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Soil Science Society of America Journal 64:1826-1833 (2000)
© 2000 Soil Science Society of America

DIVISION S-7-FOREST & RANGE SOILS

Soil Color as an Indicator of Slash-and-Burn Fire Severity and Soil Fertility in Sumatra, Indonesia

Quirine M. Ketterings and Jerry M. Bigham

School of Natural Resources, Ohio State Univ., 2021 Coffey Road, Columbus, OH 43210 USA

bigham.1{at}osu.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Summary and conclusions
 REFERENCES
 
Fire is widely used as a tool for converting forest to agricultural land in many developing countries, and correlations are thought to exist between fire severity, burned soil color, and soil fertility. To test this hypothesis, field experiments were conducted in Sepunggur, Jambi Province, Sumatra, Indonesia. Field burning a slashed 12- to 15-yr-old secondary forest caused Munsell values and chromas to decrease and hues to become yellower with increasing heat severity, especially in the top 5 cm of the soil. However, at peak surface temperatures >600°C, soil C was mostly depleted and the soil matrix was reddened. Laboratory studies showed similar results with static heating. Moreover, color changes were highly dependent on the duration of exposure at a given temperature. Fire induced the formation of aggregates with exteriors that had lower values and chromas and slightly redder hues than the interiors. Laboratory removal of organic matter from burned samples by chemical oxidation did not alter the color. Soil exchangeable Ca, Mg, and K increased with fire severity, while exchangeable acidity and Al decreased 2 wk after the burn. Soil C and N were reduced at high burn severity only. Phosphorus showed an increase in availability at low to medium fire severity and a decrease in availability at the most intense burn levels. Colors of burned areas in the field did not change significantly during the 12 wk following the burn. However, within 12 wk following the field burn exchangeable Ca had decreased to preburn levels and Al saturation had increased markedly. Using postburn color measurements to predict the spatial patterns in soil fertility was limited by the fact that fertility changed rapidly following the burn, whereas color parameters did not.

Abbreviations: CBD, citrate-bicarbonate-dithionite • REML, residual maximum likelihood analysis option


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Summary and conclusions
 REFERENCES
 
FIRE IS STILL WIDELY USED for land clearing in the tropics even though traditional shifting cultivation is declining due to increases in population pressure. Burning slash introduces within-field variability in many soil properties due to uneven distribution of ash and exposure to heat (Brouwer et al., 1993; Van Noordwijk et al., 1998b; Rodenburg, 1999; Ketterings, 1999). Recognizing and acting on this fire-induced microvariability could be an asset to the small farmer who is typically resource poor and working on marginal lands with few external inputs (Brouwer et al., 1993).

Temperature maximum and duration of exposure are major indicators of fire severity (Wells et al., 1979). Low severity fires (short exposure to <250°C) have been shown to temporarily affect soil biological and chemical properties (e.g., Nye and Greenland, 1960; Palm et al., 1996). More severe burns may alter such fundamental characteristics as texture, mineralogy, and cation-exchange capacity (Sertsu and Sanchez, 1978; Ulery and Graham, 1993; Ulery et al., 1996; Ketterings et al., 2000). The severity of a fire varies widely and depends on such variables as fuel load, fuel moisture, climatic conditions, and size of the area being burned (Wells et al., 1979; Brown, 1988; Lobert and Warnatz, 1993).

Postburn color patterns and the presence or absence of wood ash have previously been used as indicators of fire severity (e.g., Ulery and Graham, 1993; Romanya et al., 1994; Van Noordwijk et al., 1998b). Lightly burned areas (short exposure at 100 to 250°C) were characterized by incompletely combusted organic material and blackened soil. Moderate burns (300–400°C exposure for longer periods of time) reportedly consumed plant material (little residual white ash) without altering the underlying soil (Wells et al., 1979), and very severe burns (long exposure to >500°C) left white ash and reddened the soil.

Darkened soils that resulted from low- and medium-severity burns were preferred for agricultural production by farmers in Sumatra, Indonesia, because they were associated with higher crop yields and quicker crop establishment (Ketterings et al., 1999). In the same study, areas of reddened topsoil were classified as undesirable due to their perceived low fertility status and poor water-holding capacity. Areas of reddened topsoil have been commonly observed in field burns (e.g., Sreenivasan and Aurangabadkar, 1940; Boyer and Dell, 1980; Ulery and Graham, 1993), but the proportion of land affected by high fire severity is thought to be only 2 to 8% (Dyrness and Youngberg, 1957; Ulery and Graham, 1993). Although unburned, ash-covered and red combusted soils can easily be distinguished in a recently burned field, it is unknown how color patterns develop or relate to soil fertility over time.

