E L S E V I E R Field Crops Research 38 (1994) 29-36

F i e l d C r o p s R e s e a r c h

Unburnt bush fallows a preliminary investigation of soil conditions in a bush fallow and two successive crops of taro

( Colocasia esculenta (L.) Schott) in Western Samoa

D.P.C. Stewart * Department of Agriculture, University of the South Pacific, Alafua Campus, Apia, Western Samoa

Received 29 July 1993; accepted 29 June 1994

Abstract

Little research has examined shifting cultivation systems that do not burn their bush fallows. This work examines three phases of shifting cultivation (mature bush fallow and the first and second successive taro crops) on five inceptisols (Falealili, Tiavi, Etimuli, A'ana and Avele) in Western Samoa during May 1990. Traditional fanning practice was used on all sites, including not burning the bush fallows after they were cut down. The soil bulk density increased with the phases of shifting cultivation on the Falealili soil. The soil water content at both field capacity and wilting point and the available water capacity increased with the incorporation of the bush fallow, and then decreased with cropping. The flush of nutrients following the bush fallow was small, and included increases in potassium on the Tiavi soil, calcium on the Etimuli and A'ana soils, magnesium and total exchangeable base Content on the Tiavi and Etimuli soils, manganese on the Avele soil and zinc on the Etimuli soil. There were also decreases in soil phosphorus on the Tiavi and Avele soils. Changes in taro nutrient concentration with cropping included decreases in foliar nitrogen on the Falealili soil and sodium on the A'ana soil, and increases in foliar magnesium on the Etimuli and A'ana soils. More long-term research is needed on shifting cultivation systems that do not burn their bush fallows.

Keywords: Colocasia esculenta; Nutrient dynamics; Shifting cultivation; Soil physical properties; Unburnt bush fallow

1. Introduction

The shifting cultivation system is in common use in Western Samoa and on many of the surrounding Pacific islands (Vergara and Nair, 1985). In Samoa, land is normally cleared sequentially with the larger trees being cut down or ring-barked first, and then the remaining undergrowth slashed back to ground level. Land clearing commonly takes up to a year and the organic material is usually not burnt (Farrell, 1962). Taro (Colocasia esculenta (L.) Schott) is often

* Corresponding author. Present address: Department of Soil Sci- ence, Lincoln University, P.O. Box 84, Canterbury, New Zealand.

0378-4290/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSD1037 8-4290 ( 94 ) 00046- 8

planted as the main crop along with other food crops. Generally, each taro crop lasts a full year (9 months for the main crop and 3 months for the ratoon crop). A 2- to 15-year bush fallow is typically followed by 2-4 years of cropping (Farrell, 1962). The cropping phase usually includes 1-2 taro crops. The organic material from the bush fallow eventually provides large quan- tities of mineral nutrients for subsequent crops.

There are very few data on the effect of cutting bush fallows that are not burnt, and none from Western Samoa. Adedeji (1984) observed that forest regener- ation was faster in burnt than unburnt plots in Nigeria, although the bush fallow trash had been removed from

30 D.P.C. Stewart~Field Crops Research 38 (1994) 29-36

Table 1 Site and soil characteristics of the farms

Location Altitude (m) Mean annual Aspect/Topographic Slope angle Soil mapping Soil taxonomic Soil texture rainfall (mm) position (o) unit a classification b class

Moamoa 45 3200 North/River terrace 0-3 Falealili Clayey-skeletal, Oxidic, Clay loam Isohpyerthermic, Oxic, Dystropept

Lepiu 610 4600 North/Shoulder, ridge 5-10 Tiavi Fine, Mixed, Sandy clay Isothermic, Oxic, loam Humitropept

Sa'agafou 245 4400 East/Backslope, hollow 15-20 Etimuli Fine, Oxidic, Clay Isothermic, Oxic, Humitropept

Aleisa 180 3800 North-east/Toeslope, 5-10 A'ana Fine, Oxidic, Clay hollow Isohyperthermic, Oxic

