The aim of this study is to identify the chemical and physical characteristics in uncultivated soils derived from different parent materials under semiarid Mediterranean climatic conditions which favoured the formation of fragile soils. The current work is of great interest in the agriculture and environmental stakeholders for providing a “benchmark” of undisturbed soil quality regarding organic content and nutrients availability. Principal Component Analysis (PCA) was used as the primary tool to demonstrate the soil quality stage, regarding nutrient availability. The statistical analysis revealed that one of the major physicochemical characteristics such as cation exchange capacity (CEC) is controlled exclusively from mineralogy and not from organic matter. Mineralogy and bulk chemical analysis is directly related to soil parent material lithology. The availability of inorganic nutrients (macro- and micronutrients) is low and relatively identical to most of the soils. PCA shows the unusual correlation of K+ with not only illite content but also the OM in soils. The development of soils which are already of low quality in respect of organic content and nutrients is evident in Crete in most of the 54 samples investigated.
Soil is the dynamic link between the biosphere and lithosphere and constitutes a practically not renewable (very low rate of formation) natural resource, with a key role for the environment and for the agriculture. It is a key component of the Earth System since it controls the hydrological, ecological, biological, and geochemical cycles [
Apart from nutrients release through weathering processes, soil characteristics are further affected by atmospheric deposition, drainage outflow, biomass removal, and other processes such as cation exchange and organic matter decomposition, while pronounced interrelations exist between all factors mentioned above [
High geologic variability mainly appeared in tectonically active areas like boundaries of orogenetic belts. Crete is situated in the external plate of the Eurasian plate and exhibits a large variety of geomorphic and geologic features. So far, a number of papers have been published concerning soil taxonomy in Greece (i.e., [
The present study aims to identify physical and chemical characteristics in undisturbed soils of different lithology of parent rock with sparse vegetation under the influence of Mediterranean climatic conditions. We aim to clearly demonstrate the fragile nature of soils from Crete for uncultivated areas with low organic content and to supply data that are important for assessing thin soils in semiarid climates. For this reason, soil samples from uncultivated areas of different bedrocks were collected and their mineralogical characteristics, physical and chemical properties, and their nutrient content were investigated. Principal Component Analysis (PCA) was applied on chemical and physical characteristics of all samples in order to identify groups of samples with common characteristics and to investigate and to rationalize the potential correlations among soil characteristics.
The samples were collected from six areas in the island of Crete with four different bedrock lithologies. The different bedrocks are alluvial sediments (quaternary sediments) from two plateaus, ultramafic rocks, quartzite-phyllite rocks, and Neocene marly limestones. Mesozoic limestones were excluded since they are mainly situated in high altitudes with limited soil profiles. The aforementioned lithologies are present at Omalos Plateau (O) in western Crete, Lasithi Plateau in East Crete (La), Anogia in central Crete (U), Kantanos (Ka) and Kantanos-Kountoura (Kb) in western Crete, and Platanos (PL) in west Crete (Figure
Geological outline in the sampling areas.
Omalos area is situated in the west part of “Lefka Ori” mountains and its geomorphology is a typical plateau (1050 m elevation). The rock formations around the Omalos Plateau are mainly part of the Trypali limestones and less of the Plattenkalk limestones with minor outcrops of phyllites-quartzites series, whereas quaternary sediments have filled the plateau [
Lasithi soils were collected from Lasithi Plateau (353 m), which shows similar geological setting to Omalos Plateau. Limestones, mainly of Plattenkalk nappe and phyllites-quartzite group, are situated in the surrounding area of the plateau, whereas the soil samples were collected from the top of alluvial deposits.
Anogia area is underlain by ophiolites (mainly ultramafic rocks: peridotites). The most extended ophiolite outcrop in Crete exists in the area of Anogia village (701 m elevation). The ultramafic rocks comprise predominantly serpentinized peridotites. The overlying soil is mainly of limited thickness, whereas in land depressions and cavities the soil layer appears much thicker.