Soil color is heavily influenced by the type and amount of organic matter (Shields et al., 1968; Schulze et al., 1993) and Fe oxides (Bigham et al., 1978; Schwertmann, 1993), both of which are fire-sensitive components. Although correlations between color and burn severity should exist, there have been relatively few attempts to quantitatively evaluate changes in soil color with heating, perhaps due to uncertainties associated with the use of field color charts. Portable tristimulus colorimeters now offer the potential for more precise color measurements from large numbers of samples (Post et al., 1993; Ulery and Graham, 1993). The use of color transformations, such as redness rating (Hurst, 1977; Torrent et al., 1983) or redness susceptibility (LaFleur, 1970), could also provide improved relationships between color and burn severity.

In this study, we investigated the properties of some pre- and postburn soils in Sumatra, Indonesia, where slash-and-burn agriculture is still commonly used (Ketterings et al., 1999). Our objectives were to evaluate (i) the effects of fire severity on soil color; (ii) the dependence of soil color changes on the maximum surface temperature achieved and the duration of exposure to this temperature; (iii) the dependence of postburn color on soil grain size, organic matter content, and iron oxide content; and (iv) the potential use of soil color parameters for assessing fire-induced changes in soil C, N, and exchangeable Ca, Mg, K, Na, and Al with time.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Summary and conclusions
 REFERENCES
 
All studies were performed using soil samples from the Sepunggur area, Jambi Province, Sumatra, Indonesia (102° 14' E, 1° 29' S). A description of the climate, vegetation, and land use in this region is given in Van Noordwijk et al. (1995, 1998a). Soils of the study area are classified as Hapludox or Kandiudox according to U.S. soil taxonomy (Soil Survey Staff, 1999). Basic characterization data for forest soils adjacent to the primary and secondary burn sites are presented in Table 1 .


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  		Table 1 Characterization of unburned forest soil in Sepunggur, Sumatra, Indonesia

 
Field Studies
Plot research was conducted using the fields of two local farmers. In one case, the farmer performed only a primary, broadcast burn. In the second case, stacks of wood were prepared and ignited in a field that had already experienced a broadcast burn to simulate the practice of performing secondary burns for removing residual woody material (see Ketterings et al., 1999, for a description of farming practices in the area).

Primary Burn Experiment
For this experiment, a 20-yr-old secondary forest was selected. This {approx}5-ha area was slashed and burned by the farmer without relocating any of the slashed wood. Bulk soil samples (4–5 kg) were taken at 0- to 5- and 5- to 15-cm depths in locations (one per sample) where surface temperatures of 100, 300, and 600°C were reached during the burn. Surface temperature regimes were estimated in the field using heat-sensitive crayons (Cole Parmer, Vernon Hills, IL) mounted on ceramic tiles, wrapped in aluminum foil, and placed directly on top of the mineral soil prior to the burn. Samples of completely combusted, brick-like topsoil were also taken from locations where high fuel loads had produced temperatures that exceeded those measurable with the crayons (>600°C) for considerable amounts of time. Samples were taken 2 wk after the burn at 10 different locations and were composited in the field. In addition, bulk soil samples (0–5 and 5–15 cm depth) were taken in the surrounding forest and in the slashed field directly prior to burning. All samples were dried at 60°C and passed through a 2-mm sieve before further analysis.

Secondary Burn Experiment
Three wood piles were constructed following a low-severity broadcast burn of a 12- to 15-yr-old forest by the farmer. Each pile consisted of 400 kg of field dried wood on a 3 by 3 m area. The piles were ignited and surface temperatures were estimated using heat-sensitive crayons placed beneath each pile. To determine the effect of heat intensity on soil fertility and soil color parameters, samples were taken at locations where the topsoil temperature had been 600, 300, and 100°C, the latter being outside the burn spot. For comparison, samples were also taken in the surrounding forest resulting in a total of four treatments. Samples were collected during a 3-mo period at 0- to 5- and 5- to 15-cm depths starting 1 d after the burn, then 1, 2, 4, 8, and 12 wk after burning. Immediately prior to the burn, the centers of the three burn piles and the surrounding forest were sampled. All samples were taken at two depths (0–5 and 5–15 cm) resulting in 120 measurements for the entire experiment. Of these 120 samples, a subset (38 samples) was separately (i) ground to pass a 250-µm sieve, (ii) heated in a muffle furnace at 550°C for 8 h, and (iii) treated with 30% (w/w) H2O2. These treatments were done to determine changes in soil color due to grinding, reheating and organic matter removal, respectively.

