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Mixture of digestates from biogas plant and bottom ashes from biomass combustion can be a valuable source of nutrients and organic substances and an improver of adsorption capacity and water-air properties of the soil. Pelletization of such a mixture makes its composition more stable and the handling easier. However, the properties of pelletized fertilizer are affected by the external factors. The paper presents the results of the research on the impact of air accessibility during the storage of fertilizer pellets produced on the basis of digestate from biogas plants, bottom ashes derived from the biomass combustion and mineral additives (quicklime, zeolite and bentonite) on their fertilizing properties, enzymatic activity and technical parameters such as resistance to mechanical loads. The research showed that the fertilizers stored in under the roof, exposed for 1 year to the atmospheric air, contained by 5.5% more water compared to the samples shielded from the air. These more humid fertilizers were also characterized by a slightly higher nitrogen content and 1.7-fold lower enzymatic activity of dehydrogenases and 1.6-fold higher of phosphatases, but their resistance to mechanical loads, both static and dynamic was lower. On the other hand, the effects of storage conditions on the urease activity and the content of potassium, phosphorus and heavy metals were not observed. Generally, the storage of fertilizers in airtight containers is more advantageous, because under such conditions the changes in the fertilizing properties are insignificant, while their resistance to mechanical loads is much higher
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