Slagging in combustion facilities are not welcomed, as it may cause technical and operational problems as well as extra costs. Increased understanding of the critical slagging sub-processes makes it easier to suggest semi-empirical models and fuel indexes for prediction of slagging tendency of different fuels. That could open the biomass market for potentially more troublesome raw materials. The objective of this work was to determine critical ash forming processes and compositions in fixed bed combustion of phosphorus-poor biomass fuels. This was achieved by performing a systematic review of data and experience gathered from combustion experiments in a small grate burner of 36 different biomasses, chemical analysis of their bottom ashes and slags. The paper presents a discussion of the slagging tendency in phosphorus-poor biomass by combining three different slagging classifications ending up with a proposed starting point for a new slagging index. The slag (ash particles > 3.15 mm) formed during the combustion experiments has been described according to fraction of fuel ash that forms slag (wt-%), visual sintering category (1-4) and viscosity predictions. The results explain that both the fraction of melt and its viscosity is critical for the slag formation process in phosphorus-poor biomasses. Additionally, fuels with low Si/K ratio along with higher Ca concentration may form a low viscous carbonate melt not prone to form slag. Increased Si and lowered Ca concentration will increase the amount of formed silicate melt formed as well as its viscosity, thus resulting in a more sticky melt.