The present research was conducted to determine iron, manganese, zinc, and copper availability in 59 calcareous soils of southern Iran; to find relationships between micronutrient availability and some major soil properties; and to predict the micronutrient availability of soils from physiographic position, soil development, and climatic condition. Experiments were done by equilibration of 10 g soil sample with 20 ml DTPA extractant for 2 h. Results indicated that DTPA-extractable Fe, Mn, Zn, and Cu in the soils ranged 2.2–90.0, 0.6–15.4, 0.2–12.2, and 0.6–5.4 mg kg⁻¹, respectively. Generally, Zn deficiency was more widespread, followed by Fe deficiency, but Cu and Mn contents in nearly all soils were sufficient. Micronutrient availability was significantly correlated with some soil properties including pH, organic matter (OM), clay, and cation exchange capacity (CEC). Soil development, physiographic condition, and soil moisture and temperature regimes had significant effects on the micronutrient distribution and availability. The highest amount of micronutrients (except Mn) was found in Histosols, lowland soils, and soils with aquic moisture and mesic temperature regimes. On the other hand, the lowest value was measured for Aridisols and soils with aridic and ustic moisture and hyperthermic temperature regimes. It is concluded that these factors may affect soil properties such as clay, pH, CEC, OM, and calcium carbonate equivalent and thereby micronutrient cycling and distribution in the soils.

Potassium fixation capacity and mineralogical analysis of 24 representative soils, collected from southern Iran, were studied. Potassium fixation analysis was performed by adding six rates of K from 0 to 1000 mg kg⁻¹ soil in a plastic beaker and shaking for 24 h. Mineralogical analysis showed that the clay fractions were dominated by smectite, chlorite, mica, palygorskite, vermiculite and quartz. In general, the studied soils fixed 8.5–55% of the added K. The potassium fixation capacity of the studied soils was significantly correlated with smectite content (r ² = 0.87), clay content (r ² = 0.60), cation-exchange capacity (r ² = 0.79) and NH₄OAc-K. Wetting and drying treatment and incubation time had significant effects on K fixation. The average percentage increase in K fixation following the wetting and drying treatment was 24 and 30% for surface and subsurface soils, respectively. The average percentage increase in K fixation with increasing residence time was 79 and 56% for surface and subsurface soils, respectively. Because K fixation is a diffusion process, time and increased concentration of soluble K (because of soil drying) are factors affecting the rate of K diffusion from a soil solution to the interlayer positions of the expansible 2:1 clay minerals.

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.

Biochar due to its high K content may affect K forms and dynamics. The effect of biochars produced from licorice root pulp, municipal waste and cow manure at 300 °C and 600 °C on K forms, release and fixation in a loamy calcareous soil was investigated. The soil samples were analyzed after biochar application and incubation at field water-holding capacity or saturation conditions. The cow manure and municipal waste biochars increased all K forms, while licorice root pulp biochar only increased exchangeable K (10%). Cumulative K released to 0.01 M CaCl₂ for the control soil was 101 mg kg⁻¹, increasing to 108, 182 and 418 mg kg⁻¹, with the application of licorice root pulp, municipal waste and cow manure biochars produced at 300 °C, and increased to 112, 200 and 486 mg kg⁻¹ with the application of the biochars produced at 600 °C, respectively. Generally, 0.025 M CaCl₂ extracted more K than 0.01 M CaCl₂ (means of 250 vs. 230 mg kg⁻¹). Saturation had a smaller effect on K forms and dynamics; increasing non-exchangeable K. Biochar increased soil K fixation. Biochar depending on its origin and pyrolysis conditions may be used as an amendment for increasing K supplying power of calcareous soils.