Over-estimation of efficiency of weathering in tropical “Red Soils”: its importance for geoecological problems

Catena 41 Ž2000. 181–197 www.elsevier.comrlocatercatena

Over-estimation of efficiency of weathering in tropical ‘‘Red Soils’’: its importance for geoecological problems A. Bronger a,) , P. Wichmann b, J. Ensling c a

c

Geographisches Institut, UniÕersitat ¨ Kiel, D-24098 Kiel, Germany b Baumgarten 28, D-95326 Kulmbach, Germany Institut f ur ¨ Anorganische und Analytische Chemie, Johannes Gutenberg UniÕersitat, ¨ D-55122 Mainz, Germany

Abstract Weathering and soil formation rates are regarded as the main criteria of a tolerable soil loss. The efficiency of weathering in the seasonal semiarid tropics has often been greatly over-estimated especially in the geomorphologic literature in which weathering is assumed to be as fast or even faster than surface erosion. Six selected ‘‘Red Soils’’ in two intramontane basins of hyperthermic SW Nepal near the border with India, with 1500–1750 mm annual rainfall Ž5 humid months., and a ‘‘Black Soil’’ near Baroda, Gujarat, India Ž3–4 humid months. were studied mineralogically. Two of the ‘‘Red Soils’’ have TL ages between 10 and 30 ka, the ‘‘Black Soil’’ has one of about 10 ka. The yellowish silty parent material of the ‘‘Red Soils’’ is a preweathered soil sediment; it contains only small amounts of easily weatherable primary minerals: around 5% feldspars and 10–15% phyllosilicates, dominantly muscovites. Surprisingly, little pedogenic clay mineral formation could be identified. The illites and kaolinites are predominantly of detrital origin. The few non-regular mixed-layer minerals in the fine clay fraction Ž- 0.2 mm. can be interpreted as resulting from the initial stage of silicate weathering. The hematites, however, are mostly of pedogenic origin. Therefore the rubefication is a recent autochthonous process, and by itself is not a reliable indicator of strong pedogenic weathering. In the dated ‘‘Black Soil’’, only a small increase in 2:1 minerals, mainly smectites, could be found, although the content of weatherable minerals is high. These results support earlier conclusions from South India, where above a threshold of 2000 mm annual rainfall Ž6 humid months. deep weathering is a recent process leading to the formation of kaolinites over a long time interval; with 10 or more humid months per year, it leads to the formation of gibbsite. These soils are regarded as Vetusols. In the

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Corresponding author. Fax: q49-431-880-4658. E-mail address: [email protected] ŽA. Bronger..

0341-8162r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 4 1 - 8 1 6 2 Ž 0 0 . 0 0 0 9 9 - 0

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Typic and Aridic Rhodustalfs, earlier soil forming processes such as deep weathering and strong kaolinite formation have now almost ceased because of the semi-arid conditions; instead secondary carbonate is accumulating in the saprolite ŽCr. and lower part of the Bt horizons. We conclude that, as well as Ultisols, most Alfisols or Lixisols in now semiarid India are relict soils or non-buried paleosols formed in an earlier period of much moister climate. Because the rate of compensatory regeneration of soil is now in effect, almost zero, the soil erosion there is a permanent loss of the country’s most important natural resource, but it has not been recognized as such because the soils were not identified as paleosols. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Weathering; Mineralogy; Unburied paleosols; Soil erosion

