SOIL DEVELOPMENT INDICES OF SOILS DEVELOPED ON FLUVIAL TERRACES (PEÑARANDA DE BRACAMONTE, SALAMANCA, SPAIN).

ALONSO,P.1; SIERRA, C.2; ORTEGA, E.2; DORRONSORO, C.1
1. Departamento de Edafología y Química Agrícola, Facultad de Farmacia, Universidad de Salamanca.
2. Departamento de Edafología y Química Agrícola, Facultad de Farmacia, Universidad de Granada.

INTRODUCTION

Most soil properties are variables which change with time, but owing to the large number of soil properties, it is difficult to evaluate the degree of development of one soil by analyzing each property. For this reason, several authors have developed soil development indices based on variations of soil properties with respect to the parent material. Thus, Bilzi and Ciolkosz (1977) and Harden (1982) use morphological properties, whilst Walker and Green (1976) and Birkeland (1984) use properties measurable in laboratories.

The relationship between age and soil development index values in a fluvial terrace sequence and, more precisely, their distribution with regards profile depth, is the main purpose of this study.

MATERIALS

The soils studied have developed on fluvial terraces formed by the Almar river, near Peñaranda de Bracamonte village (Salamanca, Spain), situated between UTM 466694 and 471698 in the National Topographic Map nº 479.

The present climate is subhumid (precipitation 412 mm), mesic (temp. 11ºC), of the Mediterrean Continental type.

The vegetation consists mainly of holm oak of the Genisto-histricis- Quercetum rotundifoliae sigmetum series, which have been cleared in order to plant cereals and legumes.

The fluvial deposits are mainly made up of gravel and sand from the erosion of granite, slate, siliceous sediments (mainly sandstone) and quartzite rocks. The most outstanding characteristic is the abundance of small gravels particles of feldspars. A representative soil profile has been chosen from each of the seven terrace surfaces (table 1).

METHODS

The descriptions of the morphological properties and the physical and chemical analysis of the soils were conducted according to traditional methods (Soil Survey Staffs, 1951 and 1984).

Morphological indices (MI)

The morphological property indices of the soil profile have been calculated according to the scheme developed by Harden (1982). These indices are calculated by evaluating the differences in soil horizon properties and those of parent material.

The indices are calculated following these guidelines:

1) The starting point is a detailed morphological description of the soil profile.
2) Recent fluvial materials are taken as a point of reference, since they have not been subject to pedogenic processes and are assumed to have the same properties as the original material.
3) By comparing the property value of the original material with that of soil horizons, the degree of change in each horizon is calculated. In order to do this, for each change of degree, type etc. in any given property, an arbitrary value of 10 points is assigned (Table II, Harden, 1982).
4) Once all the properties changes have been give a value, the morphological index is obtained by normalizing these values on a 0 to 1 scale, by dividing the result reached for each property by the maximum value of the property considered in any of the soils evaluated.
Seven morphological indices have been calculated: i) structure (type and degree of development; ii) textural composition (textural class + type of stickiness and plasticity of wet consistence; iii) dry consistence (class); iv) wet consistence (class); v) clay films (amount, thickness and location); vi) melanization (value); vii) rubification (hue and chroma).
5) Multiplication of the value obtained in the previous step by the thickness (in cm) of the horizon and adding all of the values corresponding to all of the horizons of a given soil (a constant thickness of 2 m was used) yields the Morphological index (MI) for a single property for each profile.
6) If all the normalized values, calculated in step 4, are added up and are then divided by the number of properties considered, one obtains the combined morphological index per horizon based on several poperties.
7) Multiplying the latter values by the thickness corresponding to each horizon and then adding these products, one obtains the morphological index MI for each profile (for a standard thickness of 2 m).

Analytical indices (AI)

Other indices have been calculated from physical and chemical data according to the mIPA system (Birkeland, 1984). These indices are a modification of the Walker and Green (1976) index of profile anistropy and are calculated using the following formula:

mIPA = D/PM

where D represents the numeric difference between the property value in the horizon considered and its value in the parent material, represented by PM.

To calculate the AI we followed steps through 7 described in the previous section corresponding to the MI (step 4 - AI for each property and horizon; step 5 - AI for each property and profile; step 6 - AI for several combined properties and horizon; step 7 - combined AI for analytical properties and profile). As was done for the MI, calculations were made for normalized thicknesses of 2 m.

In this study the analytical index has been studied for five properties: % clay, % water retention at 1/3 and 15 atm, cation exchange capacity and pH.
General horizon and profile index

The morphological and analytical indices of all the properties studied can be condensed in one single value which averages all the partial indices. This general index is calculated by adding all the values of each horizon and dividing the sum by the total number of properties analyzed ( in order to do this, the analytical indices have to be previously normalized to a scale ranging from 0 to 1, the same as with morphological indices). This was done both for each horizon and the standardized profile as in steps 6 and 7.

RESULTS AND DISCUSSION

Abbreviated profile descriptions and analytical data are presented in table 1.

HORIZON DEVELOPMENT INDICES

Morphological index of texture. The distribution of texture morphological indices according depth follows two trends. First, index values tend to increase gradually with soil age and, second, the indices for different horizons of each soil tend to differ more from each other the older they are; the maximum values are reached in horizons at 40 - 100 cm deep (Figure 1).

