Effect of various covers on soil fertility under Hevea brasiliensis muell. arg. and on growth of the tree

Agro-Ecosystems, 3 ( 1 9 7 7 ) 1 4 7 - - 1 7 0

147

© Elsevier Scientific Publishing Company, Amsterdam - - Printed in The Netherlands

EFFECT

OF VARIOUS COVERS ON SOIL FERTILITY UNDER HEVEA MUELL. ARG. AND ON GROWTH OF THE TREE

BRASILIENSIS

W.J. BROUGHTON

Department of Genetics and Cellular Biology, University of Malaya, Kuala Lumpur (Malaysia) (Received 22 April 1976)

ABSTRACT Broughton, W.J., 1977. Effect of various covers on soi ! fertility under Hevea brasiliensis Muell. Arg. and on growth of the tree. Agro-Ecosystems, 3: 147--170. A survey was made of the relationship between ground covers, soil fertility, and the growth of Hevea brasiliensis. F o u r different cover management systems were widely tested in Malaysia, namely a mixture of creeping legumes (Calopogonium muconoides, Centrosema pubescens and Pueraria phaseoloides ), grasses (mostly Axonopus compressu$ with Paspalum conjugatum), a pure crop of Mikania cordata, and a naturally regenerating system representing the normal colonisation process on cleared land. Of the four systems, legumes initially had the fastest rate of growth, and generally contained more nutrients than the other covers tested. The greater nutrient return to the soil from growing a leguminous cover wasreflected in higher levels of these nutrients in rubber leaves. This, coupled with improved soil physical properties, led to an increased rate of growth of the rubber tree. Nitrogen fixation under legumes grown in association with rubber averaged 150 kg per ha per year over a 5-year period, with maximum rates of nitrogen fixation being about 200 kg per ha per year. Competition effects from both the shade and roots of Hevea caused a gradual diminution in vigour of all the covers such that they virtually died out by about the 6th year after planting. Dry rubber yield benefits in ex-leguminous plots extended for about 20 years, and amounted to approximately 4 metric tonnes more than the ~ e l d s achieved with any o f the other cover systems. In simple economic terms this means that only 4% of the monetary benefit of a legume cover policy derives from nitrogen fixation while the remaining 96% stems from post-legume effects. Two hypotheses have been invoked to explain these effects: first, that legumes re-cycle nutrients at or near the soil surface until such a stage that they can be efficiently utilised by Hevea; and second, that legumes, by processes not fully understood, cause increased Hevea r o o t proliferation which facilitates nutrient uptake.

INTRODUCTION Plantation crops are unique amongst agronomic systems in that for much of their life they do not adequately protect the ground under which they grow. Spacing versus yield requirements dictate a minimum distance between p l a n t s . T h i s m e a n s t h a t t h e r e is a l a r g e a m o u n t o f i n i t i a l l y u n p r o t e c t e d l a n d

148 under the main crop. With H e v e a b r a s i l i e n s i s Muell. Arg. for example, this amounts to as much as 75% of the planted land area (Wycherley, 1965). Traditionally, such crops are sown on cleared ground, and it takes about 5 years for the canopy to cover the ground area (Mainstone, 1970a). During this establishment phase, the soil surrounding the rubber trees is unprotected from deleterious environmental factors including: (a) s o l a r r a d i a t i o n - - which increases oxidation of humus and so causes degeneration of soil structure; increases soil temperatures; decreases soft aeration; decreases the percolating and water-holding capacity of the soil and allows increased run-offj (b) a t m o s p h e r i c p r e c i p i t a t i o n - - annual rainfall records in most of Malaysia are high but of even more importance is the intensity of precipitation. Rates of rainfall as great as 20 cm/h have been recorded (Wycherley, 1967), and the run-off from such a down-pour would obviously cause rapid, extensive and severe erosion, (c) e v a p o r a t i o n o f m o i s t u r e - - although transpiration can account for a considerable loss of moisture from the soft, plants which produce a thick mulch in their undergrowth undoubtedly keep moisture levels in the upper soil layers higher than those experienced under bare soil, (d) w i n d - - not a major factor in Malaysian conditions, although it can cause erosion particularly during the dry season on softs with higher clay contents. Many solutions to these problems have been suggeste¢l (see for example Wright, 1912; Figart, 1925; Dijkman, 1951), and some of them have been tried (Haines, 1940). By far the most satisfactory solution however, has been to sow some form of cover plant with the main crop (Watson, 1957a; Tan et al., 1961; Gray, 1969; Wycherley and Chandapillai, 1969). At the present time, covers are thought to be beneficial in that they: (a) reduce soil erosion, particularly on steeply undulating land, (b) add organic matter to the soil and conserve soil humus by direct reduction of insolation, (c) improve soil structure and aeration, (d) improve the soil nutrient status and reduce leaching, (e) improve water infiltration and hence retention of water in the soft, (f) reduce soil temperatures, and (g) minimise competition between the main crop and noxious weeds (International Institute of Agriculture, 1936; Rubber Research Institute of Malaya 1972; Broughton, 1976). Certain deleterious effects are also experienced as well, and these include the following: (a) covers often compete with the main crop for light, nutrients, and moisture, (b) covers may reduce the availability of nutrients if their litter has a high carbon/nitrogen ratio, ~ (c) covers may act as an intermediate and/or subsidiary host for diseases and pests, and

