Mobilization of non-exchangeable K by ryegrass in five Moroccan soils with and without mica

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 286 mg kg^-1 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 CaCl₂ 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.