 times for Na and K, and 100 times for Ca and Mg 
and read off the Atomic Absorption Spectrophotometer (Somasiri et al.., 1986). CI was 
estimated on the Chlor-O-Counter, using 0.1 ml of nut water (Periathamby, personal, 
communication). 

Yield 

Commencing January 1985, ripe nuts were harvested from the two mature bunches 
bimonthly and the total number of nuts recorded. Weights of the husked nuts were obtained 
from samples of 32 nuts representing each treatment. The yield of copra was determined 
using the 'copra index factor' (Mathes, 1985 - personal communication). 



M. J E G A N A T H A N 

RESULTS AND DISCUSSION 

Soils and leaf analysis - Pretreatment data 

T h e pre t rea tment soil analyt ical data in Table 1 show the exchangeab le cat ions in the 
manure circle higher than in the centres of squares. This is due to the residual fertility in the 
manure circle, while the centre of squares reflects the inherent fertility. 

Table 1 - Soil analysis - October 1984 

Parameters Sirikandura Estate Heemmeliyagara Estate 

Depth Depth 

0-25 cm 25-50 cm 0-25 cm 25-50cm 

Chemical parameters of pretreatment samples in the fertilized circle 

PH 4.2 3.8 5.5 5.1 

E.C (mmhos/cm) 51.39 55.25 46.88 52.26 

Exch. Na (meq%) 0.08 0.06 0.08 0.10 

Exch. K (meq%) 0.26 0.16 0.30 0.44 

Exch. Ca (meq%) 1.27 0.59 1.56 1.04 

Exch. Mg (meq%) 0.20 0.16 0.57 0.38 

Avail. CI (mg/kg) 27.62 27.30 24.01 30.08 

Outside the fertilized circle, in the centre of square formed by four palms 

PH 4.0 3.9 5.0 5.0 

E.C (mmhos/cm 33.39 21.75 36.08 16.34 

Exch. Na(meq%) 0.03 0.03 0.07 0.08 

Exch. K (meq%) 0.08 0.04 0.13 0.10 

Exch. Ca (meq%) 1.06 0.31 0.73 0.65 

Exch. Mg (meq%) 0.20 0.16 0.42 0.40 

Avail. CI (mg/kg) 18.94 20.40 16.06 14.33 

The mean exchangeable K level in the manure, circles of the two sites at the first depth 
was 0.28 m e q % . Earlier exper iments in lateritic soils in the wet zone have shown that in spite 
of such high K levels in the soil, the palms responded to muriate of potash (Loganathan and 
Balakr ishnamurt i , 1979). In the sub-soil (25-50 cm) the exchangeable K was much lower at 
SE than at HE, 0.16 and 0.44 m e q % , respectively, which is probably due to leaching losses 
result ing from higher rainfall in the wet zone. 

Information available on exchangeable Mg is scanty. OUagnier et al., (1983) considered 
an exchangeab le Mg level of 0.46 m e q % as high. The critical values for exchangeable K, and 
Mg in the Phil ippines are 0 .45, and 2.9 m e q % , respectively (Sant iago, 1978). Margate et al. 
(1979) did not observe a K - M g antagonism in a long-term KC1 fertilizer study on a clay loam 

4 



Studies on Potassium Magnesium Interaction 

soil in the Philippines in spite of high application of KC1 (8 kg/palm/year), with soil levels of 
0.45 meq% K and 5.3 meq% Mg. On this basis, the soils in SE are not rich. 

The data in Table 2 show the average concentrations of nutrients in the 14th leaf. With 
the exception of K in both locations and Mg at SE, the levels of the other nutrients fall within 
the sufficiency ranges as described by Loganathan and Atputharaja (1986). 

Table 2 Pretreatment leaf nutrient concentrations (J4th leaf) on the Sirikandura and 
Heemeliyagara Estates 

Nutrients (% in dry weight) 

N P K Ca Mg CI 

Sirikandura 2.02 0.15 1.09 0.38 0.16 0.30 
Heemmeliyagara 1.98 0.15 1.10 0.50 0.30 0.34 

Sufficiency 
ranges 1.9-2.1 0.11-0.13 1.2-1.5 0.35-0.55 0.25-0.30 0.30-0.40 

Sirikandura Estate Analysis of leaf samples collected in May 1988 (Table 3) showed that the 
concentrations of both N and P had remained unaltered by the differential treatments. 

Applications of K showed significant quadratic responses in leaf K and Mg, a significant 
linear increase in CI and decrease in Na and Ca. 