149 (d) tall growing covers may interfere with rubber management policies and increase the fire risk during periods of dry weather (Rubber Research Institute of Malaya, 1972). It is generally considered, however, that the positive effects of a cover policy outweigh the negative ones. As most of the adverse affects can be ascribed to certain types of undesirable covers the question to be answered is no longer one of land management, but rather the agronomic one of which type of cover to plant. Malaysia is particularly fortunate in that the Rubber Research Institute has been studying this problem since the end of the 1920s (Haines, 1931, 1932; Sanderson and Haines, 1931; Akhurst, 1932; Eaton, 1935). Indeed, the information available for the rubber-cover crop ecosystem is extremely thorough and wide-ranging. Quite possibly it represents the best studied example of the effects of cover crops on soft management, soft fertility, and subsequent development of a plantation crop. For this reason, a detailed account of the Malaysian experimentation on the cover crop--H, brasiliensis system is presented, in order to define various inter-relationships between plant and soil in this ecosystem. EXPERIMENTATION AND DISCUSSION Table I summarises the post~war Malaysian research on the effects of cover plants on H. brasiliensis. Three major research groups have been involved, two at the Rubber Research Institute of Malaysia (Watson's and Wycherley's), and one at the Dunlop Research Centre in Batang Melaka (Mainstone's). Miscellaneous findings of relevance to this work have also been reported by other workers. Although the research has centered around commercially important benefits of covers to rubber, it has been particularly thorough and detailed from an ecc~ logical viewpoint. Many different trials have been conducted utilising a broadspectrum of covers on a wide range of soil types. Omissions in the collection of experimental data have been few and concern mainly the early growth of the cover, and the decay of the cover litter. From the data available it is possible to calculate the rate of growth of at least four different types of cover (Fig. 1). As not all workers included an early harvest of the covers in their trials, a reasonable extrapolation to the first year's growth had to be made. Bearing this limitation in mind, the total area under both the green material and litter curves was integrated to give the total dry matter production over a 5-year period. Mean yearly dry matter production figures are shown in Figs 2 and 3. It will be noticed that creeping legumes attained maximum growth more quickly than any of the other covers, but also declined in growth more rapidly than any of the other covers (with the possible exception of the grasses). Natural covers yielded the most green material per hectare, followed by legumes, grasses and Mikania cordata in that order (Fig. 2). Highest litter yields were found under legumes, with natural covers and grasses ranking a close second. M. cordata again yielded poorly.

1950--1967 Regent Estate Sandy-clay-loam Mixed Batu Anam (I1)

1957--1962 Selangor River Estate, Alluvial-c~y SelaDior Series(II)

Malmtone

Watson (I)

1956--1963 Bradwal] F~latej Alluvial sand

1957--1963 Sungei Bulohj Sandy-clay-loam Rengam Series(lI )

1957--1962 Selzmg F~tate Seremhan ~ r l e s ( 11)

Period,location and soft type (series)

Group leader

Cover type x 16 ( 0 . 1 2 h a )

Cover type X 16 (0.12 ha)

Cover type × 6 (0.24 ha)

Cover type X 8 (0.20 ha)

Cover type x 8 (0.36 ha)

Plot ~ e and replication

(d)

(c)

(b)

(a)

(b)

(a)

I.~pmze inoeulation

M ikania ¢ordata(Ill) Natural (spontaneously regenerated but weeded to remove legumes and noxious weeds)

C,r~ (mixed A x o n o p u i compre~us, and Paspalum conjugatum)

Legumes Inoculation (mixed experiment Calopogonium muconoides, C. pubescens and P. p h m e o l o i d e s )

Natural No mention (spontaneously reg,~nerated, but weeded to remove legumes and noxious weeds) Legumes (mixed Centrosema pub~scens and Pueraria pheselolides )

Covers used Cover

and

%N,P,K,Ca Mg,Mn in both green material

Height, Immaturity pedod Root s Amountg,]o, ll Distributinn ~',l: Disease l°

Girth ~°''

Trunk Yields6,,

Leaves %N,P,K,Ca.Mg, Mn 12 Leaf fall"

Soil analysis ''''a~'~2 pH, %N, %C, Ex Ca. Ex Mg

Roots yield ",lodz distril~ution 12

limestone s litter s`.°'' ( fertilizer 11.1= experiments)' Litter weight6,t.p C/N ratio ~`10°1=

Mg-

Complete" '"" CIRP"" Basic s l a g " "

8oii a~alysis 4 %C, %N, CIN

Height 4

Bark thiekne~" Root development 4 Leaf tall 4 Bark renewal s

Y i e l d s *,z

Leaves j %N,P,K, Ca, Mg Canopy density Trunk z Oirth *, 4 lmm*turity period~,= ,4

Rubber

Parameters measured

All CIRP(IV) %N some (NH,),SO4 at two leveM~ ( fert~izer experiments)

Fertilizer application

Spmm~ry of the more extemlve field experimentation carried out on the relationship betwee n various ground covers and the fmbsequent growth of H. bmsiUanais in Penimmlar Malaysia

TABLE I

1 2 3 4 Table II Table HI Table IV

Fig. 5

Fig. Fig. Fig. Fig.

Fig. 6

Data used in this communlcation for:

Final 390

450 Initial 330

Initial

Initial 450 Initial 370

1950 450 1952 420 1955 400 1957 360

Tree density (numharfna) m

s Watson (1961) "Watson (1963a) 7 Watmon (1963b) ' Watson et al. (1963e) ' Watson et el. (1064a) ~°Watson et al. (1964b) it Watson et al. (1963b) 12Watson et el. 1963e) 1~Watann et el. (1964e)

4 Malustone ( 1969 )

Mxiustone (1963b)

=Malustone (1963a)

' Mximtom~ (1961)

~

O! O

circa 1958 Melaha Selangor

Others

Cover xl

1959--1968 Tanch Merch Estete, Ne3ri Sembilan, Melaka S e ~ ( I x ) Ren~am Sor/e~(II)

..

