SOIL DEVELOPMENT IN A CHRONOSEQUENCE OF FLUVIAL TERRACES OF THE ALMAR RIVER (SALAMANCA, CENTRAL WESTERN SPAIN)
C. DORRONSORO and P. ALONSO
Dpto. de Edafología. Facultad de Farmacia. Univeridad de Salamanca.
FIELD AND LABORATORY METHODS
SOIL DEVELOPMENT INDICES
MORPHOLOGICAL INDICES (MI)
ANALYTICAL INDICES (AI)
MINERALOGICAL INDICES (MineI)
RESULTS AND DISCUSSION
SOIL DEVELOPMENT INDICES VERSUS HORIZON DEPTH WITH AGE
SOIL DEVELOPMENT AGE TRENDS
It is generally accepted that most soil properties are time-dependent variables (Jenny, 1941). Therefore it is likely that soils of different ages will display their properties in different ways, especially under conditions in which the actions of other soil formation factors remain constant. Thus, the study of how the properties of soils vary with age is of great interest and may offer useful information in regard to their genesis.
Studies of soil chronosequences have focused on widely differing materials such as fluvial materials, eolian, lacustrine material, marine materials, dunes, glaciar morraines, volcanic ashes and even on anthropogenic deposits such as the slag heaps of abandoned mines. Of all these types of chronosequence, river terraces have undoubtedly been the focus of most attention since that they are very good examples of natural soil sequences whose evolution has been shaped by time. Although river terraces do cover an extended period of time, the changes occurring in their soils should not be attributed exclusively to the action of this parameter; rather, to climatic changes and those due to the development of biological organisms should also be taken into account, insofar that these will also have affected soil development.
The aims of the present work are several: i) to evaluate how the properties and development of soils change with the progression of time; ii) to define soil development age trends; iii) to analyze whether the development of the soils under study becomes fixed at a steady state or not.
The soils examined in this study were from a sequence of river terraces formed by the Almar River in the province of Salamanca (Central Western Spain).
Over geological time this river has left a typical scaled relief, with abundant horizontal surfaces placed between sharp scarps. According to Vreeken (1975), these are post-incisive sequences. The river terraces analyzed are located in the neighborhood of Macotera village (UTM coordinates: 466694 and 471698 of National Grid Map 479).
A representative profile was chosen from each of the seven geomorphological surfaces distinguished, although in some cases, due to lateral changes, two profiles were sampled (Table 1). The sequence comprised all soils from the riverbank to a terrace 64 m above the riverbed.
The fluvial deposits have thicknesses ranging between 1.5 and 5 m and are composed mainly of gravels and sands that originated from the erosion of granitic rocks, slates, quartzites and siliceous sediments.
The present day climate can be classified as subhumid (mean annual rainfall 412 mm), mesic, (mean annual T°, 11°C) of the continental Mediterranean type).
The climax vegetation is composed of oaks from the genisto- histricis-Quercetum rotundifoliae sigmetum series. In certain areas this vegetation has been altered by human activity for crop cultivation and in others it is characterized by more or less open holm-oak populations with abundant undergrowth.
The soil surfaces have been dated by Santonja et al (1976; 1982, 1984) and the IGME (1982), mainly by archaeological and stratigraphic methods. The calculated ages for these Spanish soils have been contrasted (figure 1) with the values obtained by other authors (Meixner and Singer, 1981; Harden, 1982; Busacca, 1987) for Californian soils that were formed under similar conditions to ours: fluvial terraces formed by materials originated from the erosion of granitic rocks, Mediterranean climate, with average annual precipitation of 310 to 640 mm. We illustrate the results for the rate of soil formation in figure 1. We have simply calculated these values on the basis of the proportion between the soil depth in millimeters (A hor. + B hor. + 1/2AC hor. + 1/2 BC hor. excluding CA, CB, or C horizons) and the soil age in years (soils with anomalous performances have not been represented). A strong agreement was found between the values respective to our Spanish soils in relation to the Californian soils.