1. Introduction When developing the Universal Soil Loss Equation ŽUSLE., Wischmeier and Smith Ž1978. defined a tolerable soil loss as the ‘‘maximum level of soil erosion that will permit a high level of crop productivity to be sustained economically and indefinitely’’. As sustainable production depends strongly on soil thickness, rate of weathering and soil formation has come to be regarded as the main criterion of tolerable soil loss. Evaluating a large number of publications, Auerswald et al. Ž1991. concluded that rates of topsoil formation are fairly uniform, ranging from 2 to 20 mm ay1 , and this rate is about two orders of magnitude faster than that of rock weathering. Published rates of weathering and subsoil ŽB horizon. formation, however, cover four orders of magnitude, depending on climate, vegetation, fauna, relief, and parent material, and therefore cannot be predicted for a specific site. For young Holocene soils on archeological sites in Hessen ŽGermany. on 3000 years old calcareous flood plain sediments or 350 years old calcareous colluvia, annual rates of soil development were calculated by Semmel Ž1995. to be - 0.3 mm and 0.0 mm, respectively. He concluded that, ‘‘it does not seem useful to determine a tolerable rate of soil erosion, which should not exceed the rate of soil development’’. Rates of weathering and soil formation are difficult to estimate for relict soils Žnon-buried paleosols., which cover large parts of the tropics and subtropics. These soils are broadly defined by the Commission on Paleopedology ŽCatt, 1998. as soils formed on a landscape or in an environment of the geological past, which means a changed constellation of soil forming factors, notably climate, and associated vegetation. Nonburied paleosols contain distinct evidence that the direction of soil development was previously different from that of the present. The efficiency of weathering under tropical climates has often been over-estimated especially in the geomorphological literature. For instance, Thomas Ž1978, p. 33. estimated ‘‘30–50 m of weathering in crystalline rocks within 10 5 –10 6 years, perhaps even 10 7 years’’, and Bremer Ž1981. estimated that for the formation of etchplains, ‘‘deep and uniform weathering’’ in a tropical climate is needed with rainfall exceeding 1600 mm ay1 . This uniphase process leads to weathering of rocks of different resistance at rates of 100 m in 2–3 Ma ŽBremer, 1981. to 100 m in 3–6 Ma ŽBremer, 1986, p. 104.. Later Bremer Ž1989, p. 371. postulated that the formation of a tropical ‘‘Rotlehm’’

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Žincluding oxisols. is at least as quick as that of soils outside the tropics, assuming that a fully developed soil in the temperate climatic belt is formed in 2000–5000 years. According to Budel’s theory of the ‘‘mechanism of double planation surfaces’’ ŽBudel, ¨ ¨ 1965, 1982, 1986; see also Thomas 1994, p. 288., a ‘‘basal weathering surface’’ is separated from the ‘‘wash surface’’ by a 4–10 m thick ‘‘monogenetic Rotlehm’’, especially in the Madras-Bangalore area, South India. According to Budel, this soil was ¨ formed mainly during the Holocene and later Pleistocene in a fairly uniform climate. The recent deep weathering of Peninsular Gneiss at the ‘‘basal weathering surface’’ is regarded by Budel ¨ Ž1965. as faster than erosion of the surface soil even under semiarid conditions. We investigated the mineralogy and chemistry of six ‘‘Red Soils’’ from two intramontane basins Ž‘‘duns’’. within the Siwalik system in SW Nepal near the border

Fig.1. Location of the climatic stations and soil profiles studied.

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Fig. 2. Climatic data and soil–water balances for selected stations from South India and SW Nepal.