Increasing rates of Mg applied showed a highly significant linear increase for Mg only in 
1988, the fourth year of the experiment. 

At SE leaf K reached the value of 1.49% from the premanurial 1.09% at the highest level, 
K3 (2.4 kg/palm/year), a significant improvement in K nutrition. Increasing applications of 
Mg caused significant increases in leaf Mg, from the premanurial 0.16%, to 0.22% at the Mg3 
level (1.2 kg/palm/year) still below the critical concentration of 0.25%. 

Hemmeliyagara Estate As at SE, both N and P remained unaffected by the differential K-Mg 

treatments (Table 4). 

Increasing rates of K applied showed significant linear increase in leaf K and decreases in 
leaf Na and Mg. CI showed a linear response. 

From the pretreatment 1.10% level, leaf K increased to only 1.12% at K4, still below the 
critical level of 1.2%. Unlike at SE, increasing rates of Mg caused no changes in any nutrient 
levels. 

5 



M. J E G A N A T H A N 

Table 3. Leaf nutrient concentrations after four years of differential K and Mg 
applications, at Sirikandura Estate - May 1988 

Concentration of nutrients (% in J4th leaf) 

Treatment N P Na K . Ca Mg CI 

K1 2.27 0.156 0.30 0.63 0.50 0.27 0.37 

K2 2.38 0.158 0.25 1.18 0.41 0.15 0.49 

K3 2.36 0.156 0.21 1.49 0.40 0.15 0.56 

Mg1 2.32 0.157 0.26 1.14 0.43 0.15 0.49 

Mg2 2.34 0.158 0.25 1.09 0.45 0.20 0.45 

Mg3 2.35 0.157 0.24 1.07 0.43 0.22 0.47 

Sign level 

K I - - ... * * * • * ... ... 
K q - - - • * - * * -

Mg I - - - - - ... -

CV% 3.55 2.81 11.89 7.76 9.24 9.41 9.20 

*P=0.05, ** P=0.01; *** P=0.00l Hinear response, q=quadratic response 

Table 4. Leaf nutrient concentrations due to differential applications of K and Mg at 
Heemmeliyagara Estate -January 1988 

Concentration of nutrients (% in 14th leaf) 

Treatment N P Na K Ca Mg CI 

K1 2.U 0.128 0.15 0.83 0.56 0.35 0.35 

K2 2.13 0.129 0.12 0.99 0.51 0.32 0.46 

K3 2.13 0.130 0.11 1.04 0.45 0.29 0.49 

K4 2.11 0.130 0.10 0.12 0.50 0.27 0.52 

Mg1 2.13 0.131 0.12 0.98 0.52 0.30 0.46 

Mg2 2.11 0.129 0.13 1.00 0.52 0.31 0.46 

Mg3 2.16 0.129 0.11 1.00 0.50 0.32 0.43 

Mg4 2.11 0.128 0.12 1.00 0.50 0.32 0.48 

Sign, level 

K I - - ... ... * *** ... 
K q - - - - • - -

CV% 6.21 21.77 12.89 14.95 14.95 11.75 22.70 

*P=0.05; **P=0.01; ***P=0.001 l=linear response, q=quadratic response. 

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Studies on Potassium Magnesium Interaction 

Nut water analysis 

7 

Sirikandura Estate Analysis of the nut water collected in March 1988 (Table 5) showed 
responses to increasing rates of K and Mg. K application caused highly significant positive 
responses, both linear and quadratic, in the uptake of K and CI, and a negative response for 
Na. Mg application only caused a significant linear increase in CI, which is unusual and 
difficult to explain. 

Heemmeliyagara Estate As at SE, negative linear and quadratic responses of Na to 
increasing K applications were observed (Table 6). K and Mg showed significant responses, 
both linear and quadratic, to increasing rates of K. Increased application of Mg caused a 
linear increase in K, which was unexpected, in view of the fact that usually these two nutrients 
show antagonistic effects. 

In Sri Lanka, chemical analysis of nut water has been successfully used in the study of P 
and K nutrition of coconut (Salgado, 1955; 1966). In other countries this approach has also 
been used with some measure of success (Lockhard et al.., 1969, Southern, 1956). 

Present indications are that nut water analysis can be used as an additional tool in the 
interpretation of field experiment data for Na, K, Mg and CI. Both Ca and Mg have 
functionally limited roles to perform, both in the liquid and in the solid endosperm, and 
therefore their concentration will be low and so too the changes. 