The formation rate drastically decreases with age. The soils deepen at a rate of 0.2 to 0.6 mm/year for those that are less than 1000 years old in these soil conditions. The soil that is 10,000 years old has formed at 0.1 mm/year. The soil that is 100,000 years old has formed at 0.01 mm/year. The rate decreased to 0.005 mm/year at 500,000 years old. Finally, rate decreased to 0.001 mm/year at about one million years.
FIELD AND LABORATORY METHODS
The descriptions of the morphological properties and the physical and chemical analyses of the soils were conducted according to traditional methods (Soil Survey Staffs, 1951 and 1984). The mineralogy of the sand and silt fractions was determined by X-ray diffraction.
In order to analyze the soil development trend with age, representative samples were chosen:
1) The properties of Ap horizon always refer to those of the horizon closest to the surface.
2) When two or more Bt horizons within one soil are referred to, the following have been selected:
-The maximum value of all the Bt horizons for the clay content, water retention at 0.03 and 1.5 MPa, available water and cation exchange capacity.
-For the contents in sands and silts, those corresponding to the horizon with the maximum enrichment in clays were selected
-In the case of clay, we also calculated the accumulation indices at preset depths in the 15, 50, 100, 150, 200 and 250 cm profile (cumulative values of the percentage of clay particles by horizon thickness in meters)
-The mineralogy of the sand and silt fractions was always calculated by taking the values corresponding to the second-most superficial horizon of each profile.
-Solum thickness was calculated as the sum of the thicknesses corresponding to Ap and Bt horizons (measured in centimeters) plus half the thickness of the BC and CB transition horizons .
-The thickness of the Bt horizon was also considered in this way, logically without taking the Ap horizons into account.
SOIL DEVELOPMENT INDICES
Several authors have stressed the considerable difficulty in evaluating the degree of development shown by soils due to the enormous amount of data about different soil properties that such studies usually generate. To overcome this problem, quantitative indices have been developed; using a single value these quantitative indices evaluate the degree of evolution among different soils. They can also be applied to the different horizons of a single soil (Buntley and Westin, 1965; Walker and Green, 1976; Bilzi and Ciolkosz, 1977; Harden. 1982; Birkeland, 1984 a and b). These indices were calculated by determining the intensity of the change occurring between the properties of the horizons and those of the original material.
MORPHOLOGICAL INDICES (MI)
To calculate the MI we followed the guidelines of Harden (1982), using seven properties chosen by this author; namely: i) structure (type and degree of development); ii) total texture (textural class + type of stickiness and plasticity of the wet consistence); iii) dry consistence (class;); iv) moist consistence (class); v) clay films (abundance, thickness and location); vi) melanization (color value); vii) rubification (color hue and chroma).
A detailed morphological description of the soil profile was the starting point for the calculations. Quantification of the field properties was modelled after Bilzi and Ciolkosz (1977). These authors assign points on the basis of the the difference between the properties observed in the soil horizons and the original material. The B-C index yields very good results for comparing soils developed from different original materials, although in the particular case of chronosequences developed on river terraces it is not completely satisfactory, as demonstrated by Meixner and Singer (1981). For comparative purposes, better results are obtained using the indices of Harden (1982) in large part because of the introduction of thickness and to the ability to combine several properties.
To calculate the MI, the following steps were observed (Harden, 1982; Busacca, 1987). 1) a description of the soil profile; 2) assessment of parent materials (measurement of fresh deposits or deep C horizons); 3) quantification of each field property for each horizon (assessing ten points to step increases); 4) normalization of quantified properties (division by maximum quantified property) to obtain the MI for each property and horizon. 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 yields the 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 morphological index (general properties) per horizon; 7) by multiplying the latter values by the thickness corresponding to each horizon and then adding these products, one obtains the general MI for each profile.
So that the thickness of the soil will not be overvalued in these indices, Harden and Taylor (1983), Birkeland (1984 a and b), and Busacca (1987) proposed that the values could be divided by the true thickness of each soil. In this same sense, Birkeland (1984b) suggested the use of a constant thickness for each of the soils, choosing the thickness of the deepest soil in the chronosequence as a homogenization factor, artificially prolonging the thinnest soils until the thickness of the deepest soil is reached. We obtained good results by dividing the value of the index by the true thickness of each soil (the values are not reproduced here) and also for a standard thickness of 2 m.
