Perhaps this difference between soils in the Gunung Sewu and in the Wonosari Depression should perhaps not be attributed to differences in clay content per se, but the effect this apparently has on the

entire geomorphological process resulting in subdued karstic developments, associated gentle slopes and reduced drainage. Young (1976: 190) pointed out that at a rainfall of 1000 mm or more vertisols

developed on gentle slopes in tropical limestone areas, while latosols developed on moderate, freely drained slopes.

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contributed to the formation of soils in the area (Mohr, 1938: 715). It is not clear how much of the soil material is derived from tephra, but given the very slow rate of soil formation on nearly pure limestone, it may be quite substantial2l.

Table 4 J Average values of various soil parameters for topsoils (10 cm) at various sites in the study area Site Number of samples^ Intermittently cultivated iiillsides Permanently cultivated land Forest Fallowed Cultivated^ Hillside Valley

Colour^ vdb vdb/db drb drb drb/dr/yr pH 7.0 7.2 0.2 7.3 0.3 7.3 0.2 6.6 0.4 P ( u g / m l ) 10 3.7 05 4.0 0.7 6.3 1.0 4.2 1.6 K+ (me/lOOg) 1.02 0.29 0.03 0.30 0.05 0.14 0.05 0.33 0.10 Ca2+ (me/lOOg) 34.6 37.6 8.9 29.0 6.9 20.0 1.6 14.6 2.6 Mg2+ (me/lOOg) 3.20 2.32 0.43 1.77 0.61 1.09 0.19 1.28 0.21 Na+ (me/lOOg) 0.23 0.23 0.06 0.12 0.02 0.10 0.02 0.07 0.02 CEC (me/lOOg) 39 40 8.5 32 5.9 22 1.0 20 2.0 BSd (%) 100 100 1 96 5 98 3 83 11 OMe (%) 9.3 5.6 0.6 3.6 0.1 3.1 0.0 2.2 05 N (kg/ha) 531 272 75 120 25 59 25 39 16 P r e t / ( % ) 48 63 5 61 2 55 8 56 4

Note: Standard deviations are given in italics ^ cultivated for less than five years

^ the results are based on the analysis of composite samples each consisting of five field samples per site.

^ vdb = very dark brown, db = dark brown, drb = dark reddish brown, dr = dusky red, yr = yellowish red (Munsell colours)

^ base saturation ^ organic matter / phosphate retention

Source: Field survey (See appendix 2 for basic data and methods of analysis)

A number of soil samples taken in the study area reveal the great variation in chemical soil properties with intensity of land use and topography (Table 4.3). Although the sample size is small, some patterns can be distinguished: all hillside soils are weakly

21. Yuan estimated that assuming the insoluble contents in limestone was 2.5 per cent, to form 1 m of soil, about 25 m of limestone should be denuded. Under a warm humid climate this could only be achieved in a period of 250-750 thousand years (Yuan, 1988: 4). An active volcano as the Merapi would have had at least 10,000 eruptions within the same period. If only 1 mm of ash were deposited at each eruption alone, this would amount to 10 m of ash. These calculations, of course, do not allow for accumulation due to downslope movement.

alkaline, while those in the valleys are weakly acid. There is a strong tendency for the concentration of exchangeable cations to decrease from forest and land under fallow, to permanently cultivated hillsides. In the valley bottoms, however, the levels for potassium and magnesium are again higher, but not so for calcium. There is also a clear, decrease in organic matter content from soils under forest, through soils under fallow, soils under cultivation to valley bottom soils. The same pattems applies for CEC. The CEC in soils on valley bottoms, however, is at about the same level as in soils on permanently cultivated hillsides. In the valleys, the higher clay concentrations apparendy make a larger contribution to CEC than on the hillslopes. Observations in the field indeed indicate that the soils in the valley bottom are more clayey than those on the hillslopes. Available phosphorus is highest and phosphate retention is lowest under forest, but on agricultural land there is no clear pattern.

When the above findings are compared with soil property values for Gunung Sewu soils as given by Dames, it appears that his description refers largely to conditions in the valleys. The above survey data enable us to examine in what way land use has affected soil conditions in the area. In doing so, we inevitably have to anticipate to some extent our discussion of land use and land management in the following chapters.

Clearly, the topsoil found at the forest site is by far the richest in the area. Everywhere else this topsoil, which must have covered most of the area at some stage, has been replaced by much poorer topsoils. However, where land is fallowed it seems that a process of enrichment may take place, leading to high organic matter content, high available nitrogen, and high concentrations of extractable cations released from plant litter. Due to their close contact with limestone outcrops and the presence of limestone particles, hillslope soils tend to be slightly alkaline and calcium predominates among the exchangeable cations, not just on fallowed hillslopes, but on all hillslopes. Hillslope soils may have become more prone to alkalinity over time, as they became increasingly exposed to the limestone outcrops due to erosion. The slightly lower pH in forest soil compared to the other hillslope soils supports this idea.

When hillside land is put into cultivation, organic matter content declines as a result of decomposition, involving processes of oxidation and mineralisation; the availability of extractable cations decreases due to uptake by crops, leaching and erosion. This decrease in organic matter content causes the CEC to fall as well. Permanently cultivated hillsides have much lower inputs of organic material (tree leaves) than intermittently cultivated hillslopes. Consequently, the availability of exchangeable cations also drops.

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Valley bottoms have the lowest organic matter content, as they receive little input from crop residues or vegetation. Terracing prevents larger organic material from washing down, though some of the finer material may accumulate in valley bottoms. However, it is apparently not enough to bridge the difference in organic material between valley bottoms and hillslopes. Some of the organic material is probably lost as a consequence of oxidation and leaching also. Valley bottom soils do have higher potassium and magnesium levels than permanently cultivated hillsides, which most likely results from the accumulation of upslope material. Calcium levels, however, are lower in the valley bottoms, probably because in the limestone environment on the hillslopes there is an abundant supply of calcium to replace any that is lost. Undoubtedly, the higher clay contents of valley bottom soils is a result of downslope movement too. Here, low organic matter content, the absence of limestone rock or stones, in combination with constant leaching of the bases as well as crop uptake has led to a slightly lower pH.

Toposequences similar to those just described have been reported in other limestone areas in the tropics. In a limestone region in the Malaysian Peninsula, a decrease in base saturation and a proportionally much larger decrease in exchangeable Ca values down hillslopes was noted and attributed to the differential effects of leaching (Joseph, 1968: 20). A decrease of pH with elevation was observed on a cultivated, unterraced hillslope in Central America, and was seen in connection with a decrease of CaC03 down the slope (Furley, 1987: 527). At the same site, it was also noted that, under cultivation, there had been a downward movement of carbon and nitrogen but carbon and nitrogen values tended to fall towards the lowest point of the transect. Exchangeable potassium and magnesium showed a loss on the summit and upper slope and an increase on the lower slope. The clay fraction at the bottom of the slope had increased considerably and, with organic matter content being relatively low, it largely accounted for a high CEC. However, the changes in the physical properties of the soil represented by the downward movement of clay and the decrease in soil depth on the upper slope and the increase in soil depth on the lower slope were deemed most dramatic of all (Furley, 1987: 528).

Apart from the differences described above, soil properties may also vary substantially among similar sites under a similar type of land use. According to local farmers, soil fertility is strongly associated with soil c o l o u r 2 2 . Variation is most notable

22. Appendix 3 gives an overview of some of the concepts and types used by local farmers for describing