Humitropept Vaoala 400 4100 North-west/Footslope, 5-10 Avele Fine, Oxidic, Clay

hollow Isothermic, Andic, Humitropept

aWright (1963). bSoil Survey Stuff (1990).

the unbumt plots. Sahota and Virk (1987) reported that unburnt treatments produced lower yields of potato than did burnt bush fallows, in India. Work in Austra- lian eucalypt forests has observed that forest regener- ation was faster on burnt than unburnt clear-felled sites (Lockett and Candy, 1984). Increases in soil pH, organic carbon, nitrogen, calcium and magnesium have been measured four months after cutting and burning a bush fallow, in a Western Samoan study of non-tradi- tional methods of bush clearing (Reynolds, 1976).

The aim of this paper is to examine the effect of cutting a bush fallow that is not burnt, on soil conditions and taro nutrient status. A comparison is made between a bush fallow and two successive taro crops on five different soils.

soil maps (Wright, 1963) and field profile assessment. All farms used traditional growing techniques (with the exception of one fertilizer application on the Faleal- ili soil), which included 3-5 hand weedings. The weeds are not removed from the site. At harvest the entire taro plant is removed as both the leaves and the corm are eaten.

Measurements were taken during May 1990 under mature bush fallow (approximately 10-15 years old), and under the first and second consecutive crops of taro (Colocasia esculenta (L.) Schott cv. Niue) after fal- lowing. (Niue is a popular but unimproved cultivar.)

Table 2 Soil analysis methodology

2. Materials and methods

Five farms were selected in traditional taro growing areas on the island of Upolu, Western Samoa ( 14°S, 171°40'W) (Table 1). Each farm had all three phases of shifting agriculture under investigation (mature bush fallow and the two successive taro crops after a bush fallow), growing in close proximity on the same soil type. Soil classification was as determined from

Nutrient Fraction Test used Reference measured

Carbon Organic Walkley-Black Nelson and Sommers (1982)

Nitrogen Total Kjeldahl Bremner and Mulvaney (1982)

Phosphorus Truog soluble Murphy and Riley Blakemore et al. (1987)

Remaining Exchangeable Ammonium acetate Perkin-Elmer nutrients extraction Corporation Staff

(1982)

D.P.C. Stewart~Field Crops Research 38 (1994) 29-36 31

Table 3 Soil bulk density and penetration resistance under bush fallow and two successive taro crops (n = 5 )

Soil Depth (cm) Bush fallow First crop Second crop SED

Bulk density (g cm 3)

Penetration resistance a (kg cm- 2)

Falealili 0-10 0.98 1.13 1.45 0.16 10-20 0.98 1.38 1.18

Tiavi 0--10 0.50 0.40 0.58 0.08 10-20 0.44 0.47 0.56

Etimuli 0-10 0.94 0.73 0.79 0.12 1 0-20 0.74 0.71 0.83

A'ana 0-10 0.88 1.01 0.95 0.12 10-20 0.82 0.80 0.97

Avele 0-10 0.97 0.98 1.09 0.13 10-20 0.97 0.88 1.07

Falealili 0-10 0.57 0.92 0.67 0.45 10-20 1.10 1.68 1.82

Tiavi 0-10 0.59 0.61 0.64 0.11 10-20 0.86 0.87 0.84

Etimuli 0-10 0.52 0.39 0.34 0.15 10-20 0.76 0.79 0.80

A'ana 0-10 0.40 1.00 0.70 0.17 10-20 1.30 1.05 1.15

Avele 0-10 2.03 1.64 0.93 0.34 10--20 1.83 1.69 1.67

°Adjusted means from covariate analysis with gravimetric water content. SED = Standard error of difference.

T h e t a ro c r o p s w e r e a p p r o x i m a t e l y 5 m o n t h s o l d w h e n

s a m p l e d .