ANALYTICAL INDICES (AI)
For the results of the physical and chemical analyses, we calculated the indices of Birkeland (1984). These indices are a modification of the index of profile anisotropy (IPA) of Walker and Green (1976) and are calculated thus:
where D represents the numerical difference between the value of the horizon property considered and its value in the original material, and PM refers precisely to this latter value.
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, general AI for all properties and horizon; step 7, general AI for all properties and profile). As was done for the MI, calculations were made for normalized thicknesses (AI divided by the thickness of each soil and AI for standard thicknesses of 2 m).
In this study, we calculated the AI for a single property and profile for 9 properties: the % of sand, the % of clay, water retention at 0.03 and 1.5 MPa, available water, % organic matter, % of calcium carbonate, pH and cation exchange capacity. We also calculated the general AI (all properties) for horizon and the general AI for profile.
MINERALOGICAL INDICES (MineI)
Likewise we calculated MineI as a function of the contents of quartz and feldspars of the sand and silt fractions of the soil.
To calculate these MineI we followed the same steps as for the AI, calculating the general MineI as a function of the values of the quartz/feldspars ratio.
RESULTS AND DISCUSSION
A selection of the results of the morphological, physical, chemical and mineralogical analyses of these soils is offered in the tables 2 and 3.
In this chronosequence, it is possible to observe a progressive and pronounced development of the soil with age. In the current flood plain there are Xerorthents. The soils of the Upper Pleistocene surfaces have become Haploxeralfs. Finally, in the Middle Pleistocene there are Palexeralfs.
SOIL DEVELOPMENT INDICES VERSUS HORIZON DEPTH WITH AGE
Figures 2, 3 and 4 summarize the values of the three general indices: morphological, analytical and mineralogical (MI, AI and MineI). It can be seen how the distributions of these indices follow a progressive evolution with age, and form two general developmental trends. One corresponds to the global values of the indices and the other refers to the degree of differentiation among the horizons of each soil.
A marked increase with age is observed in regard to the global values of these indices. The flood plain soils are outstanding due to their very low values. In the soils of Upper Pleistocene age, the indices corresponding to the mineral alterations continue to be spectacularly low, while those corresponding to MI and AI have moderate values. In the soils of the Middle Pleistocene age, all the indices increase as the age of the soils advances.
In regards to the degree of differentiation of the horizons of each soil, the following trend is observed: the soils of the present flood plains show very underdeveloped profiles. The soils of the Upper Pleistocene surfaces have a pronounced horizoning; these soils show a horizon of maximum differentiation situated at about 100 cm depth. In the soils of the Middle Pleistocene age the most developed horizons increase in their absolute values and become broader towards horizons above and below. Very similar distributions have been reported for the MI of Californian soils by Harden (1982, fig. 7) and Busacca (1987, fig. 3).
The MineI disclose the very low degree of alteration shown by soils of the Holocene and Upper Pleistocene ages (AM7, AM14, AM18) whereas for the other indices --MI and AI-- these soils are already clearly developed. The main process responsible for the development of these soils is clay illuviation; this process begins in the initial phase of soil development, long before mineral alteration is developed. We believe that this is due to the special characteristics of the fluvial deposits over which the soils have been formed.
First, the deposits of fluvial sands display a certain mineral stability since, on one hand, due to their size the mineral fragments are fairly well protected from weathering and, on the other, these fluvial deposits contain reasonably stable minerals, since the unstable ones would have decomposed during previous phases of erosion and river transport.
Secondly, the fluvial deposits are mostly formed of loose grains of sand and are therefore very porous, thereby enormously facilitating the infiltration of rainwater and hence the vertical transport of clay suspensions. Since they are present in small amounts, the clays are weakly retained in the soils and migrate with ease. Indeed, micromorphological study clearly shows that at the present time clay illuviation has already developed on the flood plain. Rapid clay translocation at on early stage seems to explains the absence of Inceptisols in this developmental sequence.
SOIL DEVELOPMENT AGE TRENDS
Particular types of behavior of different soil properties can be grouped in three broad categories, according to whether the value of the property tends to increase with age; decrease, or whether there is no relationship to age at all. For the first two groups, a series of subgroups can be established as a function of the intensity and constancy of the change.