Morphological index of structure. The values of this index are very similar in all soils, with the exception of the Holocene soils. Values show a rapid increase in the first 40 - 70 cm, remain almost constant to a depth of 80 - 100 cm, and decrease to zero at a depth of 100 to 150 cm and below (Figure 2). Index values tend to increase with age

Morphological index of melanization, rubification, dry and moist consistence and clay films. The indices follow similar trends as those of texture. Only indices for rubification and clay films are shown. Both seem to develops slower than texture. (Figure 3, and 4).

Analytical index of clay content. On observing this index distribution (Figure 5) it can be seen that the maximun values are to be found at a depth of 50 to 100 cm Holocene flood plain soils, lowest level, show a homogeneous distribution with depth. As soils become older the differentiation between horizons is more marked, and it can be observed that index values increase from A horizons to deeper ones and then decrease below a certain depth (approximately 1m). This a common pattern in all Pleistocene soils, but values increase with age, although once the maximun value has been reached, instead of continuing to rise, it extends to surrounding areas, thus thickening the horizon of maximum development.

Analytical indices of water retention at 1/3 and 15 bar and cation change capacity. The distribution of these indices follows the general trends explained for the clay content this demostrating their dependence on clay content. Only indices for 1/3 bar and CEC are included (Figures 6, and 7).

Analytical index of pH. This tends to de highest in A horizons, decreasing sharply in B horizons There is no clear relationship with age (Figure 8).

General horizon index. The index value portrays a gradual and systematic increase with soil age (Figure 9). As was to be expected, Holocene soils have the lowest values (less than 0.2 in the youngest soil, PB7, and less than 0.4 for soils of about 10,000 years PB6). Values are moderate in late Pleistocene soils (reaching 0.5 in some horizons of maximum development) and are very high in middle Pleistocene soils (from 0.6 to 0.7).

With respect to the differentiation between each soil horizon, this index shows a vell defined progressive increase with age. In Holocene soils (PB6) a developed subsurface horizon appears and is fully developed in late Pleistocene soils (PB5 and PB4). Finally it thickens and a clear contrast can be seen in middle Pleistocene soils (PB3, PB2 and PB1).

PROFILE DEVELOPMENT INDICES

If the increase in these indices is compared with age increase (Figure 10), it can be concluded that the development of these soils is rapid in the first stages, corresponding to Holocene soils (<10,000 years). Subsequently the formation is much more moderate (in late Pleistocene soils) and represents the soil maturity phase. Finally, a third stage can be identified where formation continue but at a very moderate rate. This corresponds to soils of the middle Pleistocene age. Since development in these soils continues, over the whole range of ages there is no evidence that in this chronosequence the soils reach a steady state. Several authors (Bockheim, 1980; Muhs, 1982; Busacca, 1987) have recently reached similar conclusions, and this can also be seen in studies of many other chronosequences (Little and Ward, 1981; Harden, 1982; Birkeland, 1984; Reheis et al., 1989; Jongman et al., 1991).

ERROR VALUES ESTIMATION

Error values for the analytical laboratory data are about 10% (Alonso, 1989), for the morphological data because of subjectivity probably twice as much. The calculated horizon and combined indices of Figs 1 to 9 have therefore errors values somewhat greater than this because they also include errors in parent material and pedon variability. The age determination of the terrace surfaces is only approximate and may vary considerably especially for the Pleistocene age soils. Hence the calculated function of Fig. 10 should be considered mainly as indicating the trend of the indices in the development of these Palexeralf soils.

COMPARATIVE STUDY OF INDICES

The correlation coefficients of the corresponding regression equations between the indices (y) and ages (x), of the soils may serve to evaluate the usefulness of these indices of development. Five regression models were tested: linear (y = a + bx), second grade polynomial (y=a + bx + cx2), logarithmic (y=a + b log x), potential (y=axb) and exponential y= abx). The highest correlation coefficient for each index is shown in Table 2.

Almost all the indices have high correlation coefficients. The AI for clay and the retention of water at 1/3 and 15 bar, together with the general profile index, the general morphological properties MI and the general analytical properties AI, best serve to evaluate the degree of development of these soils, r>0.90 for the log functions.
Often, the analytical indices have higher values than those referring to the morphology. This could be because in the calculation of the AI quantitative values were used, whereas for the MI the starting point was qualitative data.

pH and the structural MI do not seem to be a good age evaluating parameter. In the case of structure this could be due to the difficulty with an objective description of the soil structure, while pH changes with time only in the A horizon. Both are rapidly adjusting properties (Yaalon, 1971).

CONCLUSIONS

1. The majority of the morphological and analytical indices show, a progressive development with age up to the middle Pleistocene. The exception are pH and structure.

2. The degree of differentiation between the horizons of each soil becomes progressively evident gradually, developing a subsurface B horizon with the highest index.

3. The rate at which the soils develops is rapid in the first phase (Holocene soils), moderate in the second phase (Late Pleistocene soils) and extremely slow in the final phases (Middle Pleistocene soils).

4. The soils of this chronosequence show continuous changes in all properties analyzed without seeming to reach a steady state. However, development is very slow for the oldest soils.

5. Almost all the correlation coefficients of the corresponding regression equations between the development indices and the ages of the soils have high values. The AI for clay and the retention of water at 1/3 and 15 bar, the general profile index, the combined morphological properties MI, the combined analytical properties AI, are those that best serve to evaluate the degree of development of these soils.

ACKNOWLEDGEMENTS

This research has been sponsored by Spanish DGICYT (Project nº PB88-0378).

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