Yield 

Sirikandura Estate Analysis of the second year's data (1986) showed a significant 
response in terms of copra production at (P = 0.01) to increasing K applications. For the years 
1987 and 1988, these responses were highly significant at P = 0.001 (Table 7). Nut yields also 
significantly increased during these two years as a result of K application (Table 8). 

There has been an overall decrease in yield of copra, despite the increased application of 
K. The drop in the *no fertilizer' plots has been drastic, 32.4% between 1986 and 1988. 
During the same period, at the Kl and K 2 levels, the decreases have been 17.2% and 14.6%, 
respectively. Fertilizer application has to some extent arrested the decline. The fall in 
production can be attributed to adverse weather conditions, to the insufficient state of nutrition 
of the palms and to the low soil nutrient status. Table 9 gives the rainfall and yield data for the 
"no fertilizer' plots at both sites. At SE, rainfall was high, except for 1986. The lower rainfall 
in 1986 was compensated for by a better distribution and therefore yields were not adversely 
affected in 1987. 

From the nutritional aspects, it was observed that with all K application levels the K 
demand of the trees has been satisfied, and it is yet to be seen whether better yields will be 
obtained when the Mg concentrations reach the sufficiency level. It is only after K deficiency 
has been corrected that Mg manuring was found to have a positive effect on production 
(Brunin, 1970; Coomans, 1977). 



fvl. J E G A N A T H A N 

Table 5 Nut water nutrient concentrations after four years of differential application of K 
and Mg at Sirikandura Estate-March 1988 

Nutrient (mg 1 

Treatment Na K Ca Mg CI 

K1 270 1634 186 101 1870 
K2 139 2375 171 105 2095 
K3 91 2627 180 99 '2102 

Mg1 179 2195 185 100 2073 
Mg2 167 2238 179 101 2038 
Mg3 154 2202 174 105 1956 

Sign, level 
K I ... *** - - *** 

K q •** *** - - ** 

Mg 1 - - - - • « 

CV% 15.73 4.49 13.73 10.72 3.61 

*P= 0.05; ** p= 0 01 ' *** P=0.01 1= linear response, q = quadratic response 

Table 6 Nut water nutrient concentrations after four years of differential application of K 
and Mg at Heemmeliyagara Estate - March 1988. 

Nutrient (mg l-l) 

Treatment Na K Ca Mg CI 

K1 140 1918 283 156 1959 
K2 80 2128 278 138 1997 
K3 71 2201 282 130 2012 
K4 64 2196 301 142 2062 

Mg1 96 1985 292 143 1983 
Mg2 85 2120 303 143 2037 
Mg3 90 2141 276 140 2027 
Mg4 84 2197 272 141 1984 

Sign, level 
K I *•* *** - ** -

K q * * - ... -

Mg I - ** - - -

CV% 41 .44 7.55 14.92 9.74 5.48 

*P=0.05; **P=0.01; ***P=0.001 l=linear response, q=quadratic response. 



Studies on Potassium Magnesium Interaction 

Table 7. Weight of copra in different years following the start of differential annual 
applications ofK and Mg at Sirikandura Estate, with 158 palms per ha. 

Treatment Copra % Difference 
(kg/ha) (kg/ha) 

Yield 1986 

K1 1668* 100 -
K2 1882 113 214 
K3 1985 119 317 

Yield 1987 
K1 1442** 100 -
K2 1798 125 356 
K3 1843 129 401 

Yield 1988 
K1 1127** 100 -
K2 1559 138 432 
K3 1696 150 569 

*P=0.05; ** P=0.01; *** P=0.001. 

Table 8. Nut yield in different years following the start of differential annual applications of 
K and Mg at Sirikandura Estate, with 158 palms per ha. 

Treatment Nutslha % Difference 

Yield 1986 
K1 8091 NS 100 -

K2 8447 104 3 5 6 

K3 8 8 1 5 

Yield 1987 

109 724 

K1 7433 100 -

K2 8 5 4 5 115 1112 

K3 8 8 8 8 

Yield 1988 

120 1455 

K1 5728 100 -

K2 7290 127 1562 

K3 7773 136 2045 

NS = Not Significant; P=0.05; P=0.01; 
*** 

P=0.001 

9 



M. J E G A N A T H A N 

Soil analysis data for the years 1987 and 1988 at the M g 3 application level yielded 0.73 
and 0.63 m e q % Mg, respectively. These fairly high exchangeable Mg levels in the soil can be 
expec ted to raise the Mg concent ra t ions in the leaves enough to have a bear ing on future 
product ion. 