1.- Properties that increase with age
Many properties show a direct dependence with age. According to the evolution of the property with age, several groups can be differentiated thus:
1.1. Constant and regular increase throughout the chronosequence depending strongly on age: the available water and COLE (coefficient of linear extensibility) of the Bt horizon (figure 5).
1.2 Strong increase only during the first phases. Another group of properties shows strong increases with age for the Holocene and Upper Pleistocene soils, thereafter undergoing a change in their development in the final stages, corresponding to the Middle Pleistocene. This change may follow different trends:
i) Some properties continue to increase with age but more moderately (figure 6): water retention at 0.03 and 1.5MPa and cation exchange capacity of the Ap horizon (also in Dickson and Crocker, 1953; Torrent, 1976; Ahmad et al., 1977; Jongmans et al., 1991), Fed of the Bt horizon (Hendershot et al.,1979; Torrent et al., 1980; Alexander and Holowaychuck, 1983; Peña and Torrent, 1984; Arduino et al., 1984 and 1986; and Aniku and Singer,1990); solum thickness (Ahmad et al., 1977; Meixner and Singer, 1981; Little and Ward, 1981; Harden, 1982; Muhs, 1982; Alexander and Holowaychuck, 1983; Arduino et al., 1984; Chittleborough el al., 1984; Arduino et al., 1986; Busacca, 1987; Ajmone et al., 1988; Reheis et al., 1989), clay accumulations at 15, 50, 100, 150, 200 and 250 cm depth, quartz content and quartz/feldspar ratio (also in Ruhe, 1956; Barshad, 1955; Muhs, 1982).
ii) Other properties cease to increase for the soils from the Middle Pleistocene. In this situation are the silt and available water of the Ap horizon and the % of clay, available water at 0.03 and 1.5MPa and cation exchange capacity of the Bt horizons (figure 7).
A highly representative property of this type of study is the concentration of clay accumulated in the B horizon of the soils. Logically, as age increases a progressive increase in the amount of clay present would also be expected to occur. This kind of behavior has been observed both in our soils and in many others studied (Ruhe, 1956; Franzmeier and Whiteside, 1963; Janda and Croft 1967; Torrent, 1976; Ahmad et al., 1977; Pastor and Bockheim, 1980; Meixner and Singer, 1981; Dorronsoro et al. 1988; Fine et al., 1989; Reheis et al.,1989; Aniku and Singer, 1990). All of these chronofunctions can be adapted to power and logarithmic models without a value of maximum enrichment being reached after which the values remain constant, although this does occur in some cases (Peña and Torrent, 1984) and also in this investigation for the total enrichments in clay at defined depths (% clay x thickness) of 50, 100, 150 and 200 cm.
1.3 Strong increase only during the last phases. Another group of properties shows strong increases with age for the Middle Pleistocene soils. In this situation are the % of clay and nitrogen of the Ap horizon, the thickness of the Bt horizons and carbonate accumulations (figure 8). Some of these properties are not strongly age dependent. So, these trends could go to next group 3 of not age related.
2.Properties that decrease with age
These show the opposite behavior to that of the previous group; that is, the property becomes increasingly less degree of expression as the soil ages.
The properties belonging to this group are minor. All of these properties (figure 9) show strong decreases up to the Upper Pleistocene soils, thereafter decreasing very moderately (feldspar contents), or cease to increase for the soils from the Middle Pleistocene (percentages in sand of the Ap and Bt horizons) or show increase during the last phases (bulk density of the Bt horizon).
3. Properties that are not age-related.
Finally, for one group of properties it is not possible to establish any logical relationships between their degree of manifestation and the age of the soil (pH and base saturation of Ap horizon, and the silt of Bt horizon). Figure 10.
The percentages of organic matter are expected to increase with age (Dickson and Crocker, 1953; Ruhe, 1956; Viereck, 1966; Leisman, 1957; Olson, 1958; Syers et al. 1970; Mahaney, 1974), although in our soils, an increase is observed only for the very young soils; after the Upper Pleistocene the soils display a chaotic behavior, possibly due to the intense anthropogenic activity to which they have been subjected.
The absence of a dependence between the base saturation and age could be the result of land practices.