Laboratory Study
The soil samples (0–5 and 5–15 cm depth) taken from the forest adjacent to the primary burn experiment were passed through a 2-mm sieve and homogenized in the laboratory. Duplicate samples of {approx}15 g of each of the soil layers were placed in glass vials with a diameter of 2 cm and a height of 4 cm (soil column height of 3.5 cm). One batch of samples was exposed to 600°C for 1, 2, 4, 6, 10, 15, 30, 45, 60, 75, 90, 105, 120, 180, 240, 300, 360, 420, 480, 540, 660, and 930 min and then cooled to room temperature. Identical samples were exposed to 300°C for 1, 2, 4, 6, 10, 15, 30, 45, 90, 120, 240, 360, and 660 min. The experiment was conducted in two replicates. Colors were measured on the dried samples after heat exposure.

Soil Analyses
Soil Color
Soil color was measured in triplicate from air dried samples with a CR-300 colorimeter (Minolta Corp., Ramsey, NJ). Alphanumeric hues obtained in the Munsell Renotation System were converted to numerical values by using a Munsell hue circle (Chamberlin and Chamberlin, 1980) (in this conversion 10R = 10, 10YR = 20, 10Y = 30, 10GY = 40). Redness susceptibilities were calculated after heating some samples to 550°C for 8 h according to the formula of LaFleur (1970): , where hue25 is the converted hue of field-burned soil and hue550 is the converted hue after 8 h exposure to 550°C.

Soil Chemical Analyses
Field samples were analyzed for total C by the Walkley–Black procedure (Walkley, 1947). Total N was determined using the Kjeldahl procedure (Bremner and Mulvaney, 1982). Iron oxide contents were determined using dithionite-citrate buffered with sodium bicarbonate at pH 7.3 (Mehra and Jackson, 1960). Final extracts were analyzed for Fe by atomic absorption spectrophotometry. Exchangeable K, Mg, Ca, and Na were obtained by extraction with 1 M NH4-acetate at pH 7.2. Exchangeable acidity and Al were determined in 1 M KCl by titration with 0.05 M NaOH and 0.05 M HCl (after addition of 1 M NaF), respectively. Exchangeable Ca and Mg were measured by atomic absorption spectrophotometry, while K and Na were measured by flame emission. The Bray-1 procedure was used to extract labile inorganic P, and extracts were colorimetrically analyzed for total P using the ammonium molybdate method (Bray and Kurtz, 1945).

Statistical Analyses
Primary Burn Experiment
This experiment did not have true replicates because composite bulk samples were taken in the field. Each treatment was separated into four "pseudo" replicates (subsamples of the bulk sample) that were analyzed separately to assess precision of the measurements. Results of this experiment were expressed as means with standard deviations. Linear regression analyses with color parameters and iron oxide data were performed using Genstat 5 for Windows, Release 3 (Genstat 5, 1993).

Secondary Burn Experiment
This experiment was analyzed using the residual maximum likelihood analysis option (REML) of Genstat 5, Release 3 (Genstat, 1993) with a fixed treatment and time interaction effect (FIXED = Treatment*Time). Burn piles (replicates) and pile, treatment and time interactions were treated as random effects (RANDOM = Pile/Treatment/Time). Analyses were done for each sampling depth (0–5 and 5–15 cm) individually. Wald statistics were calculated to determine the significance of the models. Standard errors of difference and t tests were used to determine if treatment means were significantly different at P < 0.05. When the initial statistical analyses showed no significant effects from time of sampling, the time series were combined and an average for each replicate per treatment per layer was calculated. The experiment was then analyzed using the same REML option but without sampling time as a factor.

Laboratory Study
Means and standard deviations were calculated. Nonlinear regression analyses were computed with Tablecurve 2D for Windows, v. 2.03. No further statistical analyses were performed on these results.