Heemmeliyagara Estate Statistical analysis of the yield data for the years 1986, 1987 and 
1988, in terms of nuts and copra, showed neither a significant response to the main treatments 
K and Mg nor to any interaction between them. 

A combinat ion of factors contr ibuted to the lack of yield response, such as ~no fertilizer ' 
plots performing much better than the others at the commencemen t of the studies (Table 9) , 
and delayed application of fertilizers due to prolonged drought periods, compounded by the 
drought effect itself. 

Table 9. Yield and rainfall data and corresponding nut and copra yield in the' no fertilizer' 
treatments at the two estates. 

Year Ranfall No. of 

wet days 

Nuts/ha Copra 

(kg/ha) 

Heemmeliyagara Estate 

1984 

1985 

1986 

1987 

1988 

1815 

1262 

885 

1134 

807 

87 

55 

44 

59 

42 

11179 

9263 

5649 

5267 

2074 

1501 

751 

898 

Sirikandura Estate 

1984 

1985 

1986 

1987 

1988 

2240 

2724 

1990 

2676 

2505 

133 

164 

174 

149 

137 

9381 

8907 

8117 

6590 

1936 

1783 

1541 

1249 

Changes in the concentra t ions of Na and CI were observed in both leaf and nut water, 
resul t ing from differential fertilizer t rea tments . Both e lements have some functions in the 
nutrition of coconut , particularly CI, but discussion of such functions is beyond the scope of 
this paper. 

10 



Studies on Potassium Magnesium Interaction 

CONCLUSIONS 

Significant yield responses were obtained for the experiment at SE, albeit with an overall 
decrease in yield due to weather and nutritional factors. The experiment at HE failed to 
display any yield responses as a result of drought. 

With respect of leaf and nut water analysis, in both experiments responses to the main 
treatments K and Mg, were noticeable, with varying trends. Changes in the leaf composition 
was more pronounced at SE than at HE. 

Nut water analysis shows promise as an additional tool for interpretation of field 
experimental data. 

The presentation covers a four-year period. Statistical studies on when to conclude long-
term fertilizer trials on coconut yield consider an eight- to ten-year period as sufficient to 
understand the full response (Mathes, 1980). Both experiments are still in progress. 

ACKNOWLEDGEMENTS 

The author is thankful to Dr R Mahindapala, Director, Coconut Research Institute for 
helpful criticism in the preparation of the paper, and to Mr D T Mathes, Biometrician, Coconut 
Research Institute, for statistical advice and useful discussions. 

Thanks are due to Miss S Periathamby, Mrs D M D I Wijebandara and Mr A A Fernando, 
for the maintenance of the laboratory and field records, and to the staff of the Soils and Plant 
Nutrition Division, Coconut Research Institute, for handling the analyses. Thanks are also due 
to Mrs H M W S Athauda for typing the manuscript. 

REFERENCES 

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269-274. 

Coomans P 1977 Premiers resultats experimentaux sur la fertilisation des cocotiers hybrides 
en Cote d'ivoire. Oleagineux 32, 155-166. 

Loganathan P and Balakrishnamurti T S 1979 Effect of N P K fertilizers on the yield and leaf 
nutrient concentration of adult coconut on a lateritic gravelly soil in Sri Lanka. 
Ceylon Cocon. Q. 30, 81-90. 

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M. J E G A N A T H A N 

Logana than P and Atputharajah P P 1986 Effects of fertilizers on yield and leaf nutrient 

concentra t ions in coconut . Trop.Agric. Trindad 6 3 , 143-148. 

Lockard R G, Ballaux J C and Azucena B 1969 The results of pretreatment leaf and nut water 
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Ma thes D T 1980 A Study on when to conclude a long-term fertilizer trial on coconut yield. 
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Sant igo R M 1978 Growth of coconut seedlings as influenced by different fertility levels in 
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Sa lgado M L M 1950 A. Rep. Coconut Research Scheme for 1948, Sessional paper XXII , 
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Sa lgado M L M 1955 The nutrient content of nut water in relation to available soil nutrients 
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Somasir i L L W, Warnasiri W H, George G D and Jeganathan M 1986 Report of the Soils and 
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S o u t h e r n P J 1956 T h e f lame s p e c t r o p h o t o m e t r y : d e t e r m i n a t i o n of p o t a s s i u m , s o d i u m , 
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