In other chronosequences the pH of the soils clearly reflects the progressive acidification that atmospheric precipitation tends to produce on the surface horizon (Dickson and Crocker, 1954; Crocker and Dickson, 1957; Wilson, 1960; Cowie, 1968; Ugolini, 1968; Viereck, 1970; Bockheim, 1979; Alexander and Holowaychuck, 1983; Arduino et al., 1984; Chittleborough et al., 1984; Peña and Torrent, 1984; Aniku and Singer, 1990).
As shown in figure 11, all of the MI show strong increases during the initial phases of soil formation; as from the Upper Pleistocene soils, these increases are reduced to the minimum (MI of texture, dry consistency, and the general one for all the properties together) or may even remain constant (moist consistency and rubification MI). A similar kind of behavior, with continuous increases, and mostly in the early phases of the development of the soils has been reported for these MI by Harden (1982), Birkeland (1984b), Busacca (1987) and Reheis et al. (1989).
As occurs with the MI, the AI show a very dominant type of behavior. The values of most of the AI increase throughout the chronosequence but only do so very intensively in the case of the soils of the Holocene and Upper Pleistocene age (figure 12). Similar increases to those of these AI have been described by Birkeland (1984b) for soils of the Holocene age in New Zealand. Some of these AI are not strongly age dependent, like organic carbon and carbonate.
The MineI are also well related with age. These MineI point to a constant and intense increase with age (figure 13).
Five models of regression equations were assayed for the chronofunctions of these soils: linear (Y=a + bX), second degree polynomial (Y= a + bX + cX2), power (Y=aXb), logarithmic (Y= a + blogX) and exponential (Y= abX). Of all these models, the power, second degree polynomial and logarithmic equations proved to be the ones best represented, almost all the regressions defined for the MI and AI and also for many of the properties corresponded to these equations (tables 4 and 5). The linear model was the best fit in very few cases; the exponential equations had no representativity at all for these soils. In previous works (Birkeland, 1974 and 1984; Yaalon, 1975; Bockheim, 1980 and 1990; Little and Ward, 1981; Harden, 1982; Muhs, 1982) the equations most frequently found have been the logarithmic and power models.
In general, the correlation coefficients obtained had high values. The regression equations with the best fits for each chronofunction are shown in table 5.
Finally, it is interesting to note that very few properties of the soils studied here seem to reach steady state during their development. All of them, with the only exception of the percentages of sands, continue to undergo changes even in the oldest soils of the chronosequence. However, for a large number of properties, development manifests only during the first period, corresponding to the Holocene and Upper Pleistocene soils (up to about 100,000 years), with the rate of development slowing thereafter.
The paucity of results confirming the steady-state theory seems to be quite patent in most chronosequences investigated by different authors. Bockheim (1980) indicates that "the trends shown in this study cast some doubt as to whether soils reach a steady-state (dynamic equilibrium) with their environment. Most of the properties investigated continue to change despite the passage of as many as 106 years". Similar conclusions were reached by Muhs (1982) for the soils of marine terraces in California of up to 106 years old. Also, Busacca (1987), studying a chronosequence of river terraces of the Sacramento Valley (California) of up to 106 years concluded "Apparently none of the soils in the sequence has reached a steady state of development".
In light of the foregoing, it seems clear that most soil properties continue to develop over time and that no final stage of development is reached. Despite this, within this possible kind of development it would be necessary to take into account that during the development of a chronosequence not only are there changes in time but also modifications in climatic factors and the subordinate factor of organisms participate; these may have led to important variations in the final behavior of the soils of the chronosequence (in fact in most studies investigators should be using the term climobiochronosequences).
The following conclusions can be drawn from the results:
1.- The soil properties show several types of behavior.
1.1- As age progresses, same properties increase regularly throughout the chronosequence. This is strongly dependent on the age, COLE and the available water of the Bt horizon.
1.2- Some properties continue to increase with age but more moderately during the last phases: for example, water retention at 0.03 and 1.5MPa and cation exchange capacity of the Ap horizon, Fed of the Bt horizon; solum thickness, clay accumulations at 15, 50, 100, 150, 200 and 250 cm depth, quartz content and quartz/feldspar ratio.