M. macrophyiJa (Ill) M. mocrophyl]a vat. semi-aicta (llI) Stylosanthes gmcilis C. muconotdes C. caeruleum C. pubesee~ C. u~ramoe~is Desmodium o~lifolium M. mo~rophyl[~ (IIl) p. ph~eolo~des Tephro#i~ eandida T. vogelii

(b)

(d)

(e)

C. mueonoides C. pubeseem P. phmeoloides

Not mentioned

C. p u b e s e e n s a n d - P. phmeoloidee P. phaseoloides Inoculated with RRIM compost C. m~onoides Inoculated C. pube~we~ with RRIM P. ph~eoloides compost M. m~crophylla(m) C. pubescens and Not mentioned P. phoseolo~des A. c o m p r e ~ s P. e o v q ~ t i o u

Melastoma malabathricum Fieus ~ssu~r~o~des

odoratum M . eordate(llI)

(e)

(b)

(a)

(i)

(g) (h)

(f)

(d) (e)

(e)

(b)

Crc~piflg Probably legumes inoculated (C. pubescens P. phaseoloides Moghania maerophyllo(III) Axonopus compre~ Pasp~um conjugatum Ottoehlou nodoso Braehiaric mutico Eupatorium

Used

4 N-leveis

4 N-levels

CIRP(IV) Basic slag Mg Complete

added

Le-.ue~ %N,P,K, Mg, Ca, Mn Trunk ~ Girth Dry weight Crce~sectional area Yield Sork-thickneu Root disease *~

Soil %N, %C

%N,P in leaves

G~th ImmRturity period Yield Incidence of root disease, Leaf % Ash, %N,P,K, Mg Ca, Mn

Immaturity period

about 370

450-500

G~ %N & P in leaves

250--410

Cost 2° Yield

Girth, Costs"

N(NH~), N(NO;), pH

Soil Analysis 16 %N,P,C

%N,P,K, Mg, Ca, Mn of .or~n material and litter Root depth Root density C/N of litter

Note: (I) Some workem, particularly the Watson group, have carried out such a wide variety of experiments on different soil typm, using different fertilizer regimes L,ld so on, that it is not po~ible to list them all in detail here. Instead, representative triads have been chosen to illustrate the principal features of their experiments. Wherever possible, comparisons in this communication were based on data from experiments which used a no-fertilizer application as one of the variables. (II) see Owen (1951). (Ill) Mikanla eordeta -.M. scandan~ Moghania macrophylla = Fleminsia congesta (see Wycherley, 1960, 1963, 1963; Watson et el., 1964a). (IV) CIRP = Christmas Island Rock Phosphate.

Cover type x 4 (0.17 ha) Fertilir~er x 4 (0.17 ha) Cover type × 4 (0.19 he) Fertilizer x 4 (0.19 ha)

Cover type x 4 (0.2 ha)

1961--1967 Bukit Kajang E~tate, Melaka and R~n~n~ ~r~e~( !I ) 1961--1967 Victoria Eatate, Kedch

Singapore

1958--19~1 R.R.LM. Experiment Station Sungei Buloh and 8erdeng serles(ll)

Wycherley

2=Warrinr ( 1 9 6 9 )

~'Pu~parajch and Chellapehrajch (1969)

z0 Chew ( 1 9 6 8 )

"McIntcah (1963)

"Menon (1963)

~" Wycbe_~iey c,Lq~_~, *s Wycherley (1963) ,6 Wycherley (1965) ,7 Wycherley and Chandapilla/(1969)

O1

152

/ , ~

(a)LEGUMES

/'/\ \

l- 6-

O

Ul

, /

•~

2-

E

>" hi

iI

"-

/// /

O_J UJ rY

"-..

"/

E o

(b)GRASSES

,¢;/7 I

I

I

i

i ~

"

4

~.i

(c)MIKANIA

(d) NATURALS

8-

I,.-

N 6IE >. I1: ¢-~

/ // /

,~-

" ",2" "

//

/,~?" 1

/

/ //

i i iI / I ii//i I I/

2

3

4

5

, 1

6

. 2

, 3

, 1,

, 5

. 6

Time (yrs)

Fig. 1. Growth o f various covers when planted under H. brasiliensis. The data were plotted from those shown in Figs. 2 and 3, and extrapolated to Year 1. In this figure Year 2 corresponds to 1959, Year 3 to 1960 and so on of Figs 2 and 3 . 0 represents green material (shoots) and o the litter. See also the legends to Figs 2 and 3.

A. Nutrient analysis o f cover crops Nutrient contents of the various covers were re-calculated from Watson's data (Watson et al., 1963 a--d). This information is recorded in Figs 2 and 3. The nitrogen contents of both the shoots and litter of legumes were as much as twice that in comparable portions of the other covers. Leguminous shoots also contained more phosphorus and manganese. In fact, judged by mineral contents, legumes were only at a real disadvantage with respect to potassium. A similar situation exists with respect to the litter, although the percent nutrient content of the litter was generally lower than that of the shoots (cf. Figs 2 and 3). This could have been due either to a "holding back" of nutrients by the plant before leaf senescence, or mineralisation reducing the nutrient value of the litter. In all probability both factors are of importance, and in any event do not affect the manganese levels, for contents of this essential micro-nutrient were invariably higher in litter than in the shoots. A further calculation was made that involved multiplying the mean nutrient contents (over a 4 year period), by the mean yearly dry matter production. Data for the shoots and litter were calculated separately, summed, and'the differences in total nutrient content (kg/ha) between legumes and the other covers plotted as Fig. 4. This technique is validated by the finding of Figs 2 and 3 that whilst the amount of shoot or litter material varies with time, the

153 SHOOTS Dry Matter °I°N

%P

"IoK

°loCa

"loMg ppm Mn

1959 13(°°) 6

~

~

~ter,~...~t 1<°°)

I

e

Q

(00) ~E,

2(00)

Dry Matter °IoN

"loP

"I,K

"/,Ca

°l=Mg ppm Mn

Fig.2. Dry weights and mineral contents (expressed as percentage dry weight) of the green material (shoots) of various covers when planted under H. brasiliensis. T h e data were averaged from those contained in Tables 1 to 4 of Watson et al. (1963c). Mean shoot dry weights calculated in this manner were used to plot Fig.l, Legumes, ~ a~'asses, ~ M. cordata and [] naturals.

relative proportions of the various nutrients remain approximately constant. When the data were treated in this manner, legumes were shown to be superior to both grasses and M. cordata in terms of mineral contents. Total phosphorus, potassium, calcium, and magnesium levels of legumes were slightly lower than the 'natural' covers, b u t this was mainly a result of the large mass of dry matter produced (Fig. 1). 'Natural' covers were nutrient poor, when the mineral content was expressed on a dry weight basis (Figs 2 and 3). Leguminous litters had higher percent mineral contents than all the other covers, with the sole exception of the potassium contents of the 'naturals'. Total mean legume advantages are also shown in Fig. 4. Whilst legumes generally had higher nutrient contents, they were significantly superior in two main areas -- nitrogen and manganese contents. The nitrogen advantage a m o u n t e d to 151 kg per ha per year when compared to all the other covers.