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Summary and conclusions
 REFERENCES
 
Changes in Soil Fertility with Burning
Table 2 shows soil fertility parameters for the samples from the primary burn and from the 2-wk sampling of the secondary burn sites. The exchangeable cation (Ca, Mg, and K) concentrations and Bray-1 P at 0- to 5-cm depth increased with intensification of the fires to 600°C in the primary burn. Combustion at higher temperatures reduced Bray-1 P to below the preburn level but further increased exchangeable Ca, Mg, and K concentrations. Exchangeable Na was unaffected by fire exposure. Exchangeable Al and H, on the other hand, decreased with increasing fire severity. The Al saturation was effectively reduced from 67 to 0%. Carbon and N concentrations were variable and showed severe reductions only at temperatures >600°C. Similar results were obtained for the secondary burn. Effects were most pronounced in the surface layer (0–5 cm) and were absent (soil C, N, Bray-1 P, exchangeable Ca, Mg, Na, H) or less severe (exchangeable K and Al) at 5- to 15-cm depth. These results are similar to short-term burn effects reported in the literature (e.g., Nye and Greenland, 1960; Andriesse and Koopmans, 1984; Andriesse and Schelhaas, 1987; Hölscher et al., 1997) and support farmers' observations on the enhanced fertility of ash-covered black soil resulting from medium severity burns (Ketterings et al., 1999).


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  		Table 2 Effects of burn intensity on selected soil fertility parameters. Samples were taken 2 wk after the burn

 
Soil fertility parameters in the secondary burn experiment showed significant interactions between time and fire severity. Soil organic C (Fig. 1a) and N (results not shown) were unaffected by the low-severity fire throughout the experiment. Exposure to the moderate burn reduced soil C and N (surface soil only) during the first week following the burn, whereas reduced C and N concentrations were detected up to 28 d following the most severe burn. Exchangeable Ca (Fig. 1b) and Mg, K, and Na (results not shown) declined rapidly throughout the 90-d duration of the experiment, while Al saturation (Fig. 1c) increased with time following the low and medium severity burns. The low Al saturation 84 d after the severe burn was probably the result of a marked increase in exchangeable K due to ash relocation (results not shown). With the disappearance of ash from the surface with time, increases in Al saturation are expected. These changes indicate a decline in soil fertility that could be due to leaching losses (Uhl and Jordan, 1984; Khanna et al., 1994) and that has been a major reason for field abandonment and opening of new land for agricultural production in tropical regions (see, e.g., reviews in Nye and Greenland, 1960; Sanchez, 1976).



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  		Fig. 1 Effect of fire and climatic exposure with time (1–90 d following the secondary pile burn) on (A) soil C, (B) exchangeable Ca, and (C) Al saturation at the 0- to 5-cm depth. Numbers indicate the standard error of difference for each sampling time

 
Color Observations in the Field Studies
Colors of the topsoil (0–5 cm) and subsoil (5–15 cm) layers at the two locations were similar (Table 3) . At the primary burn site, dry colors of the two layers before burning were 0.1Y 4.9/2.8 ({approx}10YR 5/3) and 0.1Y 5.5/3.8 ({approx}10YR 6/4), respectively. Corresponding colors for forest soil near the secondary burn were 0.1Y 3.3/2.7 ({approx}10YR 3/3) and 0.1Y 3.6/3.3 ({approx}10YR 4/3). Soil color did not change significantly during the 12-wk time period following the burn, but was affected by fire severity. Burning of the topsoil (0–5 cm) at both locations caused Munsell values and chromas to decrease with increasing temperature to 600°C (Table 3). The darkening of the soil could be due to charring of the organic matter (Ulery and Graham, 1993). Similar trends were observed for value and chroma of the deeper layer in the primary burn experiment. No change in color was noted at 5 to 15 cm beneath the wood piles of the secondary burn, suggesting that elevated temperatures in the secondary burn did not penetrate deep into the soil and were probably maintained for a shorter period. Pronounced reddening was only visible when topsoil had been exposed to temperatures exceeding 600°C in portions of the primary burn. This color change was due to the thermal conversion of goethite (yellow) to maghemite (reddish brown) and hematite (red) (Ketterings et al., 2000). Reddening was accompanied by an increase in both the value and chroma of the fused soil material (Table 3), most likely due to more thorough removal of organic matter (Sertsu and Sanchez, 1978; Ulery and Graham, 1993). Such areas comprised 30 to 35% of the burned field and were much more extensive than reported by others (e.g., Dyrness and Youngberg, 1957; Ulery and Graham, 1993; Ulery et al., 1996), probably due to differences in soil properties, types of slash, slash loading, and burn conditions.