F o r e a c h p h a s e o f t he sh i f t i ng c u l t i v a t i o n c y c l e on

e a c h f a r m , f ive p e n e t r o m e t e r r e s i s t a n c e r e a d i n g s w e r e

t a k e n u s i n g a " S o i l t e s t " p o c k e t p e n e t r o m e t e r at b o t h

0 - 5 c m a n d 5 - 1 0 c m d e p t h s . A s s o c i a t e d soi l s a m p l e s

w e r e t a k e n fo r m o i s t u r e d e t e r m i n a t i o n to a l l o w c o v a r -

ia te ana l y s i s w i t h p e n e t r a t i o n r e s i s t a n c e . F i v e b u l k d e n -

Table 4 Soil moisture holding characteristics under bush fallow and two successive taro crops (0-25 cm depth, n =4)

Soil Bush fallow First crop Second crop SED

Field capacity ( % volumetric)

Permanent wilting point (% volumetric)

Available water capacity (% volumetric)

Falealili 42.0 57.2 50.7 0.7 Tiavi 44.1 62.7 60.5 0.4 Etimuli 43.7 53.5 62.1 0.2 A'ana 58.5 51.0 53.2 0.5 Avele 56.0 51.7 59.7 0.4 Falealili 32.9 42.5 37.6 0.2 Tiavi 28.6 43.7 41.3 0.5 Etimuli 31.7 34.2 37.8 0.9 A'ana 36.9 36.8 37.6 0.4 Avele 40.9 35.9 44.5 0.3 Falealili 9.1 14.7 13.1 0.8 Tiavi 15.5 19.0 19.2 0.7 Etimuli 12.0 19.2 24.3 0.9 A'ana 21.5 14.1 15.6 0.6 Avele 15.0 15.8 15.2 0.5

SED = Standard error of difference.

32 D.P.C. Stewart~Field Crops Research 38 (1994) 29-36

Table 5 Soil chemical properties under bush fallow and two successive taro crops (0-25 cm depth, n = 5 )

Soil Bush fallow First crop Second crop SED

Organic carbon (%)

pH

Total nitrogen (%)

Truog phosphorus (ppm)

Potassium (meq 100 g- i)

Calcium (meq 100 g- l )

Magnesium (meq 100 g-1)

Total exchangeable bases (meq 100 g J)

Falealili 3.18 2.65 2.64 0.33 Tiavi 8.08 9.16 8.45 0.61 Etimuli 4.58 5.83 6.20 0.79 A'ana 6.41 6.51 6.93 0.55 Avele 3.32 5.04 4.49 0.40 Falealili 6.2 6.0 6.3 0.1 Tiavi 5.5 5.4 5.4 0.1 Etimuli 5.4 5.3 5.3 0.0 A' ana 6.4 6.5 6.3 0.1 Avele 5.7 5.5 5.8 0.1 Falealili 0.322 0.324 0.300 0.044 Tiavi 0.653 0.819 0.764 0.171 Etimuli 0.312 0.365 0.353 0.067 A' ana 0.492 0.499 0.545 0.048 Avele 0.369 0.465 0.458 0.061 Falealili 223.0 339.0 196.0 74.6 Tiavi 22.0 19.5 14.2 2.2 Etimuli 5.5 8.4 5.0 3.1 A'ana 19.9 34.0 43.9 11.2 Avele 35.1 9.2 7.6 4.7 Falealili 1.610 1.010 2.130 0.396 Tiavi 0.388 0.520 0.520 0.050 Etimuli 0.290 0.860 0.380 0.260 A'ana 0.758 0.664 0.650 0.153 Avele 0.910 0.390 0.250 0.401 Falealili 20.72 21.52 22.32 0.72 Tiavi 2.42 5.26 5.40 1.53 Etimuli 0.95 1.89 1.72 0.20 A'ana 9.46 12.60 9.83 1.04 Avele 13.66 10.16 9.70 1.09 Falealili 9.20 9.25 9.22 0.05 Tiavi 0.77 1.29 1.34 0.18 Etimuli 3.77 7.03 7.13 0.61 A'ana 8.58 7.96 8.39 0.39 Avele 3.44 3.87 3.31 0.42 Falealili 31.85 32.03 33.80 14.18 Tiavi 3.62 7.19 7.30 5.74 Etimuli 5.08 9.91 9.38 7.69 A'ana 18.83 21.33 19.05 28.47 Avele 18.29 14.65 13.49 16.78