The parent rock of Kantanos area comprises metamorphic quartzite-phyllite group. Quartzite group in west Crete facies belt is characterized by an alternation of meta-greywackes, meta-sandstones, and metapelites [
Platanos area comprises mainly of Neocene marls with intercalations of limestones and sandstones. The area is situated in the most western part of Crete compared to the other sampling sites (Figure
Sampling was carried out using a special soil auger, designed for all soil types, and samples were collected from the top 20 cm. 54 samples were collected from sites chosen according to three strategic guidelines: (i) sites with uncultivated
Mineralogical composition was determined by X-ray diffraction (XRD) technique using a Siemens D500 powder diffractometer, on two grain fractions, silt (<63
The bulk chemical analysis of the samples was carried out by X-ray fluorescence spectroscopy (S2 Ranger, Bruker EDS XRF), on the less than 2 mm sample fraction. Measurements were carried out at 40 kV with an Al filter (500
Cation exchange capacity (CEC) was determined by the method described by Sumner and Miller [
The content of sand, clay, and silt of the samples (grain size analysis) was determined with the Bouyoucos method [
Results for the physical and chemical parameters of soils are presented separate for each area and then PCA analysis is applied for all samples and it is presented in a separate section. Lasithi and Omalos are presented together since they are both plateau areas and Kantanos and Kantanos-Kountoura data are also presented together since both are adjacent areas with seemingly the same soil parent material.
Omalos and Lasithi soils have been developed in plateau and the parent material is alluvial deposits as has already been mentioned. The soils have contrasting characteristics in mineralogy, in bulk chemical analysis and other physical and chemical parameters. The silt fractions (<63
Mineralogical quantitative analysis mean values for <63
<63 |
<2 | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
O | La | U | Ka | Kb | PL | O | La | U | Ka | Kb | PL | |
Calcite | 0.3 | — | — | — | — | 64.3 | — | — | — | — | — | 33.3 |
Dolomite | — | — | 2.9 | — | — | 1.9 | — | — | — | — | — | — |
Quartz | 67 | 58.2 | 9.7 | 27.4 | 22.7 | 13.2 | 4.5 | 4.4 | 2.2 | 3.1 | — | 5.3 |
Feldspars | — | 13.7 | — | 3.7 | 5.8 | 5 | — | — | — | 8.6 | — | — |
Illite | 12.1 | 17.6 | 1.3 | 33.3 | 33.2 | 15.5 | 55.8 | 60.8 | 2.8 | 23 | 71.6 | 43.5 |
Kaolinite | 20.6 | 3.2 | — | 26.5 | 26.8 | — | 39.7 | 18.9 | — | 63 | 14.8 | — |
Paragonite | — | — | 7.4 | 9.1 | 11.5 | — | — | — | — | 2.3 | 13.7 | — |
Chlorite | — | 7.4 | 21.2 | — | — | — | — | 15.9 | 46.4 | — | — | 6.5 |
Antigorite | — | — | 49.2 | — | — | — | — | — | 33.7 | — | — | — |
Talc | — | — | 8.3 | — | — | — | — | — | — | — | — | — |
Smectite | — | — | — | — | — | — | — | — | 14.8 | — | — | 11.4 |
—: not determined.
Mean concentrations for major elements content in soils (energy dispersive-XRF); measurement relative standard error was less than 5%.