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with India. In some of the ‘‘Red Soils’’ in the Deokhuri and Dang Dun ŽFig. 1., remains of late Paleolithic cultures have been found ŽCorvinus, 1985, 1987.. Five samples from two of the soils gave TL ages between 10 and 30 ka ŽZoller, this vol... The mineralogy ¨ of a Vertic Phaeozem ŽISSS-ISRIC-FAO, 1998. or Vertic Haplustoll ŽSoil Survey Staff, 1999. near Baroda in the western part of central India was also studied ŽBacker, 1989.. This soil, locally called ‘‘Black Soil’’, derived from late Pleistocene sandy loess gave a TL age of about 10 ka ŽA.K. Singhvi, Ahmedabad, India, personal communication.. These dates limit the soil forming factor time and indicate the rate of weathering in recent tropical ‘‘Red Soils’’ and ‘‘Black Soils’’, which is of interest for comparison with rates of erosion calculated for the ‘‘Red Soils’’ of South India. To distinguish recent and relict features in tropical regions, we also studied previously the mineralogy of nine surface ‘‘Red Soils’’ formed on saprolite derived from Peninsular Gneiss in South India. These soils occur in a climatic sequence from 2500 mm annual rainfall Ž10 wet months. on the windward side of the Western Ghats to 590 mm ay1 on the leeward side. The current soil temperature regime in SW Nepal is hyperthermic and the soil moisture regime is udic at the border to ustic ŽSoil Survey Staff, 1999., with a water deficit between February and May and a water surplus from July to September or October according to the monsoonal precipitation pattern. The selected meteorological stations ŽFig. 1. are Tulsipur in the Dang Dun not far from the Mahabharat range ŽLesser Himalaya., Kusum 25 km west of the Deokhuri Dun and Nepalganj in the Terai. However, the climatic record of these stations covers only 15 years ŽMinistry of Water Resources, Department of Irrigation, 1988.. The soil moisture regimes for these three stations and those for two sites in South India, Hyderabad ŽAndhra Pradesh. and Baroda ŽGujarat. were computed ŽFig. 2. using the Newhall simulation model ŽVan Wambeke, 1985..

2. Methods Particle-size distribution Žsieving and pipette method. was determined after H 2 O 2 and HCl ŽpH 4. pre-treatments and dispersion with Na 4 P2 O 7 . Total iron ŽFe t . was determined after boiling in HF and HNO 3 . Dithionite-soluble Fe ŽFe d . was determined by the DCB method according to Mehra and Jackson Ž1960., and oxalate extractable Fe ŽFe o . with an ammonium oxalate–oxalic acid mixture at pH 3.25 in darkness and 208C ŽSchwertmann, 1964.. Emphasis was given to the mineralogical composition of the sand, silt, and clay fractions. The quantitative composition of the fine and medium sand fractions Ž63–200 and 200–630 mm. was based on counts of G 300 grains, and that of three silt fractions Ž2–6, 6–20 and 20–63 mm. on 300–1500 grains. The coarse and medium clay Ž2–0.2 mm. and fine clay Ž- 0.2 mm. fractions were analysed separately after the removal of iron oxides. Samples were saturated with Mg 2q, ethylene glycol, and Kq and heated to 1108C, 4008C, and 5608C. The composition of the clay subfractions was determined by semiquantitative estimation based on the areas under selected X-ray diffraction ŽXRD. peaks. We used the weighting factors recommended by Laves and Jahn ¨ Ž1972.: illites

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and vermiculites were given a weighting factor of 1, mixed-layer minerals were given a weighing factor of 0.5, and kaolinites and smectites were given a weighing factor of 0.25. The mineral percentages by weight in each clay fraction, calculated by multiplying the estimated percentages of minerals by the weight percentage of each fraction, are consequently only estimates, in contrast to the weight percentages of the fractions ) 2 mm. To evaluate the type and intensity of weathering, it is necessary to establish the original petrographic homogeneity of the parent material from which a soil developed. One way of determining this is to use a resistant index mineral ŽBarshad, 1967., which should be present in equal amounts in all horizons and neither decomposed nor displaced by soil development. We used quartz as an index mineral because it is resistant to pedochemical weathering, at least in the young soils studied. Iron minerals, especially goethite and hematite, in the fractions - 6 mm of selected horizons of four of the six soils in SW Nepal were determined by the differential X-ray diffraction ŽDXRD. method ŽSchulze, 1981. after their relative enrichment by treatment with 5 N NaOH ŽKampf and Schwertmann, 1982.. The hematitergoethite ratio was ¨ calculated following Boero and Schwertmann Ž1987.. In addition four samples from the Bw2 and the CBw horizon of the Lalmatiya soil were analysed by Mossbauer spectra ¨ ŽBronger et al., 1983..