SED = Standard error of difference.

sity cores were t aken in each o f the th ree phases o n

each f a rm at b o t h 0 - 5 c m and 5 - 1 0 c m depths . F ive

c o m p o s i t e soil s amples were t aken f rom each phase o n

each f a r m for nu t r i en t analysis . E a c h c o m p o s i t e s ample

c o n t a i n e d five soil cores o f 0 - 2 5 c m dep th ad jacen t to

taro p lan t s ( w i t h i n a 3 0 - c m rad ius ) . ( T h e roo t ing dep th

o f taro is m a i n l y con f ined to the u p p e r soil ho r i zons

( O n w u e m e , 1 9 7 8 ) . ) The th i rd younges t l ea f o f these

p lan t s was also r e m o v e d for nu t r i en t ana lys i s ( c o u n t i n g

the u n e x p a n d e d l ea f as o n e ) . M e t h o d s used for soil

c h e m i c a l ana lys i s are g i v e n in Tab le 2. Soi l p H was

m e a s u r e d u s ing a 1:1 mix tu r e o f soil to water . Fo l ia r

D.P.C. Stewart~Field Crops Research 38 (1994) 29-36 33

samples were analyzed using a wet ash method (Wolf, 1982), and results expressed as a concentration on a dry weight basis.

Soil texture was measured on duplicate soil samples from 0-25 cm depth by the Bouyoucos method (Day, 1965) using the United States Department of Agricul- ture classification scheme (Gee and Bauder, 1986). Four texture samples were taken for each phase of the shifting cultivation cycle on each farm, from 0-25 cm depth. The < 2 mm fraction was used for soil moisture retention analysis. The available water capacity (AWC) was determined, using a pressure plate, from the difference in soil water content between "field capacity" ( - 33 kPa) and "permanent wilting point" ( - 1500 kPa).

3. Results

The bulk density increased at 0-10 cm depth with the phases of the shifting cultivation cycle on the Falealili soil (P<0 .01) (Table 3). The penetration resistance increased at 0-10 cm depth on the A'ana soil with the first crop and then decreased again (P < 0.01 ) (Table 3). In contrast the Avele soil had a decrease in penetration resistance at 0-10 cm with the phases of shifting cultivation (P < 0.05).

The soil water content at both field capacity and permanent wilting point increased on the Falealili, Tiavi and Etimuli soils with the incorporation of the bush fallow (P < 0.001 for each ) (Table 4 ). With crop- ping the soil water content at field capacity and per- manent wilting point decreased for the Falealili and Tiavi soils. The available water capacity (AWC) increased with the incorporation of the bush fallow on the Falealili, Tiavi, Etimuli and Avele soils (P < 0.001, P < 0.001, P < 0.001 and P < 0.01 respectively) (Table 4). With cropping the AWC decreased for the Falealili soil.

The soil organic carbon content increased with the incorporation of the bush fallow on the Avele soil (P < 0.01 ) (Table 5). The Falealili and Avele soil pH decreased for the first crop and then increased for the second crop (P < 0.05 for both) (Table 5). Soil phos- phorus values were low except on the Falealili soil indicating previous phosphate fertilizer use (Table 5 ). Soil phosphorus content decreased with shifting culti- vation on the Tiavi and Avele soils (P<0.01 and

P < 0.001 respectively). Soil potassium content increased with the incorporation of the bush fallow on the Tiavi soil (P < 0.05) (Table 5). Soil calcium con- tent increased with the incorporation of the bush fallow and then decreased with cropping on the Etimuli and A'ana soils (P<0.001 and P<0 .05 respectively) (Table 5). The Avele soil calcium content decreased with each phase of the shifting cultivation cycle (P < 0.01 ). There was an increase in soil magnesium with the incorporation of the bush fallow on the Tiavi and Etimuli soils (P<0.01 and P<0.001 respec- tively) (Table 5). The soil total exchangeable base content was particularly low for the Tiavi and Etimuli soils, and increased on these two soils with the incor- poration of the bush fallow.

