GENESIS, DEVELOPMENT, AND CLASSIFICATION FOR SOME SELECTED SOILS AT KURDISTAN REGION, NORTH OF IRAQ

The study area was located in the North of Iraq. Five sites were selected that have formed from the limestone parent material. One pedon dug in each site and was divided into a number of horizons. Thirty-five soil samples were collected for physical and chemical analyses. The climate of study sites were similar to the Mediterranean Sea climate which is hot dry in summer and cool humid in winter. The mean of annual precipitation, varies from one site to another. Studied soils classified as Mollisols, Inceptisols, Vertisols, and Aridisols. Study soils were relatively high clay content and its content at the surface horizons is lower than it at subsurface horizons, and soil texture was ranged between clayey to loamy, the high value for clay content indicates to soil development. Fine clay/Coarse clay ratio showed that the pathway of fine clay similar to the pathway of total clay. CEC values increased with increasing clay. Organic matter was high in the surface horizons and decrease with depth. The following pedogenic processes occurred in study soils loss, gain, leaching, illuviation, eluviation, alkalization, humification, lessivage, desalinization, calcification, decomposition,


INTRODUCTION
Pedogenic processes, particularly leaching processes, are essential to soil formation; the climate is of paramount importance in this regard, particularly rainfall and temperature (3). The stability of calcium carbonate and non-transfer or washing to the lower horizons in the soil because of the relatively low solubility, especially that calcium carbonate minerals of minerals that are concentrated in the dry areas and semi-dry as deposited metal in the case of lack of rain and increase its deposition when evaporation is higher than the amount of rain (2). Lessivage, also called argilluviation, consists of a substantial vertical transfer of fine particles ranging in size from less than 2 μm to 10 μm (9) from a superficial horizon, called the eluviated horizon or Ehorizon to another horizon called the illuviated or B horizon (25). Nevertheless, soil features are formed by pedogenesis acting on anthropogenic materials and a wide variety of (bio) geochemical processes are involved in their formation, including the bacterial breakdown of the organic material, as well as adsorption, precipitation, humification and mineralization (22). The accumulations of clays and oxides reflect the stage of pedogenic development (6). In line with this, chemical weathering and physical erosion should be coupled to the degree that mineral weathering rates depend on the availability of fresh mineral surfaces with high reactivity (17). Changes in the weathering index with depth commonly are gradual or continuous, steady and systematic for homogeneous parent rocks, reflecting continuous leaching of elements as weathering progresses on initially homogenous parent material, similar patterns have been reported in at least some instances of saprolitic profiles developed on deformed gneisses and schists (23). Illuviation of clay is considered to be an important pedogenic process in soil formation. Clay accumulation through illuviation in soils is an established fact since the presence of illuvial clay horizon known as argillic. Horizon has been recognized as a subsurface diagnostic criterion to group soils at order level in Soil Taxonomy (20). However, decomposition of organic matter and weathering of primary minerals are possible sources of exchangeable cations.
The positive correlation between clay content and soil CEC may be due to increase in clay causes increase in negative charge because clay particles regards as a source of negative charge (1), Soil organic matter is considered to be a key influence of soil quality because of its role on properties and processes in soils (13). On the other hand, Levine and Ciolkosz (15) proposed a clay accumulation index which yielded a good relationship between age and clay accumulation in (Bt) horizons. The argillic horizons are generally subsurface horizons with significantly greater percentages of phyllosilicate clay than it in surface soil horizons, the increase in clay being attributed to the illuviation process (19). The occurrence of an argillic horizon in calcareous parent material of an arid to semiarid climate could point to a palaeoprocess in stable geomorphologic conditions (10). There was no significant difference in the mean values of sand, silt and clay between rhizosphere of tree species and that of bulk soil, The surfaces and edges of mica were less affected by the weathering processes, and the edges weathering was identified with different thickness of weathered edges related to the intensity of weathering that were exposed (4). This study aimed to explain soil formation depending on the pedogenic processes, specify soil development, and classification of study soil according to the key to soil taxonomy as adopted for use by USDA. We try through this study to answer questions about how soil development depending on the particular criteria and measurement soil properties that will be used in order to determine soil evolution in the study area because there are very few studies was conducting in this field.

MATERIALS AND METHODS Study area:
The study area is located in the Kurdistan region, North of Iraq and five sites were selected that have soils formed from limestone parent material (Semeel, Fayde, Gerderash, Alton-Kopri, and Wermawa) ( Table.1 and Figure 1). The climate of study sites is similar to the Mediterranean Sea climate which is hot dry in summer and cools humid in winter. The mean of annual precipitation, varies from one site to another, in Semeel (503.53) mm, Fayde (293.83) mm, Gerderash (371.08) mm, Alton-Kopri (281.87) mm and Wermawa (723.42) mm. Table 1. GPS reading of pedons in studied sites Figure 1. Map of northern Iraq show the study sites Field work and preparation of soil samples Soil samples were collected from five sites in the Kurdistan region, northern Iraq, by digging one pedon in each site (5 pedons). Depending on soil morphological properties, each pedon was divided into a number of horizons according to soil survey staff (21), and thirtyfive soil samples were collected, ground and sieved with using (2) mm sieve, soil samples were air-dried in order to prepare it for physical and chemical analyses Laboratory analysis Particle size distribution was determined by the hydrometer method as showed by Klute (14). Bulk density was determined as described by Black (7). The total porosity of soil samples was calculated according to Danielson and Sutherland (8). Fractionation of total clay and the coarse clay was separated from fine clay by centrifugation with according to Jackson (12). Soil pH and EC measured in (1:1) soil water suspension by using pH-meter and EC-meter (18). Total carbonates were measured by the calcimeter apparatus. Soil organic matter was determined by wet combustion (24), and Cation exchange capacity measured depending on the method of Polemio and Roads method (16).