3. Results and discussion 3.1. Nepal and western India Detailed descriptions including micromorphology, pedochemical and mineralogical results of all six soils in Southwest Nepal are given by Wichmann Ž1993., and those of nine ‘‘Red Soils’’ mostly Benchmark Soils ŽMurthy et al., 1982. from South India were given earlier by Bruhn Ž1990.. The results of the pedochemical and mineralogical investigations are summarized in Table 1 and Figs. 3–7. The main points worth emphasizing are the following. Ž1. The yellowish-brown to brown parent materials ŽTable 1: Munsell notations. of the ‘‘Red Soils’’ in both intramontane basins contain only small amounts of easily weatherable primary minerals: around 5% feldspars and 10–15% phyllosilicates, dominantly muscovites. In the clay fractions, there is also a little kaolinite. Therefore the parent material of all six, more or less, rubefied soils is considered to be strongly preweathered soil sediment. Although the silt content is mostly very large in five of the six soils and only slightly less in the Gidhniya soil ŽFigs. 3–5., any eolian addition is probably small, because until anthropogenic deforestation occurred only a few decades ago, the area was covered by a ‘‘subtropical1 moist deciduous forest’’, which contained Shorea robusta ŽSal., Terminalia tomentosa, Ficus and Cedrela species, etc. ŽSchwein1

In Indian literature, the boundary between subtropical and tropical climate follows the annual 248C isotherm.

Table 1 Some chemical properties and forms of iron in two Typic Dystrochrepts and a Typic Hapludalf from SW Nepal Depth ratio

Horizon

pH ŽH 2 O.

CEC Žmmolr100 g clay.

Fe d Ž%.

Fe o Ž%.

Fe d r Fe t

Fe o r Fe d

Munsell notation Ždry.

Lalmatiya soil (Typic Dystrochrept, Deokhuri Dun) 20 AB 6.1 14.2 55 Bwl 6.1 15.5 95 Bw2 6.2 15.1 140 Bw3 6.6 15.1 180 Bw3 6.7 15.8 230 CBw 6.8 17.1 ŽB.Ctg 370 6.7 15.2

4.04 4.35 4.50 4.45 4.45 4.51 3.84

2.21 2.17 2.36 2.39 2.32 2.41 2.09

0.08 0.09 0.08 0.08 0.08 0.09 0.05

0.55 0.50 0.52 0.54 0.52 0.53 0.54

0.04 0.04 0.03 0.03 0.03 0.04 0.02

3.75YR4r8 3.75YR4r8 3.75–2.5 YR4r8 5YR5r8 5YR5r8 5YR5r8 7.5YR5r8

Kurepani soil (Typic Dystrochrept, Dang Dun) 20 AB 5.2 16.2 50 Bwl 5.1 16.0 90 Bw2 5.4 14.9 115 Bw2 5.5 15.9 155 CBg 5.5 16.3 205 CBtg 5.6 16.6 600 2C 5.8

4.60 4.76 5.01 5.04 4.80 4.56 3.70

2.52 2.56 2.72 2.82 2.63 2.57 2.17

0.09 0.07 0.10 0.09 0.10 0.09 0.12

0.55 0.54 0.54 0.56 0.55 0.56 0.59

0.04 0.03 0.04 0.03 0.04 0.04 0.06

5YR5r8 5YR5r8 5YR5r7 5YR5r8 6.25YR5r7 6.25YR5r7 10YR7r3

Gidhniya soil (Typic Hapludalf, Tui Õalley, Dang Dun) 10 A 5.9 25.5 35 Btl 5.8 16.8 60 Bt2 6.3 16.5 100 BC 5.8 17.7 155 C 5.5 21.5 550 2C 6.4 18.3

1.66 2.84 2.60 2.48 1.99 4.14

1.10 1.72 1.70 1.40 1.25 2.82

0.07 0.08 0.06 0.04 0.04 0.06

0.66 0.61 0.65 0.56 0.63 0.68

0.06 0.05 0.04 0.03 0.03 0.02

10YR6r4 7.5YR5r7 7.5YR6r6 10YR6r6 10YR7r6 10YR6r7

Hematiter goethite ŽDXRD.