Shifting cultivation decreased the soil iron content of the Etimuli and A' ana soils (P < 0.001 and P < 0.01 respectively), but the Tiavi soil iron content increased (P < 0.01 ) (Table 6). Soil manganese increased with shifting cultivation on the Avele soil (P < 0.001 ), and also increased on the Tiavi soil after the first crop

Table 6

Soil micro-nutrient status under bush fallow and two successive taro crops (0-25 cm depth, n =5)

Soil Bush First crop Second SED

fallow crop

Iron (ppm) Falealili 7.54 6.32 5.48 0.85

Tiavi 3.63 6.75 6.50 0.86

Etimuli 3.50 1.69 1.30 0.44

A'ana 2.92 2.14 1.97 0.23

Avele 8.56 8.78 8.38 0.83

Falealili 47.7 61.9 65.5 8.6 Manganese

(ppm)

Copper (ppm)

Zinc (ppm)

Tiavi 45.4 33.8 69.5 10.1

Etimuli 37.9 41.7 34.4 3.2

A'ana 157.0 123.0 137.3 19.9

Avele 96.8 153.4 173.8 11.4

Falealili 1.83 2.25 2.22 0.19

Tiavi 2.34 1.99 2.34 0.23

Etimuli 1.66 0.72 0.92 0.10

A'ana 5.58 5.70 5.81 0.45

Avele 0.13 0.24 0.31 0.04

Falealili 2.87 2.53 2.92 0.29 Tiavi 1.37 1.66 2.14 0.29 Etimuli 0.56 1.27 1.75 0.15

A'ana 3.23 4.35 4.16 0.74 Avele 2.47 3.25 3.12 0.37

SED = Standard error of difference..

34 D.P. C. Stewart/Field Crops Research 38 (1994) 29-36

Table 7 Foliar macro-nutrient concentration of taro in two successive crops after a bush fallow (n = 5)

Table 8 Foliar micro-nutrient concentration of taro in two successive crops after a bush fallow (n=5)

Nutrient Soil First crop Second crop SED Nutrient Soil First crop Second crop SED

Nitrogen (%) Faleal i l i 2.55 2.23 0.11 Tiavi 3.62 2.69 1.21 Etimuli 2.29 2.35 0.28 A'ana 2.37 2.68 0.34 Avele 2.92 3.11 0.28

Phosphorus (%) Falealili 0.452 0.458 0.090 Tiavi 0.281 0.291 0.047 Etimuli 0.670 0.630 0.135 A'ana 0.594 0.637 0.028 Avele 0.480 0.344 0.101

Potassium (%) Falealili 2.30 3.79 0.9t Tiavi 4.69 4.71 1.07 Etimuli 5.66 4.85 0.55 A'ana 5.34 6.27 0.47 Avele 8.87 7.10 0.93

Calcium (%) Falealili 1.40 1.32 0.14 Tiavi 1.52 1.24 0.22 Etimuli 0.54 0.65 0.09 A'ana 2.59 2.96 0.34 Avele 2.12 2.16 0.19

Magnesium (%) Falealili 1.12 1.11 0.08 Tiavi 1.37 1.33 0.04 Etimuli 0.52 0.74 0.08 A'ana 1.54 2.11 0.16 Avele 0.57 0.69 0.07

Sodium (%) Falealili 0.014 0.013 0.006 Tiavi 0.010 0.023 0.015 Etimuli 0.011 0.009 0.002 A'ana 0.022 0.008 0.003 Avele 0.010 0.013 0.003

SED = Standard error of difference.

( P < 0 . 0 5 ) (Table 6). There was more soil copper

present under the bush fallow than under the taro crops

on the Etimuli soil ( P < 0 . 0 0 1 ) , whereas there was

more soil copper under the taro crops of the Avele soil

( P < 0 . 0 0 1 ) (Table 6). Soil zinc concentration

increased with shifting cultivation on the Etimuli soil

( P < 0 . 0 0 1 ) (Table 6).