RESULTS AND DISCUSSION Physical and chemical properties
The results of the particle size distribution indicated a relatively high clay content of all the soil samples and its content at the surface horizons is lower than it at subsurface horizons for all pedons. On the other hand, the distribution of silt particles has a reverse pattern of clay content that was decreased with depth as a result of alluvial deposits while the sand content has different patterns of distribution in all studied pedons. Commonly, soil texture was ranged between clayey and loamy. Soil bulk density increased with depths except for the pedon (2) at the Fayde site, which was decreased with depth. The highest value of bulk density was (2.18 Kg m -3 ) in the deep horizon (C) of pedon (5) at Wermawa site, this may be due to increasing clay content, which acts to fill the spaces between pores, addition to increase the bonding strength between the soil particles and increase the values of bulk density. Concurrently, the high porosity values of surface horizons as a result of high decomposition of organic matter and caused the good aggregation of the soil structure in contrast to the low porosity values in subsurface horizons, resulting from mechanical migration for clay (lessivage) from upper horizons to subsurface soil horizons. The high porosity in surface soil horizons and low porosity in subsurface horizons indicate increasing soil development (Table, 2). The values of CEC ranged between (10.3-27.7 Cmolec kg -1 ) and the Cation exchangeable capacity values increased with increasing clay content and the highest value was (27.7 Cmolec kg -1 ) in pedon (5) at Wermawa site in (Bk1) horizon which has clay content (625.3 g kg -1 ), and this may be due to the lessivage process that was happened in this pedon with a high amount of clay content, and the clay particles are small size, that has a high specific surface area, therefore can be adsorbed high amount of cations on its surfaces, in turn, causes high CEC values whereas the lowest value of CEC was (10.3 Cmolec kg -1 ) found in the surface horizon (Ap) in pedon (3) at Gerderash site which has clay content (184.8 g kg-1). All studied pedons are exhibited a similar trend in pH value with increasing depth that was ranged (7.8-8.9). The surface horizons had lower pH values than the subsurface horizons because of has relatively high organic matter content. Similarly, the pH values of the studied soils were slightly alkaline as a result of increasing calcium carbonate and pH tends to rise slightly with increasing depth. Generally, studied soils were non-saline and low EC values because of were derived from limestone parent material and consider calcareous soils. The low content of soluble salts was attributed to the desalinization process as affected by rainfall and existing of calcium carbonate (Table, 3). The amount of organic matter in studied soil samples ranged between (0.68-23.1 g kg -1 ), and the results indicated that the amount of organic matter was high in the surface horizons and decrease with depth; this is due to the accumulation of a high amount of plant residuals (litter) at the upper part of the soil, and increasing organisms activity in the biosphere at roots zone which in turn caused decomposition of litter and increasing humified organic matter (humus) in the surface horizon for soil, particularly at optimum soil moisture regime and optimum soil temperature regime, hence, it can be concluded that the humification process is closely related with (A) horizon in studied soils. Total carbonates content values were ranged between (84-320 g kg -1 ), the highest value was found in the buried horizon (Ab1), in pedon (3) at Gerderash site, it may be due to the origin of parent materials, whereas the lowest value of carbonates showed at Semeel site because of high precipitation amount which led to leaching and translocation of carbonates out of soils body. Studied soils consider calcareous and have a high content of carbonate minerals as a result of the calcification pedogenic process, in turn, these soils had a high buffering capacity (Table, 3).  Table 3. Chemical properties of studied soils

Clay distribution
The results indicated to increase the clay content with increasing depth and this may be due to clay formation or lessivage process as a result of leaching. The highest value of clay (629.8 g kg -1 ) found in the underlying horizons (Ck) and (A2) in pedon (2) and (5) at Fayde and Wermawa sites respectively, and the lowest value (184.8 g kg -1 ) was found in the surface horizon (Ap) in pedon (3) at Gerderash site. The increase in clay content in underlying horizons may be due to the parent material of studied soils (Table, 2 in study sites, correspondingly clay content increased with depth in the studied pedons; this is due to leaching process as a result of the rainfall effect. On the other hand, in pedon (3) at Gerderash site the leaching of clay includes two stages, in the first stage accumulation of clay content at depth (100) cm at gravel layer, in the second stage clay translocate as a result of leaching process in buried soil and increasing accumulation of clay content in subsurface horizons of studied pedon at Gerderash site (Figure, 2).

