Thermal admittance and diffusivity

I'm having some trouble interpreting the graphs for thermal diffusivity and admittance vs water content. For thermal admittance, I'm confused as to why it increases exponentially for the mineral layer vs linearly for the organic layer. And for thermal diffusivity, I don't understand why it barely changes with water content for the organic soil, but increases exponentially before levelling off in the mineral soil?

Dexter, when interpreting thermal diffusivity or thermal admittance, go back to the components. 1) Thermal conductivity - how it changes with water content and mineral versus organic materials; and 2) soil heat capacity. If you understand the components you can follow the relationships for thermal admittance(which tells us about changes in surface temperature) and thermal diffusivity (which relates to subsurface temperature response).