Sample | Na2O (%) | MgO (%) | K2O (%) | CaO (%) | TiO2 (%) | MnO (%) | Fe2O3 (%) | Al2O3 (%) | SiO2 (%) | P2O5 (%) | LOI (%) | SUM |
---|---|---|---|---|---|---|---|---|---|---|---|---|
O | 0.6 | 0.5 | 1.2 | 1.0 | 0.4 | 0.1 | 3.8 | 8.4 | 83.1 | <0.05 | 1.0 | 100.1 |
La | <0.2 | 3.8 | 1.7 | 0.9 | 0.6 | 0.2 | 6.2 | 23.3 | 61.6 | 0.4 | 1.6 | 100.1 |
U | <0.2 | 26.4 | 0.5 | 1.2 | 0.4 | 0.3 | 11.1 | 6.2 | 40.9 | <0.05 | 13.8 | 100.6 |
Ka | 1.5 | 0.5 | 1.6 | 0.02 | 0.8 | 0.04 | 4.9 | 14.2 | 72.1 | 0.1 | 4.2 | 99.9 |
Kb | 1.00 | <0.2 | 1.8 | 0.4 | 0.8 | 0.03 | 4.1 | 14.5 | 62.2 | <0.05 | 14.9 | 99.7 |
PL | <0.2 | 2.5 | 1.7 | 26.4 | — | 0.1 | 2.3 | 4.7 | 24.2 | — | 38.4 | 100.2 |
—: not determined.
Table
Average, minimum, maximum values, and standard deviation (due to spatial variability) of the physicochemical parameters measured for all soils are illustrated in Table
Average values in soils from investigated areas for pH, electrical conductivity, CEC, macronutrients (
pH | EC |
CEC (meq/100 g) | K+ (mg/kg) | Mg2+ (mg/kg) | Ca2+ (mg/kg) | Cu2+ (mg/kg) | Mn2+ (mg/kg) | Fe2+ (mg/kg) | Zn2+ (mg/kg) | P- |
N- |
OM% | |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
O (9 samples) | |||||||||||||
|
|||||||||||||
Average | 6.4 |
187 |
4 |
50 |
64 |
5 |
0.5 |
10 |
6 |
0.3 |
1.1 |
2.6 |
0.64 |
min | 5.0 | 75 | 1 | 23 | 13 | 0.2 | 0.1 | 1 | 0.1 | 0.1 | 0.4 | 0.8 | 0.19 |
max | 8.4 | 487 | 6 | 80 | 281 | 15 | 1 | 26 | 26 | 0.5 | 2.2 | 5.3 | 2.86 |
stdev | 1.4 | 133 | 2 | 8 | 83 | 6 | 0.2 | 8 | 9 | 0.2 | 0.7 | 1.5 | 0.87 |
|
|||||||||||||
La (11 samples) | |||||||||||||
|
|||||||||||||
Average | 7.4 |
364 |
9 |
114 |
228 |
27 |
0.7 |
17 |
18 |
0.72 |
1.5 |
3.0 |
1.47 |
min | 7.1 | 210 | 3 | 67 | 64 | 18 | 1 | 7 | 8 | 0.45 | 1.3 | 0.2 | 1.02 |
max | 7.5 | 487 | 16 | 172 | 506 | 75 | 2 | 33 | 40 | 1.22 | 2.4 | 5.0 | 1.66 |
stdev | 0.2 | 77 | 4 | 28 | 170 | 24 | 0.4 | 8 | 9 | 0.24 | 0.4 | 1.8 | 0.23 |
|
|||||||||||||
U (8 samples) | |||||||||||||
|
|||||||||||||
Average | 7.4 |
279 |
12 |
66 |
1611 |
4 |
0.7 |
7 |
17 |
0.3 |
0.5 |
7.5 |
0.44 |
min | 7.0 | 151 | 5 | 8 | 397 | 0.1 | 0.1 | 1 | 4 | 0.2 | 0.4 | 5.6 | 0.02 |
max | 8.5 | 485 | 21 | 157 | 2572 | 11 | 2 | 15 | 31 | 0.7 | 0.7 | 10.3 | 1.00 |
stdev | 0.5 | 134 | 6 | 52 | 845 | 4 | 0.6 | 6 | 11 | 0.2 | 0.1 | 1.9 | 0.29 |
|
|||||||||||||
Ka (7 samples) | |||||||||||||
|
|||||||||||||
Average | 5.6 |
292 |
2 |
61 |
101 |
10 |
0.3 |
5 |
7 |
0.4 |
0.7 |
2.9 |
0.38 |
min | 4.9 | 153 | 1 | 24 | 24 | 0.6 | 0.2 | 0.2 | 4 | 0.3 | 0.4 | 2.4 | 0.11 |