1.3- Other properties cease to change in the soils from the Middle Pleistocene. In this situation are the silt and available water of the Ap horizon and the % of clay, water retention at 0.03 and 1.5MPa and cation exchange capacity of the Bt horizons.
1.4- Still other properties show a strong increase only during the last phases (Middle Pleistocene and older soils). In this situation are the % of clay and nitrogen of the Ap horizon, the thickness of the Bt horizons and carbonate accumulations.
1.5-Other properties decrease with age: with strong decreases up to the Upper Pleistocene soils, thereafter decreasing very moderately (feldspar contents), or cease to increase for the soils from the Middle Pleistocene (percentages in sand of the Ap and Bt horizons) or show an increase during the last phases (bulk density of the Bt horizon).
1.6- Other properties are not age-related. Within this group there are two subgroups. One refers to those properties that remain constant (base saturation of the Ap horizon), and another to those that vary chaotically: organic matter and pH of the Ap horizon and the silt of the Bt horizon.
2.- The horizon development indices show marked increases in their values with the advancing age of the soils and also show a progressive differentiation over time for the horizons of the soil profile.
3.- The soil development indices show very good correlations with age. In the vast majority of cases the tendency to increase falls off strongly for the oldest soils of the Middle Pleistocene age.
4.- In most cases the properties and development indices continue to evolve constantly throughout the chronosequence without steady-state being reached; however, the soils of the Middle Pleistocene age do reach a state of evident maturity from which development processes very slowly.
The authors are grateful to the Spanish DGICYT for the financial support received for this study (PB88-0378)
AHMAD, M.; RYAN, J. and PAETH, R.C. 1977. Soil development as a function of time in the Punjab River Plains of Pakistan. Soil Sci. Soc. Am. J. 41: 1162-1166.
AJMONE MARSAN, F.; BARBERIS, E. and ARDUINO, E. 1988. A soil chronosequence in northwestern Italy: morphological, physical and chemical characteristics. Geoderma. 42: 51-64.
ALEXANDER, E.B. and HOLOWAYCHUK, N. 1983. Soils on terraces along the Cauca River, Colombia, I. Chronosequence characteristics. Soil Sci. Soc. Am. J. 47: 715-721.
ANIKU, J.R.F. and SINGER, M.J. 1990. Pedogenic iron oxide trends in a marine terrace chronosequence. Soil Sci. Soc. Am. J. 54: 147-152.
ARDUINO, E.; BARBERIS, E.; CARRARO, F. and FORNO, M.G. 1984. Estimating relative ages from iron oxide/total-iron ratios of soils in the western Po Valley, Italy. Geoderma. 33: 39-52.
ARDUINO, E.; BARBERIS, E.; AJMONE, F.; ZANINI, E. and FRANCHINI, M. 1986. Iron oxides and clay minerals within profiles as indicators of soil age in northern Italy. Geoderma. 37: 45-55.
BARSHAD, I. 1955. Soil Development. En: Chemistry of the soil. Ed.: F. E. Bear. Reinhold, N. Y. 373 pp.
BILZI, A.F. and CIOLKOSZ, E.J. 1977. A field morphology scale for evaluating pedological development. Soil Sci. 24: 45-48.
BIRKELAND, P.W. 1984a. Soil and Goemorphology. Oxford University Press. Inc. N. Y. 372 pp.
BIRKELAND, P.W. 1984b. Holocene soil chronofunctions, Southern Alps, New Zealand. Geoderma. 34: 115-134.
BOCKHEIM, J.G. 1979. Relative age and origin of soils in eastern Wright Valley, Antarctica. Soil Sci. 128: 142-152.
BOCKHEIM, J.G. 1980. Solution and use of chronofunctions in studying soil development. Geoderma. 24: 71-85.
BOCKHEIM, J.G. 1990. Soil development rates in the Transantarctic Mountains. Geoderma, 47: 59-77.
BUNTLEY,G.J. and WESTIN, F.C. 1965. A comparative study of develpmental color in a Chesnut-Chernozem-Brunizem soil climosequence. Soil Sci. Soc. Am. J. 29: 579-582.