154 LITTER Dry Iv~tter %N

°I.P

%K

°/,,Co

%M9 ppm Mn 1959

~

I a(°°) _~Z(OO)

G, oo,:

1960

~a(oo)~

196, 13c°°' Z (00)

"~)

B $

~°°~ ~

MeQns(1959to 1962)

~

Dry Matter °I°N

%P

"/,K

%C0

%Mg

I

3(007

Z(O0)

ppm Mn

Fig.3. Dry weight and mineral contents (expressed as percentage dry weight) of the litter of various covers when planted under H. brasiliensis. The data were averaged from those contained in Tables 1 to 4 of Watson et al. (1963c). Mean litter dry weights calculated in this manner were used to plot Fig.1. [] Legumes, ~ grasses, [] M. cordata and [] naturals.

Note too that this does n o t include excess nitrogen in the roots (about 20% of the total -- Hamilton and PiUay, 1941), possible losses of nitrogen to the atmosphere, and the release of nitrogen by decaying roots. Nor should it be forgotten that this a m o u n t of nitrogen is accreted on a m a x i m u m o f only 75% of the total land area. This estimate is therefore conservative, but it is an extremely reliable one. It represents the mean nitrogen surplus (over a 5-year period and a large land area), minus t h a t which could conceivably be extracted from the soil as NO;, NH~ etc. The rate of dinitrogen fixation might seem excessive, but it is in fact near the lower observed limits for tropical legumes (Table II). It can readily be p u t into perspective by assuming that, as this represents actual nitrogen added to the soil per year, it is equivalent to adding 720 kg/ha of

155 N

P

K

Co

Mg

Mn ~

N

- P

K

Ca

Mg

Mn~

+80

0-8

+6O

0.6

+40

0.4

+20

'0.2 0.0

(a)

-20

"2

v

e-

.o

E

uo

E

0.8

+6OH

06

'~'

+407

0,4

~J~

+20-

0-2

o~

0.0

-20

(b)

O~

+60

OS

+20v///J

II

H

0 ~"

E

O LJ

0.2

"~

0.0

"r"

(c) LEGUMES VS. NATURALS

z!

2

2

+8O

0.8

+6O

0.6 ' ~

+40

0.4

'~

0.2

U

+2O

[-7

0

.~J

er-

0-0

-2O

(d)

LEGUMES

VS. ALL OTHERS

Q tO

v C

+40-

-2O,

P

[3

.9

LEGUMES VS. MIKANIA

+8O

!

e"

"~'

+8o

0I

Z

LEGUMES VS. GRASSES

)

+140 +lz,o

+100

.1.0

+6o

0.8

+60

.0.6

+40

"0.4

+2.0

0,2 0.0

(e) TOTAL LEGUME ADVANTAGE N

P

K

Ca

Mg

Mn e

Fig. 4. Comparison of the nutrient contents of legumes and other covers. Total contents of each nutrient were calculated as follows: (a) the shoot (or litter) dry weight was calculated by integrating (from year 0 to year 5) the area under the appropriate curve shown in Fig. 1. (b) Total yields obtained over this 5-year period were averaged and multiplied by the mean nutrient contents (Figs 2 and 3) to give total nutrient contents in kg per ha per year. (c) Differences in total nutrient contents between legumes and other covers were plotted as Fig. 4(a) to (d). (d) Fig. 4(e) was obtained by plotting the difference in combined total nutrient contents ~shoots plus litter) between legumes and the other covers. Figures on the right vertical axis apply to Mn only.

156 TABLE H Observed rates of nitrogen fixation in tropical creeping legumes Legume

Habitat (and locality)

Nitrogen excess (kg/ha)

References

Glycine javanica

Cover crop (Kenya)

200

Centrosema pu bescens

Pot (Malaysia)

235

Jones (1942) see also Henzell and Norris(1962) Watson (1957b)

C. pubescens intermixed with Cynodon plectostachyus

Pasture (Nigeria)

280

Moore (1962)

Pueraria phaseoloides

Cover crop

650

Jaiyebo and Moore (1963)

Calopogonium muconoides plus C. pubescens plus P. phaseoloides

Cover crop

170 (maximum)

Watson et al.(1963a)

C. pubescens with Panicum maximum

Pasture (Australia)

100

Bruce (1965)

Desmodium uncinatum

Pasture (Australia)

125

HenzeU et al. (1966)

Phaseolus atropurpureus or Stylosanthes humilis

Pasture (Australia)

20--290

Henzell (1968)

P. atropurpureus

Pasture (Australia)

44--81

Vallis (1972 )

C. muconoides C. pubescens P. phaseoloides

Cover crop (Malaysia)

151 (average) 200 (maximum)

this communication

(Malaysia)

a m m o n i u m sulphate (21% nitrogen). Furthermore, atmospheric nitrogen is slowly and continuously added to the ecosystem by legumes. Fertilizerapplication is at best only periodic, and, due to a variety of environmental influences, is extremely wasteful of nitrogen which is the major limitation to plant growth in tropical communities (Date, 1973). B. N u t r i e n t analysis o f H. brasiliensis laminae I t has b e e n e s t a b l i s h e d t h a t l e g u m i n o u s covers c o n t a i n h i g h e r m i n e r a l c o n t e n t s in t h e i r foliage t h a n a n y o f t h e o t h e r c o m m o n l y u s e d covers. With

157

%N

°I,P

%½

"/,Co

"I,Mg

PPm Mn E Q. e~

v t-

v

0 Q.

E

E .3 o

0 U

Z

(b)