max | 6.5 | 450 | 3 | 162 | 209 | 31 | 0.4 | 20 | 14 | 0.7 | 1.1 | 3.6 | 1.07 |
stdev | 0.6 | 99 | 1 | 47 | 60 | 12 | 0.1 | 7 | 3 | 0.1 | 0.2 | 0.5 | 0.32 |
|
|||||||||||||
Kb (11 samples) | |||||||||||||
|
|||||||||||||
Average | 6.2 |
603 |
3 |
200 |
127 |
11 |
0.4 |
19 |
15 |
0.7 |
0.8 |
11.6 |
1.09 |
min | 5.1 | 130 | 0.8 | 67 | 109 | 2 | 0.1 | 1 | 3 | 0.2 | 0.4 | 5.3 | 0.41 |
max | 7.0 | 1697 | 7 | 374 | 143 | 33 | 1 | 90 | 57 | 3.1 | 1.2 | 34.7 | 1.80 |
stdev | 0.6 | 477 | 2 | 24 | 11 | 10 | 0.3 | 25 | 16 | 0.8 | 0.3 | 10.3 | 0.60 |
|
|||||||||||||
PL (8 samples) | |||||||||||||
|
|||||||||||||
Average | 7.9 |
1496 |
11 |
90 |
146 |
136 |
0.9 |
2 |
6 |
0.7 |
1.0 |
17.1 |
0.44 |
min | 7.5 | 448 | 8 | 39 | 27 | 28 | 0.3 | 0.3 | 3 | 0.4 | 0.4 | 13.0 | 0.10 |
max | 8.3 | 2990 | 21 | 140 | 197 | 559 | 2 | 5 | 7 | 1 | 3.1 | 24.3 | 1.50 |
stdev | 0.3 | 1014 | 4 | 33 | 51 | 187 | 0.5 | 2 | 2 | 0.3 | 1.0 | 3.9 | 0.48 |
Lasithi samples have higher CEC values (8.52 meq/100 gr), compared to Omalos soils CEC (4 meq/100 gr) (Table
Anogia soils contain abundant serpentine in both 63
The chemical composition of Anogia soils resembles that of ultramafic rocks with SiO2 content < 45% and high MgO concentration (26.4%) (Table
Anogia soils exhibit an average pH value of 7.4. Electrical conductivity is rather low (279
Anogia samples display the highest cation exchange capacity (12 meq/100 g) compared to the CEC of the other soils (Table
Kantanos and Kantanos-Kountoura soils contain abundant illite, kaolinite, and quartz (Table
Ka soils exhibit higher concentration of SiO2 (72.1%) compared to Kb soils (62.2%) (Table
Low pH values are observed for soil samples from Ka and Kb (5.6, 6.2), which are related to SiO2-rich parent materials (Phyllites-Quartzites). Electrical conductivity exhibits spatial variability for both areas and the average value is higher for Kb soils (Ka: 292, Kb: 603
Ka and Kb soils show low CEC (Ka: 2.1 meq/100 g, Kb: 3.1 meq/100 g). Moreover, soils from Ka and Kb areas are sandy loam and sandy clay loam, respectively.
Platanos soils contain abundant calcite in both the silt and clay fractions which is related to sedimentation in shallow marine basins [
On Table
PL soils exhibit the highest pH values compared to other soils (in average 7.9, standard deviation 0.3) which can be related to the high content of basic cations (Ca2+: 125 mg/kg) in extraction solution (Table
Platanos samples exhibit the second higher cation exchange capacity (CEC) compared to other soils (11 meq/100 gr). Grain size distribution is that of clay loam for PL soils.
