Kinetics of non-exchangeable potassium release in selected soil orders of southern Iran

Non-exchangeable K release can greatly influence soil K fertility. This study was carried out to study the release of non-exchangeable K from 22 surface and subsurface soils of southern Iran using successive extraction with 10 mmol/l of CaCl₂ and oxalic acid in a period of 2−1368 h at 25 ± 1°C. Alfisols, Aridisols, Entisols, Inceptisols, Mollisols, and Histosols were among the studied soil orders. Illite, smectite, chlorite, vermiculite, kaolinite and palygorskite clay minerals were identified. The amount of non-exchangeable K varied from 95 to 506 mg/kg. Results showed that CaCl₂ and oxalic acid released 60 and 55% of non-exchangeable K from soils, respectively. The discontinuity in slope was found when the cumulative amount of released K was plotted versus time, supporting the multi-reactive nature of K exchange sites. The cumulative K release in soils ranged from 87 to 300 mg/kg for CaCl₂ and 78 to 300 mg/kg for oxalic acid which was well described by simplified Elovich and power function equations. The b constant of simplified Elovich as an index of non-exchangeable K release rate was in the range of 10 to 36 mg/kg for CaCl₂ and 11 to 36 mg/kg for oxalic acid. The highest b constant was measured in Inceptisols and Alfisols, while the lowest values were found in Entisols and Histosols. In conclusion, the K release pattern was similar for both extractants and affected by clay content and type, and soil depth.

Effects of zeolite and vermicompost applications on potassium release from calcareous soils

Zeolite and vermicompost applications to soil may improve K fertility of soils. For this purpose, these materials were added to five representative calcareous soils collected from southern Iran. The treatments included (1) control, (2) 20 g/kg zeolite, (3) 20 g/kg vermicompost, and (4) 20 g/kg vermicompost + zeolite (1:1). The samples were incubated for 90 days, and the concentrations of soluble, exchangeable, and non-exchangeable K and K release rate to 0.01M CaCl₂ during 240 min (16 successive 15 min extractions of soil samples with CaCl₂ solution) were determined. Results indicated that zeolite application increased significantly the concentration of soluble and exchangeable K. Vermicompost application had a positive effect on all forms of K. Cumulative K release was also affected by vermicompost application. Comparison of experimental data to different kinetic models indicated that Elovich, power function, and parabolic diffusion models described well kinetics of K release from all soil samples to 0.01M CaCl₂. The b values of Elovich equation had significant relationships with NH₄OAc- and HNO₃-extractable K. It is recommended that for K fertility management of calcareous soils, organic and inorganic amendments application to soils should be taken into consideration.

Factors affecting potassium release in calcareous soils of southern Iran

Experiments were conducted on 48 representative surface and subsurface soils collected from different climatic and physiographic conditions in southern Iran to assess factors affecting potassium (K) release and to find the best fitting models to describe the release kinetics of K to 0.01 m CaCl2. Mineralogical analysis showed that smectite, mica, chlorite, and palygorskite were the main clay minerals in the studied soils, whereas quartz, mica, and feldspars were abundant in the silt and sand fractions. Results indicated that cumulative K released ranged from 26.7 to 260 mg/kg (mean 176 mg/kg) for surface soils and from 37.9 to 198 mg/kg (mean 130 mg/kg) for subsurface soils. The predominant source of K in the soils appeared to be mica, because there was a significant relationship between cumulative K released and mica content. The percentage of non-exchangeable K release to CaCl2 during 1496 h was also correlated with the ratio of mica in the coarse fractions to total mica in the whole soil. Results showed that the Elovich equation adequately described the reaction rates of K release from surface and subsurface soils and suggested a heterogeneous diffusion process. We concluded that mica content and its particle size, soil depth, and some soil properties are the main factors controlling K release rate. On the other hand, we found no significant relationships between K release rate and climatic conditions, or physiographic positions.

113 ESTIMATING SOIL. POTASSIUM SUPPLY TO MEET PROCESSING TOMATO REQUIREMENTS

A 1993 survey of 50 commercial processing tomato fields in California revealed widespread potassium deficiency, as determined by tissue K levels below existing sufficiency standards and the occurrence of vine necrosis consistent with K deficiency. Soils from these fields were analyzed for exchangeable K by ammonium acetate extraction, and for K release rate by a 7 day incubation procedure (1:10 soil:. 01 M CaCl2 at 25°). Soil K release rate was more highly correlated with tissue K at midseason than was exchangeable K. These soils were further examined for K fixation capacity. Three g soil was blended with 3 ml 10 meq K as KNO3, allowed to dry, incubated for 7 days in a 1:10 soil: H2O solution, then extracted in 1 N NH4Cl; added K not recovered was considered fixed. Percent K fixation ranged from 0 to 82%. These data suggest that the inconsistent response of processing tomato to K application in numerous California trials may be related to a) the reliance on extractable K analysis to characterize soil K supply and b) no consideration of soil K fixation capacity in determining K application timing and method.