0.2

.6

v

w-

D. ~

0.1

P ~.~

i5

0.0

§

tO

U

I[

°IoN

"I.P

°I,K

%Co

°l, Mg

ppmMn

Fig. 5. Mean nutrient contents o f young H. brasiliensis laminae grown under different cover regimes. In (a) the data were averaged from those shown in Watson et al. (1964a, Table 6), Watson et al. (1964b, Table 4 and 5 without fertilizer treatments only), and Wyeherley (1965). Fig. 5(b) shows the mean differences in nutrient contents between H. brasiliensis laminae grown under leguminous as opposed to other covers. B Legumes, m grasses and [] M. cordata.

leaf senescence and abscission, as well as the eventual death of the covers due to competition from the developing rubber stand, these nutrients are returned to the soil via litter. Accordingly, one would expect higher proportions of these nutrients in rubber grown in association with legumes than would be so in the non-legume case. That this is indeed so is apparent from Fig. 5. Leaves from rubber trees grown in association with legumes had higher contents of every mineral except potassium. Often the differences were not large, but generally the authors w h o collected the prime data found significant effects. For example, the difference between legumes and all other covers with respect to manganese (calculated from Figs 2 and 3) was about 1 0 0 ppm (0.01% on a dry weight basis). N o t surprisingly therefore, rubber trees grown in association with leguminous covers had, on average, about 8 ppm more manganese in their leaves. Similarly, the presence of leguminous covers enhanced laminae nitrogen contents by 0.17%.

C. Growth of H. brasiliensis The trend towards an improved nutrient status of H. brasiliensis grown in association with legumes is continued when one considers actual growth of the tree. Leguminous covers allow increased dry weight production (Fig. 6a),

158 (a) DR

-~o

120'

A -30 uE v ¢-20 .--~

90¸ t~1

60¸ 30,

-10

Time (yrs)

/

5 0

0.5

.7 03 03

PcmeI

.5

~ Panel A

-5

ti

06

-3

Z 0.5¸

Time (yrs) 4.0 100%

018

10115 16123 25130 33138 ~250

(¢) % T A P P A B I L I T ~

.Q O

I

4Y

75%

e'~ 50%

30%

20O0

0

o.~

Time (yrs)

1.o

1.s

10oo

750

>"

500 250 0

1

z

3

~

5

s

7

5

s

lO

Time (yrs)

Fig. 6. (a) Increase in estimated total dry weight of I-I. brasiliensis when grown with leguminous (+) as compared to other covers (Axonopus compressus, Paspalum conjugatum and M. cordata) (o). The data were taken from Wycherley (1965, Table 2). (b) Increase in girth of H. brasiliensis when grown with leguminous (+) as compared to other covers (grasses, M. cordata and "naturals") (o). Data are means of those taken from Watson et al. (1963c, Tables 21 to 24 -- experiments without fertilizer only) and Wycherley (1965, Table 3). (c) Increasedn thickness of renewed H. brasiliensis bark when grown in association with leguminous (+) as opposed to "natural'covers (o). The data were plotted from those shown in Mainstone (1969, Table 6). The data for each treatment are significantly correlated. (d) Number of rubber roots per unit soil depth. Trees grown in association with legumes (+), trees grown in association with grasses (o). The figure was redrawn from Fig. 3 of Mainstone (1969). (e) Effects of leguminous versus non-leguminous covers (grasses plus M. cordata plus naturals) on the percent tappability of tL brasiliensis. The data were averaged from those shown in Table 10 ("no fertilizer" treatments) of Watson et al. (1964b). (f) Effects of leguminous (+) versus "natural" (o) covers on the dry rubber yield of H. brasiliensis. The figure was plotted from the data contained in Table 1 (low nitrogen treatments only) of Mainstone (1969).

159

increased trunk girth (Fig. 6b), increased tree height (Mainstone, 1969), and increased thickness of bark-renewal (Fig. 6c). Once again these differences are significant, and with the exception of tree height (which allows increased wind damage), are commercially desirable. Legume covers also decrease the immaturity period by about 1 year (Fig. 6e), and caused increased latex yields lasting for at least 10 years (Fig. 6f). Mainstone has been the only worker to continue his studies well into the maturity phase, and he found that the increased girth noted in Fig. 6b, was less marked but still present 10 years later (Mainstone, 1969). D. Nutrient status o f the soil

The increased nutrients that are available from the growth of leguminous covers, might be expected to make at least a transitional appearance in the soil before being re-absorbed by H. brasiliensis. Carbon/nitrogen ratios of 12--15 : 1 are generally considered to indicate nitrogen assimilation and fixation by soil-microflora. Guha and Watson (1958) showed, for soils and litters similar to those upon which the present discussion is based, that a carbon/ nitrogen ratio of 20 : 1 or lower is necessary for the rapid mineralisation of nitrogen. Of the four covers examined by Watson et al. (1963c) only the legumes attained this ratio (Table III). Both the grasses and naturally regenerated covers had C/N ratios in excess of 40, and the lowest value for M. cordata was 27. Further, the C/N ratio for legumes continually declined to reach 12.5 : 1 when the cover was beginning to suffer from the competitive effects of H. brasiliensis. For these reasons we may use C/N ratios as a measure of the probable rate of decay of the litter, and hence the availability of nutrients through re-circulation. Over the immaturity period, the quantity of nutrients in the litter, and the C/N ratio probably account for a large degree of the variation in tree growth (Fig. 7). Litter with a favourable C/N ratio will be rapidly mineralised. T A B L E HI

Carbon/nitrogen r a t i o s o f litter accumulating f r o m t h e g r o w t h o f various covers s o w n in association w i t h H. brasiliensis. T h e d a t a were averaged f r o m those presented in T a b l e s I t o I V o f W a t s o n e t al. ( 1 9 6 3 c ) . Year

Legume

Grasses

M. cordata

"Naturals"

1959 1960 1961 1962 Means

17.2 16.2 13.4 12.5 15

43.3 57.6 42.8 39.0 46

23.2 39.2 23.7 21.7 27

35.3 49.7 36.2 37.7 40

M e a n = 38

160

3.8"

(a)

p >0-001

(b)

p>O-05 '50

"~

3.7"