1.9

0.7

0.5 0.8 0.6 0.6 0.5

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Fe t Ž%.

0.6

0.1

187

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Fig. 3. Mineralogical composition of a Typic Dystrudept ŽLalmatiya soil, Deokhuri Valley, SW Nepal..

furth, 1957; Puri et al., 1989.. Even in the cooler and drier latest Pleistocene, the area was covered by a moist deciduous forest; this forest type occurs in nearby parts of India today with - 1100 mm rainfall ŽDas Gupta, 1976; Bronger, 1998; Fig. 4..

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Fig. 4. Mineralogical composition of a Typic Dystrudept ŽKurepani soil, Dang Valley, SW Nepal..

Ž2. The percentages of quartz by weight are approximately constant throughout the profiles of five of the six soils from Nepal. The Lalmatiya and Kurepani profiles are shown in Figs. 3 and 4 as example. This suggests that the soil parent materials were petrographically homogeneous. In the Gidhniya soil, the percentage of quartz by weight is greater in the A horizon than the Bt1 and then increases downwards to the BC and C horizons ŽFig. 5.. Consequently, we can only indicate trends in pedogenic mineral weathering and not calculate weathering balances for this profile. In all soils surprising little clay mineral formation could be identified. The illites and kaolinites are predominantly of detrital origin. The few non-regular mixed-layered minerals in the fine clay fraction Ž- 0.2 mm. could have formed during the initial stage of in situ weathering. Only in the Gidhniya soil do the amounts of illuviation argillans in thin sections Žca. 2%. of the Bt horizon and the increase in clay content from the A to the Bt horizon ŽFig. 5. allow the soil to be classified as a Typic Hapludalf ŽSoil Survey Staff, 1999, p.

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Fig. 5. Mineralogical composition of a Typic Hapludalf ŽGidhniya soil, Tui ŽDang. Valley, SW Nepal..

29–34. or Haplic Luvisol in the World Reference Base ŽWRB. ŽISSS-ISRIC-FAO 1998.. In the other five soils, the increase in clay content from the A to the Bw horizons is very small ŽFigs. 3 and 4.; illuviation argillans were usually detected in thin sections in small amounts occurring only at the transition from the subsoil to the parent material, although the soil moisture regime in this area shows a distinct water surplus during the

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Fig. 6. Mineralogical composition of a Vertic Haplustoll ŽPurohit soil near Baroda, Gujarat, India..

191

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Fig. 7. Mossbauer spectra Ž58K. of the Lalmatiya soil, Deokhuri Valley, SW Nepal. ¨

summer monsoon period ŽFig. 2.. Therefore the soils should be classified as Typic Dystrudepts ŽSoil Survey Staff, 1999. or as Dystric Cambisols in the WRB ŽISSSISRIC-FAO, 1998.. Ž3. In the Vertic Phaeozem ŽWRB. or Vertic Haplustoll derived from Late Pleistocene, sandy loess near Baroda, an area with a typic ustic soil moisture regime ŽFig. 2., there is only a small increase in 2:1 minerals, mainly smectites ŽFig. 6., although the content of weatherable minerals is high. Ž4. The high Fe drFe t values through all three profiles ŽTable 1. suggest that primary iron-bearing minerals have been strongly weathered, but the Fe drFe t and Fe d values differ little from those of the C horizon. Therefore, the large amounts of Fe are mainly

