Sewage sludge post-treatment by means of an incineration is an efficient way to remove all pathogens and organic contaminants. Moreover, sewage sludge calorific value, similar to a low-grade brown coal one, enables to consider sewage sludge incineration as a waste-to-energy process. To assure that sewage sludge incineration post-treatment is an effective way for sewage sludge phosphorus utilization, the understanding of incineration residues agricultural behavior is necessary. To achieve such understanding, an employment of relevant techniques is crucial. Therefore, we questioned the agricultural behavior of sewage sludge incineration products. In our study, we questioned also the relevancy and the corresponding value of experimental tests, which are commonly used for predicting\nsewage sludge incineration products phosphorus agricultural utilization efficiency. It was found that sewage sludge incineration residues might need another post-treatment, which would increase bioavailability of present phosphorus, prior to their agricultural application. It was also found, that currently used methods evaluating phosphorus bioavailability in sewage sludge based recycled sources do not provide sufficient information about the agricultural behavior of these materials.