The chemical analysis (XRF) is presented in Table
(a) PCA of chemical analysis (Na2O, MgO, K2O, Fe2O3, MnO, CaO, Al2O3, SiO2, and LOI). (b) PCA of macronutrients availability (N-NO3, P-PO4) and physical parameters (clay content: clay%, organic content: OC%, and cation exchange capacity: CEC). 54 soil samples from Omalos Plateau (O), Lasithi Plateau (La), Kantanos (Ka, Kb) Anogia (U), and Platanos (PL). Variable axes are depicted in the figure.
Figure
(a) PCA analysis of macronutrients availability (Ca2+, Mg2+, and K+) and physical characteristics (clay content: clay%, organic content: OC%). (b) PCA analysis of macronutrients availability (Ca2+, Mg2+, and K+). 54 soil samples included from Omalos Plateau (O), Lasithi Plateau (La), Kantanos (Ka, Kb) Anogia (U), and Platanos (PL). The variable axes are depicted in the figure.
Micronutrients (Zn2+, Cu2+, Fe2+, and Mn2+), clay content, and OM% are the variables for the PCA depicted in Figure
(a) PCA analysis of micronutrients availability (Zn2+, Cu2+, Fe2+, and Mn2+) and physical characteristics (clay content: clay%, organic content: OC%). (b) PCA analysis of micronutrients availability (Zn2+, Cu2+, Fe2+, and Mn2+). 54 soil samples from Omalos Plateau (O), Lasithi Plateau (La), Kantanos (Ka, Kb) Anogia (U), and Platanos (PL). The variable axes are depicted in the figure.
As it is expected, parent material influences primary mineral phases of soils. Two main reasons impose differences in the soil samples from Omalos Plateau and Lasithi Plateau and these are differences in the parent material and differences in the amount of precipitation in those areas. The presence of feldspars and chlorites and the lower quartz content in La soils compared to O soils are attributed to differences in the lithology of the weathering products (Table
The silt fraction of Anogia soils exhibits characteristic primary mineralogy with serpentine and talc as inherited from ultramafic rocks (Table
Ka and Kb soils have quartz, feldspars, and phyllosilicates (paragonite) in silt and clay fractions which are related to phyllites-quartzite parent material. Ka soils exhibit differences in secondary minerals compared to Kb soils. Ka soils contain lower illite in clay fraction compared to Kb soils, which is attributed to differences within the phyllites and quartzites series (Kb soils less acidic: low presence of quartzites) since both sampling areas are in the same elevation. This is an evident that soils from different lithology exhibit different content of secondary mineralogy like illite under the same climatic conditions (same amount of rain).
Platanos soils show mineralogy which is characteristic of the calcaric Neocene sediments (parent material) with abundant calcite in both silt and clay fractions. The presence of smectite in the PL soils is linked to MgO presence in chemical analysis. PL soils contain illite, which might be secondary and/or primary sedimentary mineral.
Smectite is present in soils with different lithological characteristics (i.e., ultramafic: U, alluvial: La, and limestone: PL). This is an indication that factors like precipitation and drainage of soils affect concentration of basic cations (i.e., Mg2+ and Ca2+) favouring the formation of smectite. This is also evident from bulk chemical analysis where U, PL, and La exhibit significant Mg2+ concentrations (Table
Bulk chemical analysis of soils reflects the main characteristics of the parent materials (Table
Soil texture shows that sedimentary parent material exhibits finer grain size soils (PL, O, and La), whereas soils with nonsedimentary parent material are coarser grained (Ka, Kb, and U). Parent material of the PL soils (soils with finer grains) is a biochemical sedimentary rock and the deposition of fine grain calcite is common [
PCA analysis showed strong interrelation between potassium content, organic matter, and illite content (Figure
(a) Potassium availability versus % illite corrected content calculated from both silt and clay fractions normalized to 100%. (b) Potassium availability versus organic matter %. Fitted lines and
Identical results can be inferred for Mg2+ availability; thus, all samples apart from U exhibit the same Mg2+ and there is again a small difference between O and La samples as it is for potassium. Calcium shows no statistical difference in average values among all soil samples and that is also the case for the soils with calcaric parent material. Someone could infer that the
Figure
OM content is similar in most of the soil samples and it is slightly lower than typical Mediterranean soils (1 to 1.5%) [
Figure
PL, U, and La soils exhibit high pH values, due to the high content of basic cations like Mg2+ and Ca2+, whereas soils like Ka and Kb exhibit low pH, due to high content of silicon and depletion of basic cations. O and La soils exhibit different pH values despite their similar parent materials, due to differences within the alluvial deposits as already have been mentioned and/or the higher precipitation in Omalos Plateau. The dependence of CEC on pH is related to the presence of exchangeable sites on the edges of the silicate minerals and oxides and on the competitive adsorption of H+ in exchangeable sites [
Finally the present study shows the fragile soil development in different lithological units of Crete. To visualize the low nutrient capacity of uncultivated soils in Crete, we consider the fertilization requirements of olive trees which are a widespread cultivation in Mediterranean area [
Availability of nutrients in uncultivated soils in Crete and recommended values for fertilization in olive trees (recommended values obtained from TDC-OLIVE European project). Values in parentheses are standard deviation.
