Clay Mineralogy Relationships with Potassium Forms in Different Physiographic Units

Potassium is an essential element for plant growth and its importance in agriculture is well known. Total soil potassium reserves are generally large; although the distribution of K forms differs from soil to soil as a function of the dominant soil clay minerals present. The objectives of the present study were: (i) the content, forms, and distribution of K as a function of clay mineralogy in different physiographic units; (ii) to investigate the relationship between K fractions and their physiographic units.
The study area is located in south of eastern Azarbaijan province, north of Iran. The region stands between 37° 45' and 38° 00' N latitudes and 46° 00' and 46° 15' E longitudes. The present climate of the region has dry and hot summers, cold and wet winters; with an average rainfall of 334 mm. soil moisture and temperature regimes of study area are xeric and mesic, respectively. Site selection for studied pedons was based on a reconnaissance survey from different physiographic units, namely, pedons 1, 2 and 3 located on Piedmont Plain (PP), pedons 4, 5 and 6 on Rivera Alluvial Plain (RAP) and pedons 7, 8 and 9 on Low Lands (LL). Soils were described and classified based on soil survey manual and keys to soil taxonomy. Samples were air dried, crushed and passed through a 2mm sieve. Particle size distribution, organic matter, Calcium Carbonate Equivalent (CCE), pH, Electrical Conductivity (EC) and Cation Exchange Capacity (CEC) and different forms of K (total, soluble, exchangeable and nonexchangeable) were determined. X-ray diffractograms were obtained through a Shimadzu XRD 6000 defractometer employing a Ni-filtered CuKa radiation source from oriented clay (40 kV, 30 mA). The content of clay minerals was estimated according to Biscaye method.
All soil samples were calcareous, calcium carbonate content was between 10 and 17%, with relatively high clay content, ranging from 18 to 36%. Soil organic carbon contents were between 0.3 to 1.9%. Land use affected the amount of organic matter so the pedons located on piedmont plain showed more organic matter than other units. Variation in soil characteristics were considerable, which was most affected by physiography. The variation was also especially noticeable in clay minerals and K pools. Soluble K, ranging from 6 to 14 mg/kg, had higher content in piedmont plain than other units. Nonexchangeable k was between 345 to 545 mg/kg and piedmont plain had higher amount because of existing more illite. Exchangeable K, ranging between 278-416 mg/kg had highest content in piedmont plain and lowest content in river alluvial plain. The release rate of non-exchangeable K is the result of the type and particle size of K bearing minerals and soil conditions. The XRD data for the less than 2 μm fractions of the studied soils indicated that the soils were similar in their clay mineralogy, mainly consisted of illite, smectite, kaolinite and chlorite, but were different in content. Illite and smectite were higher in Piedmont Plain (PP) and Low Lands (LL) had higher content of smectite in comparison with River Alluvial Plain (RAP) due to low drainage condition. Several factors such as differences in geomorphological conditions and clay mineralogy between physiographic units caused the differences in K forms. The statistically significant relationship between clay content and most forms of k was because of high specific surface of clays. A highly significant positive relationship between non exchangeable K and illite content (r2 = 0.81, P