Foliar nitrogen concentration decreased with succes-

sive crops on the Falealili soil ( P < 0 . 0 5 ) (Table 7).

Plant magnesium concentration increased with succes-

sive crops on the Etimuli and A 'ana soils (P < 0.05 and

P < 0.01 respectively) (Table 7). Foliar sodium con-

centration decreased with cropping on the A 'ana soil

( P < 0 . 0 1 ) (Table 7). The concentration of manganese and zinc in leaves

decreased with successive crops (Table 8). The

Iron (ppm) Falealili 156 217 Tiavi 108 117 Etimuli 261 242 A'ana 200 138 Avele 215 379

Manganese (ppm) Falealili 78 74 Tiavi 120 127 Etimuli 329 257 A'ana 95 130 Avele 236 212

Copper (ppm) Fa lea l i l i 11.44 Tiavi 9.70 Etimuli 8.84 A'ana 7.15 Avele 7.15

Zinc (ppm) Falealili 23.7 Tiavi 19.6 Etimuli 17.6 A'ana 22.5 Avele 29.7

89 34 40 65 59 11 26 30 15 34

9.15 1.88 6.70 2.80 7.15 1.25 7.25 0.93 7.45 0.87

27.9 5.0 14.7 1.7 17.7 3.6 24.9 2.6 15.0 3.1

SED = Standard error of difference.

decrease in manganese was significant on the Etimuli

soil (P < 0.05), and the decrease in zinc was significant

on both the Tiavi and Avele soils ( P < 0 . 0 5 and

P < 0 . 0 1 respectively). However on the A 'ana soil,

foliar manganese concentration increased with succes-

sive crops ( P < 0.05). The foliar iron concentration

increased on the Avele soil with cropping (P < 0.05)

(Table 8 ).

No nutrient deficiencies were observed in the taro

crops.

4. Discuss ion

The low bulk densities of these soils was because of

their abundance in organic matter, 1:1 type clays, and

sesqui-oxides. These substances promote aggregation

and produce a very porous and well structured soil

(Young, 1976). Small changes in bulk density and high

variability meant that only the large increase in bulk

density on the Falealili soil was statistically significant.

This soil is 42% sand and is on fiat land so it is partic-

ularly susceptible to compaction. This increase in bulk

density with cropping results from the clearing of bush,

D.P.C. Stewart~Field Crops Research 38 (1994) 29-36 35

exposure of soil to the environment, and trampling compaction, and can affect plant growth by reducing porosity and increasing resistance to root penetration ( S anchez, 1976). Increases in soil bulk density of 0.10 g cm- 3 have been measured after clearing and burning a bush fallow (Popenoe, 1957). In contrast to when bush fallows are burnt, unburnt bush fallows may cause a decrease in bulk density after they are cut down. This was apparent on the Etimuli soil.

The addition of organic material increased the total soil porosity and hence the soil water content at field capacity and AWC. The proportion of small pores cre- ated by the addition of organic matter varied, and hence the variable effect on the soil water content at wilting point. Soils which produced more small pores ( < 0.2 /zm in diameter) than larger pores, had an increase in soil water content at wilting point and vice versa. This effect could in part result from differences in soil tex- ture. Cropping had a rapid deleterious effect on soil pores and hence the decrease in magnitude of the soil water content at both field capacity and wilting point with cropping on most soils.

The increase in organic carbon from the addition of organic matter was small. It was greatest on the soils with greater rainfall and from higher elevations, because of the slower rate of mineralization in these conditions. A greater increase may have occurred if the bush fallow had been cleared over a shorter period. When a bush fallow is not burnt, more organic carbon would be added to the soil, but it is only slowly mixed into the soil so it may initially affect only the soil surface. Increases in soil organic carbon of 4% have been recorded from burning a bush fallow (Reynolds, ! 976). Wright ( 1963 ) recorded similar organic carbon values (5-6%) for three of the soils used in this study (Falealili, Tiavi and A'ana soils) in a cropping regime. The amount of soil organic carbon measured in this study under bush fallow was considerably lower than the 20-23% recorded for the topsoil of two of these soils (Falealili and A'ana), under permanent forest ! Wright, 1963), indicating a long-term decline with the shifting cultivation system.

