With the depletion of fossil fuel, the exploitation and utilization of biomass have attracted attention widely. Unfortunately, high concentrations of alkali metals, especially K, in biomass result in serious slagging. Therefore, considering the co-existence of unnoticed and inevitable molten slagging as well as the highlighted alkali-induced slagging, the synthetic effects of kaolin and soil on both slagging are conducted by means of chemical fractionation and ash fusion testing. Addition of kaolin and soil into biomass can convert soluble-K into insoluble-K effectively, and thus reduce alkali-induced slagging. Moreover, soil, attributed to low-cost and wide range of sources, can be slagging inhibitor instead of kaolin. In addition, K2O-SiO2-Al2O3 ternary diagrams constructed by addition of K2O, SiO2 and Al2O3 into biomass underestimate IDT and over-estimate FT, and they should be constructed according to the biomass ash properties, rather than the simulated ash by adding various compounds into biomass.