Sites | Available K+ kg/ha† | Recommended K+ kg/ha | Available Mg2+ kg/ha | Recommended Mg2+ kg/ha | Available Ca2+ kg/ha | Recommended Ca2+ kg/ha | Available P-PO4 kg/ha | Recommended P kg/ha | Available N-NO3 kg/ha | Recommended N kg/ha |
---|---|---|---|---|---|---|---|---|---|---|
O | 0.13 (0.06) | 0.17 (0.07) | 0.01 (0.01) | 0.003 (0.002) | 0.007 (0.004) | |||||
La | 0.30 (0.07) | 0.59 (0.13) | 0.07 (0.06) | 0.004 (0.002) | 0.008 (0.005) | |||||
U | 0.17 (0.14) | 300 | 4.19 (0.77) | 20–34 | 0.01 (0.01) | 360–714 | 0.001 (0.0002) | 9–16 | 0.020 (0.005) | 150 kg/ha |
Ka | 0.16 (0.12) | 0.26 (0.06) | 0.02 (0.03) | 0.002 (0.001) | 0.008 (0.001) | |||||
Kb | 0.52 (0.21) | 0.13 (0.01) | 0.03 (0.03) | 0.002 (0.001) | 0.019 (0.026) | |||||
PL | 0.24 (0.09) | 0.38 (0.05) | 0.33 (0.49) | 0.003 (0.003) | 0.045 (0.010) |
Conclusions from the results of the chemical and mineralogical analyses in undisturbed soils in Crete can be summarized to the following. Primary mineralogical phases and bulk chemical analysis of soils reflect parent material lithology; however, PCA revealed that most of the soils showed identical macronutrient availability irrespectively of the parent rock lithology. Only the case of Mg2+ shows higher availability in limited samples from ultramafic parent rock. Most of the sampling sites show that micronutrients availability has reached a common stage and is low due to low organic content. Only few samples show higher micronutrient availability related possibly to local lithological differences without significantly statistically changing the overall arguments. PCA revealed that potassium is low and it is controlled from both the secondary minerals and the organic content in soils. Cation exchange capacity is low and it is connected to smectite presence, edges of minerals, and pH and no relation is identified to organic matter, with the exception of one sample from Lasithi Plateau. Nutrients like nitrogen, phosphorous, Mn2+, Fe2+, Cu2+, and Zn2+ are not correlated with the parent material, and all soils exhibit common behaviour while the availability in those nutrients is low. Organic content in undisturbed soils in Mediterranean semiarid climatic conditions is low and it should be considered in future perspectives of soil modeling studies, soil management, and soil protection. Identically, macro- and micronutrients have been washed out in the undisturbed soils due to missing host sites like organic matter.
The authors declare that they have no competing interests.
The project was cofunded by the European Social Fund and National Resources EPEAEK II-PYTHAGORAS. The authors are thankful for the economic support.