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inherited from the soil parent materials. Room temperature Mossbauer spectra of ¨ selected samples show only a quadruple doublet typical of paramagnetic FeŽIII., which persists in the 5 K spectra. However, the low-temperature spectra are dominated by well-resolved magnetic hyperfine sextets, indicating the presence of goethite and hematite Žexamples in Fig. 7.. This changeover from a doublet to a sixline pattern as the main component of the spectrum upon cooling is expected for very small antiferromag˚ . which exhibit superparamagnetism ŽKundig netic particles ŽF 200 A et al., 1966; ¨ Bronger et al., 1983.. The relative amounts of hematite and goethite can be estimated from the peak areas of the different components in the Mossbauer spectra at 5 K. In the ¨ Lalmatiya soil, for example ŽFig. 7., the hematitergoethite ratio is much greater in the Bw2 horizon than the CBw horizon, which is in agreement with the DXRD results ŽTable 1.. Also, because the hematite particles are very small, they are partly if not mostly of pedogenic origin. This suggests that the rubefication is a recent autochthonous process, and by itself is not a reliable indicator of strong pedogenic weathering. 3.2. Southern India The results for the Nepal profiles confirm our earlier work on the ‘‘Red Soils’’ ŽDigar and Barde, 1982; Murthy et al., 1982., which cover an area of 720,000 km2 in South India ŽKrantz et al., 1978.. Most of the ‘‘Red Soils’’ are classified as Rhodic Lixisols ŽISSS-ISRIC-FAO, 1998. or as Typic and Aridic Rhodustalfs ŽSoil Survey Staff, 1999., though the latter subgroups reflect the present day climate, notably rainfall. The remaining two soils are a Dystric Nitosol or Typic Rhodudult and an Orthic Ferralsol or Typic Hapludox. Both are on the windward side of the Western Ghats, where the udic soil moisture regime has not changed much during the Quaternary period. Results for soil chemical properties, the mineralogy of sand, silt and clay fractions, and micromorphology of the soils and their parent materials ŽBronger and Bruhn, 1989; Bruhn, 1990. can be briefly summarized as follows. Ž1. Above a threshold of 2000 mm annual rainfall Ž6 humid months., deep weathering in a Lixisol Žclose to an Acrisol. or Udic Rhodustalf is a recent process leading to formation of kaolinites. Above 2500 mm at 900 m a.s.l. Ž10 humid months. in the Dystric Nitosol or Typic Rhodudult and the Orthic Ferralsol or Typic Hapludox weathering has resulted in formation of gibbsite. According to Cremaschi Ž1987. and Catt Ž1998., these soils would be considered as Vetusols or old non-buried soils which have undergone very similar processes of soil formation under a similar constellation of soil forming factors Žespecially a constantly moist climate. over at least several 100 ka. Ž2. On the leeward side of the Western Ghats under decreasing rainfall Ž1500–1000 mm. the base saturation of the saprolite increases and so do the amounts of 2:1 clay minerals in the saprolite and soils. Despite the decreased weathering intensity here, a broad spectrum of weathering features is present in the Rhodic Lixisols or Typic Rhodustalfs. These include kaolinized biotite flakes, boxwork crystals of strongly weathered hypersthene, garnet and hornblende, and single quartz grains with hematite infillings Ž‘‘runiquartz’’ of Eswaran et al., 1975 and Schnutgen and Spath, ¨ ¨ 1983.. For reasons discussed below, these are interpreted as relict features inherited from an earlier period of wetter climate.

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Ž3. In even drier climates with annual rainfall - 800 mm Že.g., in two Rhodic Lixisols or Aridic Rhodustalfs near Hyderabad., pedogenic kaolinites are still dominant, though with increasing amounts of smectites, illite-smectite intergrades, and illites. Ž4. These changes in clay minerals with the increase in present-day rainfall seem to reflect a sequence of climatic desiccation in the past, perhaps as a consequence of the increasing distance of the Indian plate from the equator fortified by uplift of the Western Ghats since the Late Tertiary. The only change in clay mineralogy that can be related to the present climate is formation of illites; smectites, mixed-layer minerals and especially Koalinites are apparently relict features. Ž5. In three soils near Hyderabad and Coimbatore, which are today in an aridic ustic soil moisture regime, secondary calcite has accumulated in the saprolite and lower B horizons. In association with the pedogenic kaolinites, this carbonate accumulation is good evidence for a change to more arid conditions. We therefore conclude that most Rhodic Lixisols or Typic and Aridic Rhodustalfs in now seasonally semiarid India are relict soils or non-buried paleosols formed in an earlier period of much wetter climate than the present.