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  		Table 3 Effect of burn severity on field colors of <2-mm sieved soil

 
Laboratory Study
Redness Susceptibility
A significant increase in redness susceptibility occurred with increasing temperature of exposure for the topsoil (0–5 cm) samples (Fig. 2) . A similar but nonsignificant trend was observed for the deeper materials. An increase in redness susceptibility confirms that the samples became yellower with exposure to increasing field temperatures to 600°C. However, this result seems counterintuitive if redder colors are expected with burning (LaFleur, 1970; Sertsu and Sanchez, 1978; Ulery and Graham, 1993; Ulery et al., 1996; Anda et al., 1998). Temperatures of 400°C were high enough to redden the soil in a study by Sertsu and Sanchez (1978), while in our study surface temperatures of >600°C were necessary to cause reddening. Although the mineralogical composition and organic matter content of the soil will play an important role in fire-induced Fe oxide conversion and color changes (Ketterings et al., 2000), the duration of exposure to elevated temperatures must also be considered.



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  		Fig. 2 Redness susceptibility (R = hue25 - hue550, where hue25 is hue of field-burned soil and hue550 is the hue after 8 h of exposure to 550°C) of surface (0–5 cm) and subsurface soil (5–15 cm) exposed to different peak surface temperatures during a broadcast field burn. Alphanumeric Munsell hues were converted to numerical values by using a Munsell hue circle (Chamberlin and Chamberlin, 1980) before calculating redness susceptibility. With this conversion 10R = 10, 10YR = 20, and 10Y = 30. Different letters (surface soil only) indicate significant differences at P < 0.05

 
Changes in Color with Time of Exposure to Elevated Temperatures
Trends in the field colors and redness susceptibility can be understood by examining changes with time with static heating of forest soil from the primary burn at 300 and 600°C in a laboratory oven. In this experiment, topsoil (0–5 cm) hues initially became yellower with heating, and the effect was much more pronounced at 600°C than at 300°C (Fig. 3a) . The yellowing also persisted longer at the higher temperature (250 vs. 125 min). Eventually, the heated samples became redder than the starting soil at both temperatures. Similar results were obtained with the subsoil (5–15 cm) at 600°C; however, duration of the yellowing was shorter and the final hue was redder. At 300°C, no change in subsoil hue was observed.



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  		Fig. 3 Effect of static heat exposure over time on (A) Munsell hue, (B) Munsell value, and (C) Munsell chroma. Alphanumeric Munsell hues were converted to numerical values by using a Munsell hue circle (Chamberlin and Chamberlin, 1980). With this conversion 5YR = 15, 5Y = 25, and 5GY = 35

 
The samples rapidly darkened (lower Munsell value) with heating (Fig. 3b). No recovery over time was noted at 300°C; however, samples heated at 600°C became lighter (higher Munsell value) as organic C was volatilized. Once again, this effect was delayed in the topsoil relative to the subsoil. A similar delay was observed with chroma of the 600°C samples, and the final chromas obtained after 930 min of heating differed significantly between the two layers (Fig. 3c).

The pronounced changes observed at static temperatures in the oven experiments demonstrate that the duration of exposure to elevated temperatures must be considered to understand the evolution of soil color over short time periods (<4 h) at moderate to high temperatures in the field. Other factors affecting combusted soil colors may include aggregate formation, organic matter content, and mineralogy of the soil.

Effect of Aggregate Formation on Combusted Soil Color
The textures of combusted soils usually become coarser as a result of the melting and fusion of primary mineral grains into aggregates (e.g., Sertsu and Sanchez, 1978; Ulery and Graham, 1993; Ketterings et al., 2000). If aggregate formation shields primary particles from combustion, grinding should result in color changes. Table 4 shows the effect of grinding aggregates from 38 samples representing all temperature treatments and both layers at the secondary burn site. The results suggest that the aggregates were coated with pigments that decreased the value and chroma. Increases in value and chroma with grinding were largest for the forested sites and decreased with increasing heat intensity (data not shown). Hues of the field burned samples were unaffected by grinding.


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  		Table 4 Effects of grinding (to pass a 250-µm sieve) and organic matter removal (treatment with 30% H2O2) on colors of field-burned and oven-reheated 2-mm sieved soil (n = 38)

 
Increases in the Munsell values of ground samples after reheating at 550°C in an oven were similar to those of the field-burned samples. Chroma increases due to grinding were generally twice as large in the oven-combusted samples than in the field-burned samples. Hues increased slightly with the largest increases at the highest fire intensities (data not shown). These results indicate that pigmentation within the aggregates was not uniform, even when heating was performed under controlled conditions. Aggregation apparently resulted in physical protection of the soil material. Therefore, grinding of combusted soils may result in lost or modified information.

