Biochar as an eco-friendly substance, depending on its origin and production conditions, may have significant effects on soil properties, nutrient availability and K dynamics of arid and semi-arid soils. Biochars produced from wheat and corn residues, pyrolyzed at 250, 450 and 650 °C during 2, 4 and 8 h, were applied to four representative calcareous soils. First, 2 g of each biochar was added to 100 g of each soil and incubated under field capacity for 90 days. Then the soil samples were analyzed for soil properties, nutrient availability and K fractions. All biochars increased soil CEC, ECe and available K, P, Fe, Zn, Mn and Cu. The biochars produced at higher temperature had larger effect on soil properties, P and K availability, and smaller effect on Fe, Mn and Zn availability. The used biochars increased the amounts of soluble, exchangeable, non-exchangeable and HNO₃-extractable K to 143–348, 316–605, 597–1,064 and 1,056–2,012 mg kg⁻¹, respectively. The wheat biochar had larger effect on soil K than the corn biochar. Potassium saturation percentage increased with biochar application due to the larger effect of biochar on exchangeable K than CEC. The biochars increased the percentage of soluble K due to the limited capacity of exchange sites and clay interlayers for K adsorption. Biochar had different effect on the experimental soils; and the content of soluble K in the coarse-textured soils after biochar application increased more, than other K fractions. It was concluded that biochars, particularly the wheat biochar produced at higher temperature, have potential for improving K management of K depleted soils.

Different K pools in 197 surface and subsurface soil samples of southern Iran were determined to accurately assess factors affecting K pools distribution. Mineralogical analysis indicated that smectite, mica, chlorite, and palygorskite were the major minerals in the clay fractions. We obtained significant relationships between exchangeable, non-exchangeable, mineral, and total K with mica content, CEC, calcium carbonate equivalent, and clay content. Although many researchers have stated that mineralogical composition determines K status of soils, our studies indicated that some characteristics such as soil moisture and temperature regimes, soil orders, depth, agricultural activity, leaching, and some soil properties like particle size distribution, soil salinity, calcium carbonate equivalent, and cation exchange capacity could also affect K equilibration in the soils. Generally, Histosols and coarse-textured Entisols had the lowest contents of different K pools, and Alfisols and Vertisols had the highest values. On the other hand, soils with xeric moisture and mesic temperature regimes had the highest contents of different K pools. Further research is required to study the relationship between these findings with plant response and uptake of K.

The release of non-exchangeable potassium from 16 calcareous soils collected from southern Iran was examined. For this study, a method was used involving a total of 14 successive 15-min extractions with 0.01 M CaCl₂ as an extractant. Cumulative potassium released from soils was in the range of 144–509 mg kg⁻¹ for surface and 115–445 mg kg⁻¹ for subsurface soils. The studied soils were categorized into two groups based on the cumulative K released, clay mineralogy and soil taxonomy. (I): Smectitic soils including Vertisols, Vertic subgroups and some Alfisols and Inceptisols; (II) other soils including Mollisols, Entisols, and other subgroups of Inceptisols and Alfisols. Cumulative K release was significantly (p < 0.01) higher in Group I than in group II. The release kinetics of non-exchangeable K showed the best fit to Elovich, parabolic and power function kinetic models. The Elovich and parabolic release rate constants were significantly (p < 0.05) higher in Group I than in Group II. Greater K release rate constants in Group I soils could be due to the heavy soil texture and domination of expansible clay minerals.