Figure 2. Vertical distribution of clay in studied soil pedons Fine clay/Coarse clay
The ratio of (Fine clay/Coarse clay) was used as an indicator of soil development because of clay particles became finer with less weathering for soil particles. Results of (Fine clay/Coarse clay) ratio were ranged between (0.15-1.34), the highest value was noticed in the lower horizon (Ck) in pedon (4) at Alton-Kopri site (1.34), while the lower value was noticed in the upper horizon (Ap) of pedon (2) at Fayde site (0.15) ( Table, 2). Generally, the values of (Fine clay/Coarse clay) ratio of all studied pedons irregularly increased in lower horizons and have a lower value in upper horizons, these values were increased with increasing the depth that refers to the effect of climatic conditions especially precipitation which related with occurring of many soilforming processes such as weathering, lessivage, leaching, illuviation, and elluviation, these processes occur over a period of time, the period of formation and development of illuviation in soil horizon that was riches with clay as indicated by Alonso et al. (5). In studied pedons (1, 2, and 4) at Semeel, Fayde, and Alton-Kopri sites, the values of (Fine clay/Coarse clay) ratio increased with increasing depth as a result of rainfall that caused transportation, movements, and mechanical migrations for clay particles from upper horizon to the lower horizons when the leaching process occurs. On the other hand, in pedon (3) at Gerderash site, the ratio of (Fine clay/Coarse clay) ranged between (0.33-0.88) the highest value was shown in (Ck) horizon which has a high value, as a result, the movement of clay particles from upper horizons and accumulated in the lower horizon that caused by lessivage process, while in pedon (5) at Wermawa site the ratio of (Fine clay/Coarse clay) ranged between (0.16-0.49) which has the lowest value in the upper horizon that was meant occurring loss process and translocation of fine clay particles from the surface horizons to lower horizons as a result of rainfall. Increasing the ratio of (Fine clay/Coarse clay) led to the dominance of the pedogenic processes such as losses, leaching, gains and in turn soil development Increasing the ratio of (Fine clay/Coarse clay) due to increasing fine clay content as compared with coarse clay as a result of increasing weathering process that causes reducing the size of soil particles particularly of clay fraction, hence can be concluded the value of (Fine clay/Coarse clay) ratio directly proportional with pedogenic processes such as loss and gains, leaching and lessivage. The ratio of (Fine clay/Coarse clay) and its content and distribution is an important criterion for indicating the illuviation horizon, and soil development additionally considers as an indicator of some subsurface diagnostic horizons such as Argillic horizon (18). On the other hand, results of (Fine clay/Coarse clay) ratio showed that the pathway of fine clay similar to the pathway of total clay in study soils which insure the activity of some pedogenic processes and assist in the development process through the movement of clay particles from surface horizons and accumulated it in the subsurface (illuvial) horizons, in turn, these horizons were enrichment with clay content. Movement of clay increased continuously with depth in some study pedons for example in (Bt1) and (Bt2) horizons. Indeed the fine clay particles have the ability of movement more than other soil particles which gives an obvious indicator as we noticed through the displaying the results of clay motion in studied soil pedons, which helps in increasing (Fine clay/Coarse clay) ratio in illuvial horizon comparison with the elluvial horizon, these results agreed with the similar results of Gunal and Ransum (11).

Dominance of pedogenic processes and development of studied soils
Depending on the chemical and physical analyses of studied soils, the following pedogenic processes can be indicated (loss, gain, leaching, illuviation, eluviation, alkalization, humification, lessivage, desalinization, calcification, decomposition, and synthesis). These are considered the major processes for soil formation and represent important criteria in studying soil development, according to these processes, can be differentiated from different soil horizons in studied pedons because these processes occur in vigorously. The results proved that the studied pedons developed, because of the occurrence of many important pedogenic processes that were previously mentioned. On the other hand, soil-forming factors have an important role in developing studied soils, particularly climate which overlaps with other factors. The intensity of these factors specifies the activity of soilforming processes that were dominant in studied pedon. The ratio of (Fine clay/Coarse clay) was used as a quantitative criterion to specify soil development, depending on the percentage of fine clay and coarse clay and transportation it through soil pedon. The results of (Fine clay/Coarse clay) indicated the development of studied soils as a result of pedogenic processes that were happened and led to the arrangement with re-distributions of soil particles particularly, clay particles because of elluvation and illuviation processes, in B horizon. Mainly, the soil development depends on the amount of precipitation hence; climate factor (precipitation and temperature) was responsible for soil-forming (pedogenic) processes that caused the development of studied soils. Soic classification: Studied soils were classified depending on the soil properties and climate conditions at family level according to the soil taxonomy that is adopted for use by USDA-NRCS (21) as showed in the table (4).

CONCLUSIONS
Can be concluded from this investigation the study soils were calcareous and contain a high amount of carbonates, slightly alkaline, nonsaline with occurs humification process that was closely related with (A) horizon. The high value of clay content indicated to soil development and the clay content increase towards subsurface horizons of soil pedon as a result of mechanical migration through increasing precipitation. Fine clay/Coarse clay ratio is used as a good criterion to specify soil development. This study showed many important pedogenic processes that happened in soil pedons such as loss, gain, leaching,