Most of Khan's claims about having an endless supply of Potassium in the soil seem to be based primarily on the context of Mollisols in the midwestern USA. Has he looked into sandy soils with subsoil aluminum saturation toxicity in the equatorial humid tropics? I wonder if his statments apply to extremely leached soils in tropical areas. Is it possible to have soil profiles and parent materials that are absent K or do not contain sufficient quantities of K? Is ICP-OES an accurate method for testing total (elemental) K content of a soil sample? I tested a soil sample with this method and it showed 90mg/kg K, which sounds extremely low to me considering that method is supposed to take into account any and all forms of K present: exchangeable, fixed, mineral. At least that is my understanding of ICP-OES. Perhaps there is a limitation of ICP-OES when it comes to testing K?

'promosm' 🤭