Potassium, K, element is volatile, and its precipitation is not the same as refractory U and Th elements in the Earth. A measurement of the K elements in the Earth is of interest to understand the chemical evolution of the Earth. Furthermore, the discussion on the Ar40-K40 system of the air, and the crust and upper mantle is one of the supports for the depleted and enriched two-layer mantle structure. The detection of K40 neutrinos may lead to new knowledge of the Earth. Previously only U and Th geoneutrinos can be detected with the inverse beta process with a 1.8 MeV threshold. K40 geoneutrinos are hard to discover for its low energy and high solar neutrino background. In this work we found that Liquid scintillator Cherenkov neutrino detectors can be used to detect the K40 geoneutrinos. Liquid scintillator Cherenkov detectors feature both energy and direction measurement for charge particles. With the elastic scattering process of neutrinos and electrons, K40 geoneutrinos can be detected without any intrinsic physical threshold. With the directionality, the dominant intrinsic background originated from solar neutrinos in common liquid scintillator detectors can be suppressed. With the studies of MeV electrons Geant4 simulation, quantum and detection efficiency, and Cherenkov direction reconstruction algorithm, it is found that we can detected K40 energy geoneutrinos with 3 standard deviations with a kilo-ton scale detector. We are on the cutting edge to reveal this geoneutrino component in the near future. The result and relevant features will be reported.