There was considerable variability in soil nutrient results. Increases in all the nutrients measured in this study have been recorded from burning a 17 year old bush fallow (Seubert, 1975). The amount of nutrient addition ranged from 67, 6 and 75 kg ha- ~ for nitrogen, phosphorus and potassium respectively, down to 0.3

kg ha- 1 for copper. Even greater additions of nutrients from burning bush fallows have been measured, rang- ing from 90-800 kg ha-1 of potassium, 275-3000 kg ha-~ of calcium and 30-180 kg ha-~ of magnesium (Nye and Greenland, 1960, 1964; Sanchez, 1976). Similarly, Reynolds (1976) recorded increases in the soil of 0.4% total nitrogen, and 4.5, 2 and 6 meq 100 g-~ respectively of calcium, magnesium and total exchangeable bases, from burning a bush fallow in Samoa. An unburnt bush fallow should contribute sim- ilar amounts of nutrients to the soil as a burnt bush fallow, except for greater amounts of carbon, nitrogen and sulphur, because of reduced gaseous losses. It appears that unburnt bush fallows do not provide as large a flush of inorganic nutrients as burnt bush fal- lows. When burning is not used, organic nutrients in the debris from the bush fallow must be mineralized by soil micro-organisms before they are available for crop growth. This provides a more gradual release of nutri- ents. Burning also could destroy some soil organic mat- ter, releasing more inorganic nutrients. The decrease in soil phosphorus with shifting cultivation may be because of the rapid fixation of phosphorus by sesqui- oxides as it is mineralized. Reynolds (1976) recorded decreasing levels of phosphorus with the incorporation of a burnt bush fallow in Samoa. The decrease in soil iron and copper content on the Etimuli soil with the incorporation of the bush fallow could be the result of the lower pH increasing the plant availability and there- fore plant uptake of these ions.

The absence of increases in other soil nutrients may be partly attributed to some nutrients still being immo- bilised by the flush of soil organisms following the incorporation of the bush fallow. Significant leaching losses of the more mobile ions such as nitrate, would also have occurred during the prolonged bush clearing and early stages of the crop. Variability because of the uneven distribution of the organic material added to the soil could have also masked trends in soil fertility fur- ther.

One would expect a gradual decrease in foliar nutri- ent concentration with successive crops, as nutrients are removed in the harvested crops and are lost by leaching and erosion. This was not always evident in this study because of the shormess of the cropping phase, and some of the soils being moderately fertile. Decreasing calcium and iron concentrations of taro corms over three successive taro crops have been

36 D.P.C. Stewart/Field Crops Research 38 (1994) 29-36

recorded in Fij i (Bradbury and Hol loway , 1988).

Shif t ing cul t ivat ion systems with unburnt bush fal-

lows could be more sustainable than burnt ones. This

is because the soil is less prone to eros ion when burning

is not used result ing f rom the pro longed trash cover.

Also, nutrients are less l ikely to be leached f rom

unburnt bush fa l lows that have been cut, because o f the

s lower release o f nutrients by mineral izat ion. Finally,

gaseous losses o f carbon, ni t rogen and sulphur are

reduced when burning is not used.

5. Conclusion

The soil fo l lowing unburnt bush fa l lows has

improved physical properties. The flush o f nutrients

fo l lowing an unburnt bush fa l low is small and appears

to be less than that fo l lowing a burnt bush fal low. Fur-

ther study is needed compar ing burnt and unburnt bush

fal lows, to de termine more precisely the effect o f cut-

t ing an unburnt bush fa l low on soil physical propert ies

and nutrient release.

Acknowledgements

Thanks to L.M. Stewart for assistance in the field,

K. Chand for laboratory analysis, and R. Chase, I.S.

Cornforth, B.C. Jacobs, S. Kubuabola and C.S. Weer -

aratna for their comment s on the manuscript .

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