BUSACCA, A.J. 1987. Pedogenesis of a chronosequence in the Sacramento Valley, California. U.S.A. I. Application of a soil development index. Geoderma. 41: 123-148.
COWIE, J.D. 1968. Soil-age relationship in the Manawatu District, New Zealand. Trans. 3th Int. Congr. Soil Sci. Adelaide. 4: 113-120.
CHITTLEBOROUGH, D.J.; WALKER, P.H. and OADES, J.M. 1984. Textural differentiation in chronosequences from eastern Australia. I. Descriptions, chemical properties and micromorphologies of soils. II. Evidence from particle-size distributions. III. Evidence from elemental chemistry, Geoderma 32: 181-248.
CROCKER, R.L. and DICKSON, B.A. 1957. Soil development on the recessional moraines of the Herbert and Mendenhall Glaciers, southeastern Alaska. J. Ecol., 45: 169-185.
DICKSON, B.A. and CROCKER, R.L. 1953. A chronosequence of soils and vegatation near Mt. Shasta, California.II. The development of the forest floors and the carbon and nitrogen profiles of the soils. J. Soil Sci. 4: 142-154.
DICKSON, B.A. and CROCKER, R.L. 1954. A chronosequence of soils and vegatation near Mt. Shasta, California.III. Some properties of the mineral soils. J. Soil Sci. 5: 173-191.
DORRONSORO, C.; DEL ARCO, J. y ALONSO, P. 1988. Micromorfología de una cronosecuencia de suelos en la zona de Calvarrasa (Salamanca). Ana. Edaf. y Agrob. XLVII: 351-370.
FINE, P., SINGER, M.J., LA VEN, R., VEROSUB, K. and SOUTHARD, R.J. 1989 Role of pedogenesis in distribution of magnetic susceptibility in two California chronosequences. Geoderma, 44: 287-306.
FRANZMEIER, D.P. and WHITESIDE, E.P. 1963a. A chronosequence of podzols in northern Michigan, II. Physical and chemical properties. Mich. Agric. Exp. Stn. Q. Bull. 46: 21-36.
HARDEN, J. 1982. A quantitative index of soil development from field description: examples from a chronosequence in central California. Geoderma. 28: 1-28.
HARDEN, J. and TAYLOR, E.M. 1983. A quantitative comparaison of soil development in four climatic regimes. Quat. Res. 20: 342-359.
HENDERSHOT, W.H.; SINGLETON, G.A. and LAVKULICH, L.M. 1979. Variation in surface charge characteristics in a soil chronosequence. Soil Sci. Soc. Am. J. 43: 387-389.
IGME. 1982. ¨Mapa Geológico de España¨. 1:50.000. Hoja nº 479 (Peñaranda de Bracamonte).
JANDA, R.J. and CROFT, M.G. 1967. The stratigraphic significance of a sequence of noncalcic brown soils formed on the Quaternary alluvium of the northeastern San Joaquin Valley, California. In: R.B. Morrison and H.E. Wrigth, Jr. (Editors), Quaternary Soils. Desert Res. Inst., Univ. Nevada, Reno, pp. 157-190.
JENNY, H. 1941. Factors of soil formations. McGraw-Hill. N.Y. 281 pp.
JONGMANS, A.; FEIJTEL, T.; MIEDEMA, R.; VAN BREEMEN, N. and VELDKAMP, A. 1991. Soil formation in a Quaternary terrace sequence of Allier, Limagne, France. Macro and micromorphology, particle size distribution, chemistry. Geoderma 49: 215-239.
LEISMAN, G.A. 1957. A vegetation and soil chronosequence on the Mesabi iron range spoil banks, Minnesota. Ecol. Monogr. 27: 221-245.
LITTLE, L.P. and WARD, W.T. 1981. Chemical and mineralogical trends in a chronosequence developed on alluvial in easthern Victoria, Australia. Geoderma. 25: 173-188.
MAHANEY, W.C. 1974. Soil stratigraphy and genesis of neoglacial deposits in the Arapaho and Henderson cirques, central Colorado Front Range. En Quaternary Environments. 197-240. Ed.: W. C. Mahaney. Proc. Symp. Geogr. Monogr., 5 York Univ., Atkinson College, Toronto.