.g r-

"rl .g z

~D 3.s-

.&0

3.4-

°/o N in Litter

% N in Litter

1~o (C)

=!o

Jo p>O-O5

,I0

2'0

(d)

~o p>O.05

54-

'50

52'~ v

50v r-

46"

t9

44"

40

42" 40"

ppm ~

lbo

in Litter 2bO

CIN Ratio of Lith=r 3bo

ib

sb

Fig. 7. Correlations between various nutrient contents of legume litter and different growth parameters of H. brasiliensis. The figures were plotted from data of Wycherley (1965). (a) % nitrogen in Heuea laminae = 3 . 3 7 2 + 0 . 1 0 6 × % nitrogen in litter, (b) girth ( c m ) ffi 1 4 . 6 8 + 1 . 2 1 5 x % nitrogen in litter, (c) girth ( c m ) ffi 1 4 . 6 8 + 0 . 0 2 0 6 X m a n g a n e s e c o n t e n t of litter (ppm) and (d) girth (cm) = 60.24 - 5.05 In x.

All the nutrients, b u t particularly nitrogen itself, will stimulate growth of Hevea upon liberation.

Wycherley (1965) advanced the hypothesis that the most important relationship is that between tree growth and nutrient turnover (inversely proportional to the C/N ratio). In Wycherley's trials, manganese was the nutrient in most limited supply, and he found a good correlation between overall growth and manganese in the litter (Fig. 7c), and between growth and the C/N ratio of the litter (Fig. 7d). Of course, nitrogen deficiency most often limits growth (see Date, 1973), and there are many recorded instances o f H e v e a growth responses to nitrogenous fertilizer (see for example, Watson et al., 1963c; Mainstone, 1969; Pushparajah and Chellapah, 1969). Laminae contents of nitrogen appeared to be satisfactory in all the experiments reported above however (see Chan et al., 1972). Turning to the nutrient status of the soil itself (Table IV), we find that all mineral contents, with the exception of nitrate-nitrogen, were the same or lower in soil under legumes than under any of the o~her covers. This was also true of the pH and moisture contents. The differences in nitrate-nitrogen were quite significant, and amounted to an increase of 250% over that under the

161

TABLE IV Effect of leguminous and other covers (grasses plus M. cordata' or grasses plus M. cordata plus "naturals ''2) on the fertility of soil under H. brasiliensis. The data are averaged from those presented by Watson et ai. (1964a) (Table 5) 1, Watson et al. (1963c) (Table 25) 2 and Watson et al. (1963b) ~

Soil depth (cm)

Legumes

Others

parameter

pH ' %C 1 % Total N 1 N (NH~) (ppm) 2 N (NOs) (ppm) 2 Total P (ppm) = available P (ppm) = Exchangeable K 1

0--15

30--45

0---15

30--45

5.4 2.3 0.17 4.2 3.0 260 28 0.06

5.3 1.2 0.08 1.8 2.4 150 14 0.02

5.6 2.5 0.17 4.6 1.2 290 33 0.08

5.4 1.3 0.09 2.2 1.0 160 13 0.03

(me/lO0 g) Excnangeable Ca ~

1.6

0.31

1.8

0.92

(me/100 g) Exchangeable Mg 1

0.11

0.03

0.16

0.04

(me]lO0 g) % Moisture 3

19.1

17.6

29.4

19.6

other covers at b o t h soil depths (see also Watson et al., 1964c). Quite possibly this is the reason for the elevated nitrogen levels observed in the laminae of Hevea. One must remember, however, that leaves from rubber grown in association with legumes contain not only higher levels of nitrogen, b u t all the other minerals (with the exception of potassium) as well (cf. Fig. 5). The leaf area index will affect the total amounts of nutrients held in the canopy, b u t obviously higher soil nutrient levels cannot be the reason for these differences. So, again, we are forced to conclude that it is the turnover of nutrients, rather than the absolute amounts which are important. Accordingly the following interpretation is suggested. When the rubber trees are young, their rooting systems are undeveloped, and probably do n o t extend far into the inter-rows. Covers quickly develop over the inter-row area, and protect the soft. Through their normal growth, covers 'save' or 'bank' nutrients within their roots and shoots. Gradually, the canopy of H. brasiliensis begins to cover the inter-rows, and competition causes a gradual death of the covers. At the same time rubber roots grow o u t and under the covers (Soong, 1970). Eventually, due to the normal physiological processes of ageing, to increased root competition, and to the shading effect of the rubber canopy, the covers begin to defoliate. Litter accumulates and is mineralised providing it possesses a satisfactory C/N ratio. The rubber, with its n o w more extensive r o o t system, is better able to withdraw nutrients previously contained in the cover. Or, in

162 8

?

After 2 years At ptanting

6

r

.-~ 4

Groun(

_ k._......... i.......................

level

i

r-2

'7 I I' I

I

I

I

I

I

I

I

I

I

t

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

After 6 years

16

14 After

12

4 years

lO E v .=_=8

level t- lGround

~-4

I

6

'

:.

'

~

'

J

o

I

~

I

Planting distance (m)

I

4

I

~

;

'

I

~

I

~

I

J

o

I

2

:.

I

Planting distance (m)

Fig. 8. D e v e l o p m e n t o f a t y p i c a l H. brasiliensis a n d l e g u m e c o v e r s t a n d over t h e first 6 years o f g r o w t h . E s t i m a t e s o f t h e r a t e o f g r o w t h of t h e a b o v e - g r o u n d p o r t i o n o f t h e r u b b e r tree were t a k e n f r o m M a i n s t o n e ( 1 9 7 0 a ) . R a t e s o f g r o w t h o f t h e r u b b e r r o o t s were c a l c u l a t e d f r o m d a t a c o n t a i n e d in S o o n g ( 1 9 7 0 ) . S p r e a d o f lateral r o o t s f r o m t h e t r e e was e s t i m a t e d as: a t t h e e n d o f 2 years - - 4.4 m, at t h e e n d o f 4 years - - 6.6 m, a n d a f t e r 6 years - - 9.9 m. D e v e l o p m e n t of t h e a b o v e - g r o u n d p o r t i o n o f t h e l e g u m i n o u s c o v e r c r o p was c a l c u l a t e d f r o m Fig. 1.