4. Geoecological conclusions Historically, the whole of India was under forest cover except the driest parts of the Rann of Kutch and the Thar Desert ŽGaussen et al., 1978; Puri et al., 1990; Bronger, 1998; Fig. 1.. Today about 46–50% of the area of India is under agriculture, and only 22–23% is forest ŽAgarwala, 1985; TATA, 1992.. However, much more than half of the forestland has been degraded to savannas by human activity ŽMisra, 1980.. So the area of true forest is now scarcely 10% of the total area ŽMisra, 1980; Meher-Homji 1989., even if the ‘‘open forest’’ areas are included ŽBronger, 1998; Fig. 2.. Most of the forests are of ‘‘tropical moist deciduous’’ and ‘‘tropical dry deciduous’’ types ŽAgarwala, 1985, p. 53–54.. The extensive deforestation together with the widely used dryland farming system, in which the land is fallowed for 5–7 months, favours soil erosion, especially under monsoon rainfall patterns. The annual soil loss by water erosion of vertisols under forest is - 3–5 t hayl , but under the dryland farming system it has been estimated as 65 t hayl ŽNarayana and Babu, 1983., equivalent to 43.3 cm 100 ayl Žassuming a soil bulk density of 1.5 g cmy3 .. Abrol Ž1990, iii. calculated a similar rate of soil loss by water erosion. In their map of iso-erosion rates of India, Singh et al. Ž1990. show soil losses of 10–) 20 t hayl ayl , equivalent to 6.7–13.3 cm 100 ay1 , in large parts of the Vertisol region. The average annual soil loss of 40 t hayl ayl by water in the ‘‘Red Soil’’ area under dryland farming ŽNarayana and Babu, 1983. may be too high, as the map of Singh et al. Ž1990. shows soil losses ) 10 t hayl ayl for large parts of the ‘‘Red Soil’’ area; under forest, it is only 0.5–5 t hayl ayl , though under shifting cultivation it may exceed 40 t hayl ayl , equivalent to 26.7 cm 100 ayl . As a result, the depth of the soil cover in large parts of India, south of the Ganga plain Že.g., in the Satpura Hills., is only 50–100 cm. In an even larger area between Madhya Pradesh, Orissa, and Andhra Pradesh, the depth of the soil cover is only 20–50

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cm ŽDas Gupta, 1980., mainly because of erosion following deforestation in historical times. It may be accepted that soil erosion can be tolerated from a crop production viewpoint provided soil formation keeps pace with it to compensate for the losses. However, in semiarid India the soil development processes have changed so that the present rate of compensatory regeneration of soil is in effect almost zero. The soil erosion there is consequently a permanent loss of the country’s most important natural resource, but it has not been recognised as such because the soils were not identified as paleosols inherited from past periods of wetter climate. The underlying relict saprolite ŽCr horizon. is even more susceptible to soil erosion, as it has little structural stability because of its small clay content, absence of organic matter, and much smaller amounts of pedogenic iron oxides as cementing agents ŽScholten, 1997.. Therefore anthropogenic deforestation causes accelerated soil erosion including erosion of saprolites and as a consequence stripped inselbergs Ž‘‘bornhardts’’. are coming to the surface in many parts of South India.

Acknowledgements We thank Dr. G. Corvinus, Kathmandu, Nepal for guidance in the field and stimulating discussions and the Deutsche Forschungsgemeinschaft for supporting field, micromorphologic and laboratory work ŽGrants Br 303r19; 21-1 and 21-2..

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