Effect of Organic Matter and Iron Oxides on Soil Color
Treatment of a subset of ground samples from the secondary burn with H2O2 to remove organic matter did not significantly change soil color parameters (compare <250 µm to 30% H2O2 samples in Table 4). These results suggest that the color was caused by the Fe oxides rather than the organic matter. However, treatment with H2O2 could not remove all residual C; on average, 11 g kg-1 C remained. This fraction was probably dominated by charcoal that could not be further oxidized and may have affected the overall color of the soil.

The same sample set contained 12 to 48 g kg-1 of citrate-bicarbonate-dithionite (CBD)-extractable Fe, which is commonly taken as a measure of the Fe-oxide content. Although Fe oxides are important pigmenting agents, CBD-extractable Fe produced very low correlation coefficients with color parameters for the field samples (data not shown). On the other hand, additional exposure of field samples to 550°C for 8 h resulted in strong correlations between color parameters of the combusted samples and CBD-extractable Fe content prior to combustion in the laboratory (Fig. 4) . The observed trends (decreasing hue and value but increasing chroma) with increasing Fe content are also consistent with the results shown in Fig. 3 for surface and near-surface materials after heating at 600°C for long time periods.



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  		Fig. 4 Soil color parameters of field burned soil after recombustion at 550°C for 8 h as a function of the amount of citrate-bicarbonate-dithionite-extractable Fe. Alphanumerical Munsell hues were converted to numerical values by using a Munsell hue circle (Chamberlin and Chamberlin, 1980). With this conversion 5YR = 15 and 10YR = 20

 

   Summary and conclusions
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Summary and conclusions
REFERENCES
 
Surface temperatures >=600°C were achieved in both field experiments, but the primary broadcast burn produced higher average intensities in this study. Most likely, locations selected under the primary burn were exposed to elevated temperatures longer than the soil under the secondary burn. Burning caused measurable changes in soil color and fertility.

Concentrations of most nutrient elements (exchangeable Ca, Mg, K, and Bray-1 P) taken 2 wk after the secondary field burns were significantly higher than in the forested controls at corresponding depths. Effects were most pronounced within 5 cm of the soil surface and were strongly attenuated or lost within 15 cm. Similar trends were observed in the primary burn. Exchangeable Al and extractable acidity, by contrast, were reduced by fire exposure. Soil C and N were significantly reduced in combusted (reddened) soil only. Within 2 mo after the burn, exchangeable cations had decreased to preburn levels, whereas Al saturation had increased markedly.

Munsell values and chromas decreased and hues became yellower with heating to 600°C. At higher temperatures, soil C was mostly depleted and reddening of the soil matrix occurred. Laboratory studies showed that pronounced changes in color parameters developed with time when the soil was subjected to static heating. As in the field, Munsell hues became yellower as values and chromas decreased with short-term heating at 300 or 600°C. Reddening at 600°C did not occur until after {approx}45 min of exposure. This result suggests that most areas in the farmers' fields were not exposed to high temperatures long enough to pass the point at which hues redden and values and chromas increase. Initial darkening of soil color probably occurs when the soil organic fraction burns, whereas reddening develops when large wood fragments are in direct contact with the soil and combustion is of long duration. In contrast to soil fertility, colors of burned areas in the field did not change significantly during the 12 wk following the burn.

The usefulness of postburn soil color measurements for identifying heat-induced changes in soil fertility is limited by the fact that fire severity is determined both by peak temperature and the duration of exposure. Therefore, one color measurement could represent different combinations of time and temperature. Because soil fertility parameters do not react to different temperature–duration combinations as soil color does, the usefulness of field color measurements for predicting soil fertility is limited. Rapid changes in soil fertility (but not color) following the burn further limit the predictive value of color measurements.


   ACKNOWLEDGMENTS
 
This research was funded by a Scholarship from the Mervin G. Smith International Studies Fund, an Ohio State University Graduate School Alumni Research Award, and two projects within the International Center for Research in Agroforestry Southeast Asia Regional Program: the Smallholder Rubber Agroforestry Project (CIRAD/GAPKINDO/ICRAF) and the Alternatives-to-Slash-and-Burn project supported by the Global Environment Facility with United Nations Development Program sponsorship. We are grateful to Mr. Sahroni, Mr. Zulkifli, and the Kepala Desa of Sepunggur for allowing us to conduct our study in their fields and to their families for their support, meals, and water supply that prevented one of us (QMK) from reaching skin color hues approaching 10R.

Received for publication October 6, 1999.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Summary and conclusions
 REFERENCES
 




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