The mineralogy of Vertisols and Mollisols, associated in a hydrotoposequence in the western part of the phosphate plateau of Morocco (High Chaouia), was studied using x‐ray diffraction, interlayer swelling with alkylammonium cations, chemical analysis, and infrared spectroscopy. The swelling behavior of Li‐ and K‐saturated clays demonstrated the beidellitic character of the expandable 2:1 layer silicates that dominate the clay mineralogy of these soils. The beidellite has a heterogeneous layer charge, as measured by the swelling with alkylammonium cations, that ranged from 0.50 to 0.66 charges per half unit cell. The swelling of Li‐saturated clays with alkylammonium cations after heating at 300 °C confirmed the tetrahedral location of the charge. Structural formulas calculated for the fine clays showed that the beidellite is similar in all three soils and confirmed that these clays have a high charge density and are Fe rich. Infrared spectroscopy demonstrated that these beidellites contain substantial amounts of Fe3+ in the octahedral sheet. Kaolinite, chlorite, palygorskite, and quartz were also present, especially in the coarse‐clay fractions and in the subsoil. The presence of this Fe‐rich, high‐charge beidellite in the soils helps explain the behavior of soil K and also offers an explanation of K soil test interpretation problems.

Forty‐four soil clays and four reference phyllosilicate clays were used to establish qualitative and quantitative relationships between clay mineral characteristics and K fixation. The soil clays were obtained from Vertisols, Vertic Mollisols, Haploxerolls, Xerochrepts, and a Palexeralf, from three semiarid regions of northwestern Morocco. The soil clays contained beidellite, montmorillonite, vermiculite, and illite in widely varied abundances. Both total and tetrahedral cation‐exchange capacity (CEC) were measured. Octahedral CEC was calculated by difference. Tetrahedral CEC was determined after saturation with Li and heating at 300 °C to reduce octahedral charge to near zero. Potassium fixation was determined in both wet (1:4 soil/solution, 16 h on a shaker) and dry conditions (after drying for 24 h at 70 °C) using three different levels of added K. Total CEC was well correlated with tetrahedral CEC (r2 = 0.76, P < 0.001). About half of the smectitic and vermiculitic soil clays have >50% of their total charge due to tetrahedral charge. The amount of K fixed Increased with added K, whereas the percent K fixed relative to total added K decreased. The K fixation by the clays studied was best correlated with total CEC for a given amount of K added. It was also well correlated with tetrahedral CEC. Simple regression analysis did not show any significant effect of octahedral CEC. A multiple regression analysis, however, demonstrated that both components of total CEC, tetrahedral and octahedral, have a simultaneous effect on K fixation. Tetrahedral CEC explained more of the variation (64%) than octahedral CEC (36%). This greater effect was attributed mainly to the proximity of tetrahedral charge to the interlayer space of 2:1 phyllosilicate clays.

The 2:1 layer silicate mineralogy of a Vertic Haplaquoll of north‐western Minnesota was studied using x‐ray diffraction and determination of cation exchange capacity (CEC). The x‐ray diffraction behavior of Li‐ and K‐saturated samples demonstrated that a highcharge smectite with tetrahedral charge dominates the mineralogy of both the coarse and the fine clay fractions. The high CEC and the large difference between Li‐CEC and K‐CEC confirmed the x‐ray results. Alkylammonium‐clay complex basal spacings show two phases of 2:1 clays; a high charge (about 0.9 molc per ½ unit cell formula) phase concentrated in the coarse clay and an Fe‐rich beidellite with a charge of 0.4 to 0.5 molc per ½ unit cell formula. The high charge phase largely represents K+ ‐depleted mica formed during the analytical procedure. This phase may also contain some soil vermiculite. The results presented in this study suggest that beidellite is a product of mica weathering, and that during weathering soil mica is transformed first to a vermiculite then to a beidellite, preserving the tetrahedral character of the charge.

Intensive cropping of Italian ryegrass (Lolium multiforum L.) in pots was used to assess the contribution of non-exchangeable K to plant uptake. The soils used were: two soils high in mica (illite) developed on recent alluvium plus two smectitic (beidellitic) soils and a soil of mixed mineralogy rich in mica. Four K treatments were used (0, 28.6, 143, and 286mgkg -~ soil) with 8 successive monthly cuttings. A response of plant K uptake to added K was observed in all soils. Both 1.0 M NH40Ac and 0.2 M CaCI 2 extractable K were depleted to a minimum level specific for each soil. The minima were lower in the old upland soils compared to the young alluvial soils. Uptake of K by Italian ryegrass induced K release from the non-exchangeable K to replenish the plant available pool of K ions. The release of mica interlayer K in the alluvial and in the high K smectitic soil supplied sufficient K to plants even under intensive cropping. The rate of mobilization of interlayer K was low in the smectitic soil with lower K. The lowest release rate was in the old high mica soil. Iron coatings may have inhibited mobilization of interlayer K. The rates of mobilization cannot be predicted from mineralogical and K-extraction data only. The rates of K uptake and the rates of K release by ryegrass under intensive cropping are potential values which can be used for modelling K availability to plants in the soils studied.

The purpose of this study was to investigate the impact of redox-induced changes in the organization of the clay fraction of a bulk vertisol using transmission electron microscopy. Chemical and X-ray powder diffraction (XRD) analyses indicated that the oxidized clay was composed of 32% kaolinite and 68% non-pure smectitic material, mostly a dioctahedral beidellite with octahedral Fe, according to Quantarg2 and DecompXR models.The cation exchange capacity of the soil increased from 26.1 to 65 cmolc+ kg-1 due to structural iron (FeStr) reduction and dissolution of oxide coatings. Transmission electron micrographs revealed dramatic changes upon reduction. Oxides were dissolved and the smectite increased in particle darkness, lateral extension, thickness, compactness and stacking order. These changes were interpreted to be a consequence of sorption of ferrous Fe and reduction of FeStr, as found in previous studies on pure Fe-bearing smectites.