other words, the legume cover serves as a 'bank' -- accumulating nutrient capital which is subsequently made available to the rubber. Legumes are particular good 'banks' for several reasons including: (a) they rapidly expand over newly planted areas with the result that as much soil as possible is canvassed for 'deposits', (b) they provide 'interest' in the form of added nitrogen, and (c) nutrient 'withdrawals' are speedily affected because

l

i

~FTER 12 MONTHS

L

J

~

@

Horizontal Distance (rn)

i

4

i

L~

2

~1

i

i

L~

~

ETER 24 MONTHS

@

AFTER 6 MONTHS

L~

2

i

~ 4

k

L~

@

6

i

5U

5

Fig. 9. D e v e l o p m e n t o f a typical H. brasiliensis and l e g u m e cover stand over the first 2 years o f growth. Rates o f d e v e l o p m e n t o f ~he rubber tree and o f t h e above-ground p o r t i o n s o f the l e g u m e cover were calculated as described in Fig. 8. G r o w t h o f the Legume rooting s y s t e m was c o m p u t e d from data c o n t a i n e d in Chandapillai ( 1 9 6 8 ) (data for C. pubescens, P. phaseoloides w e r e ~veraged). Arrows have b e e n used t o represent: L - - l e a c h i n g o f nutrients from the soil; u - - u p t a k e o f nutrients from the soil particularly by legumes; R - - return o f nutrients to the soil f r o m the l e g u m e litter ( m u l c h ) ; and N~ - - nitrogen f i x a t i o n b y the Legumes. The relative size o f the arrows and the n u m b e r o f arrows is used to represent the i m p o r t a n c e o f these processes.

5

e-

U

E

,T PLANTING

-Iorizontat Distance (m)

164 of the low C/N ratio of leguminous litter. A schematic illustration of this is shown in Figs 8 and 9. In Fig.8, the development of a typical rubber stand during the first 6 years of establishment is represented diagrammatically. Whilst vertical growth of the roots is slow, they rapidly develop in a horizontal direction and cross in the inter-rows somewhere between 1 and 2 years after planting. Legumes begin to fix nitrogen any time after the first 2 or 3 weeks of growth (Broughton et al., 1977), and start to accumulate litter after a b o u t 6 months (Fig. 9). As observed earlier, the legumes rapidly cover the ground surface (see also Figs 1 and 9), and speedily canvass the soil for nutrients to a b o u t a depth of i m (Fig.9). Legume litter accumulation probably reaches a maximum 12--18 months after planting (Fig. 1). As this litter begins to decay, the rubber roots are n o w ideally placed to absorb the returned nutrients (Fig. 9). This process intensifies during the ensuing months, and reaches a climax somewhere between the 2nd and 3rd year of growth (Fig. 1). After this the legumes begin to decline in vigour, and b y the 6th year, covers of all sorts represent only a minor portion of the total flora of the rubber plantation (Figs 1 and 8). It can easily be seen from Fig.8 that their demise is probably due to many factors including a substantial diminution in available light at ground level and to intense competition between both rubber and cover roots for nutrients and moisture. This hypothesis easily explains the benefits that accrue to rubber when planted with legumes during the immaturity phase, b u t one perplexing fact remains. The yield benefits from a leguminous cover extend well into the maturity phase - - l o n g after the legumes have disappeared. Malnstone (1969) d o c u m e n t e d this effect, and the metric transformation of his legume cover yield benefit equation (both low and high fertilizer treatments) is Yield benefit (dry rubber -- kg/ha) = 377.1 - 20.3 × time (year). This means that the yield benefit will expire after 18.6 years of exploitation. Where possible in this communication however, discussion has been based on 'no fertilizer' treatments, in the belief that differences obtained in this manner better reflect inherent cover performance. A low fertilizer treatment was included in Mainstone's trials, and if only these data are used, the regression becomes Yield benefit (dry rubber -- kg/ha) = 483.1--23.1 x time (year), with the yield benefit expiring after 20.9 years. Malnstone did not use this equation since (a) the rubber came from different panels on the tree and (b) the linear correlation is low, b u t it serves to further exemplify the magnitude of the legume-cover benefit. If one accepts a yield benefit expiry time of a b o u t 20 years, then this covers almost the entire commercial lifetime of H. brasiliensis -- 5 years immaturity, and 20 to 25 years yielding. To explain these post-legume benefits to the rubber tree, one must invoke an additional hypothesis. Hevea root numbers over much of the soil orofile are increased in the presence of legumes (Fig. 6d and Mainstone, 1969; 1970b).

165

Watson et al. (1964a) recorded similar data, with even greater differences due to legumes. Exactly how legumes stimulate the rooting of Hevea is not known, but three facts have emerged which are important in this connection: (a) the roots of H. brasiliensis are known to be widely associated with a vesiculararbuscular mycorrhiza (Endogone) (Wastie, 1965), (b) some tropical legumes respond remarkably to mycorrhizas (Crush, 1974), (c) addition of phosphate stimulated Hevea root proliferation (Watson et al., 1963d), and (d) the physical properties of soil under rubber are enhanced by the growth of legumes (Soong and Yap, 1976). Endogone is known to be particularly efficient at transmitting nutrients from the soil to its host plant (Mosse, 1973). Undoubtedly, a particular combination of Endogone, Rhizobium and cover crops stimulates root proliferation of Hevea. Presumably this, plus the improved soil nutrient status, gives the longterm basis of stimulation by legume covers. Indirect support for this hypothesis comes from two sources. First, higher levels of every mineral (except potassium) are found in rubber leaves grown in association with legumes, but the levels of these same nutrients in the soil are, in fact, lower. This is particularly true of total nitrogen and manganese -minerals with which the legume advantage is greatest. It can only be explained by facilitated removal of nutrients from the soil under legumes. Second, Mainstone (1969) in his 10 year follow-up on the benefits of legumes found that the ground vegetation was sparser in ex-legume plots. This was not due to a denser rubber canopy (differences due to various covers had disappeared) but presumably to increased competition from rubber roots. CONCLUSIONS