MEIXNER, R.E. and SINGER, M.J. 1981. Use of a field morphology rating system to evaluate soil formation and discontinuities. Soil Sci. 131: 114- 128.
MUHS, D.R. 1982. A soil chronosequence on quaternary marine terraces, San Clemente Island, California. Geoderma. 28: 257-283.
OLSON, J.S. 1958. Rates of succession and soil changes on southern Lake Michigan sand dunes. Bot. Gaz. 119: 125-170.
PASTOR, J. and BOCKHEIM, J.G. 1980. Soil development on moraines of Taylor Glacier, Lower Taylor Valley, Antarctica. Soil Sci. Am. J. 44 : 341- 348.
PEÑA, E. and TORRENT, J. 1984. Relationships between phosphate sorption and iron oxides in Alfisols from a river terrace sequence of Mediterranean Spain. Geoderma. 33: 283-296.
REHEIS, M.; HARDEN, J.; MACFADDEN, L. and SHROBA, R. 1989. Developments rates of Late Quaternary soils, Silver Lake Playa, California. Soil Sci. Am. J. 53: 1127-1140.
RUHE, R.V. 1956. Geomorphic surfaces and the nature of soils. Soil Sci. 82: 441-455.
SANTONJA, M. y QUEROL, A. 1976. Estudio de industrias del Paleolítico inferior procedentes de una terraza del Tormes (Galisancho, Salamanca). Zephirus. XXVI-XXVII: 97-109.
SANTONJA, M.; QUEROL, A. y PEREZ GONZALEZ, A. 1982. El yacimiento de La Maya I y la secuencia paleolítica del valle del Tormes. Actas de la II Reunión Regional de Geología del Duero (Salamanca, 1979), 2ª parte: 641-662. Temas Geológico-Mineros, VI, I.G.M.E.
SANTONJA, M. y PEREZ GONZALEZ, A. 1984. Las industrias paleolíticas de La Maya I en su ámbito regional. Ministerio de Cultura. Excavaciones arqueológicas en España. 347 pp.
SOIL SURVEY STAFF. 1951. Soil survey manual. SCS-USDA. U. S. Government Printing Office, Washington. DC.
SOIL SURVEY STAFF. 1975. Soil Taxonomy. Agriculture Handbook 436. USDA, Washington. DC. .
SOIL SURVEY STAFF. 1984. Procedures for collecting soil samples and methods of analysis for soil survey. Soil Survey Investigations Report No. 1.U.S.D.A., Washington, D.C.
SYERS, J.K.; ADAMS, J.A. and WALKER, T.W. 1970. Accumulation of organic matter in a chronosequence of soils developed on wind-blown sand in New Zealand.J. Soil Sci. 21: 146-153.
TORRENT, J. 1976. Soil development in a sequence of river terraces in northern Spain. Catena. 3: 137-151.
TORRENT, J.; SCHWERTMANN, U. and SCHULZE, D.G. 1980. Iron oxide mineralogy of some soils of two river terrace sequences in Spain. Geoderma. 23: 191-208.
UGOLINI, G.C. 1968. Soil development and alder invasion in a recently deglaciated area of Glacier Bay, Alaska. In: Biology of Alder. U.S. Dep. Agric., For. Serv.., Pac. Northwest For. Range Exp. Stn., Portland, OR. 115-140.
VIERECK, L.A. 1966. Plant succession and soil development on gravel outwash of the Muldrow Glacier, Alaska. Ecol. Monogr. 36: 181-199.
VIERECK, L.A. 1970. Forest succession and soil development adjancent to the Chena River in interior Alaska. Arct. Alp. Res. 2(1): 1-26.
VREEKEN, W.J. 1975. Principal kinds of chronosequences and their significance in soil history. J. Soil Sci. 26: 378-394.
WALKER, P.H. and GREEN, P. 1976. Soil trends in two valley fill sequences. Aust. J. Soil Res. 14: 291-303.
WILSON, K. 1960. The time factor in the development of dune soils at Soth Haven Peninsula, Dorset. J. Ecol. 48: 341-359.
YAALON, D.H. 1975. Conceptual models of pedogenesis: can soil-forming functions be solved?. Geoderma. 14: 189-205.