In general terms we may summarise the legume benefits as follows: (a) legumes are efficient at protecting soil from other environmental influences as a result of their rapid ground coverage, (b) legumes act as a 'bank' for nutrients, allowing a large pool of essential elements to be rapidly accumulated; legumes also fix nitrogen; and because of their favourable C/N ratio, allow rapid return of these nutrients to the soil, (c) legumes established within the inter-rows of young rubber increase aggregation of finer soft particles; increase the average size of the soil aggregates; increase total soil porosity; and they increase the permeability of the soft to the downward movement of water (Soong and Yap, 1976), (d) a complicated and extremely important interaction probably occurs between Hevea and legumes allowing increased proliferation of rubber roots and (e) as a result of (a) to (d) increased productivity and yields of H. brasiliensis accrue. The benefits of legume covers may be less marked in rubber grown on the better soil types (the response will be less on the coastal clays than on the inland soils) (Watson et al., 1964b and cf. Broughton, 1976), and can be

166 TABLE V Total potential Peninsular Malaysia import and export savings derived from the use of leguminous covers under H. brasiliensis Parameter

Data

Reference

Import savings (a) Planted rubber area

1,740,000 ha

Mid-Term Review Second Malaysia Plan. Calculated from (a) assuming 5 year immaturity and 25 year yielding Differences between legume and non-legume recommendations for rubber contained in Pushparajah et al. ( 1 9 7 4 ) averaged over the 5 year immaturity period. Imperial Chemical Industries, March 1975

(b)

Annual replanting area

(c)

A m m o n i u m sulphate* requirement

0.4 metric tonnes/ ha/year

(d)

Cost of ammonium sulphate I m p o r t (b) × (c) × (d) saving

$M630/metric tonne =$M14,616,000

(e)

Export earnings (f) Increased yield

(g)

Area yielding

(h) (i)

Dry rubber price Export earnings

58,000 ha

about 4 metric tonnes dry rubber/ 20 years = 200 kg/ year 1,450,000 ha $1.20/kg (f) × (g) × (h) =$M348,000,000

see this communication Section D. Difference between total planted area and total immature area May 1975

(e) ÷ (i) Total

= $M362,616, 000

*Note: Whilst this communication has demonstrated that leguminous covers are capable of providing almost twice as much nitrogen (as the value listed here) to the rubber per year, the figure of 0.4 metric tons has been used since it foUows the Rubber Research Institute of Malaysia's recommendations.

partially offset by higher rates of fertilizer application (Pushpamjah and CheUapah, 1969). However, in a follow-up to these experiments, Pushparajah and Tan (1976) reported that it took 11 to 15 years after cover establishment before yields of dry rubber in nitrogen-fertilized grass plots were comparable to those in unfertilized legume plots. By this time an additional 840 to 1080 kg (as N) of nitrogenous fertilizer had been added directly to the soil surrounding the trees in the grass plots. This further exemplifies the legume benefit to the rubber crop and could be used as the basis of a cost--benefit analysis.

167 On the other hand, many factors control the cost of legume covers to the planter. Rather than consider t h e m in detail, Table V simply indicates the total potential monetary benefit to Malaysia over import (fertilizer) and export (dry rubber) costs. When considered in this manner, legume benefits are truly enormous. Nutrient re-cycling and nitrogen fixation are important, b u t b y far the greatest legume benefit appears to be the increased rooting capability of Hevea, and amounts to approximately 96% of the total monetary benefit accruing from a legume cover policy. Similar legume benefits are obtainable with other plantation crops. Comparable post-legume benefits are observable in experiments with the oil palm, Elaeis guineensis Jacq (see Broughton, 1976), although detailed soil and r o o t analyses have y e t to be made. The situation with respect to c o c o a (Theobroma cacao L) (see Mainstone, 1971) and tea (Camellia sinensis (L.) O. Kuntze) is more complex in that cover or shade crops are only required for a restricted period during the establishment phase. Furthermore, these effects have not been well documented. Nevertheless, legumes would also seem to play an important role with these crops. It may well be that the use of legumes and judicious agronomic management practices can provide long-term benefits deriving from a multitude of soil-related factors in many tree-cover crop ecosystems. ACKNOWLEDGEMENTS The author wishes to thank Mr. B.J. Mainstone, Dunlop Research Centre, Batang Melaka, Malaysia for permission to publish Fig. 6d and for critically reading the manuscript. Mr. C.K. John of the R u b b e r Research Institute of Malaysia made available much information on covers and rubber. Amir bin Salleh drew the figures. Financial assistance was provided b y the International Foundation for Science (Stockholm) and the University of Malaya. REFERENCES Akhurst, C.G., 1932. The carbon and nitrogen contents of some natural covers. J. Rubber Res. Inst. Malays., 4: 131--139. Broughton, W.J., 1976. Effect of various covers on the performance of Elaeis guineensis Jacq. on different soils. In: Proc. Malaysian Int. Off Palm Conf., in press. Broughton, W.J., Hoh, C.H., Behm, C.A. and Tung, H.F., 1977. Development of the nitrogen fixing apparatus in tropical legumes. Submitted to Aust. J. Plant Physiol. Bruce, R.C., 1965. Effect of Centrosema pubescens Benth. on soil fertility in the humid tropics. Queensl. J. Agric. Anim. Sci., 22: 221--226. Chan, H.Y., Soong, N.K., Woo, Y.K. and Tan, K.H., 1972. Manuring in relation to soil series in West Malaysian mature rubber growing plantations. Proc. Rubber Res. Inst. Planters Conf. 1972, pp. 127--139. Chandapillai, M.M., 1968. Studies of root systems of some cover plants. J. Rubber Res. Inst. Malays., 20: 117--129. Chew, C., 1963. A realistic approach to the use of legumes as a cover plant. Plant Bull.

Rubber Res. Inst. Malays., 68: 150--154.

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