We evaluated the influence of moisture content and water activity in poultry or swine compost on the caking of bulk blending fertilizers. Caking tendency was assessed when compost was mixed with primary bulk blending raw materials—namely ammonium sulfate, diammonium phosphate, and potassium chloride—stored for 4 weeks at 30°C and 85％ relative humidity. The caking tendency correlated more strongly with water activity than with moisture content. Conversely, anticaking effect is indicated with lower water activity, as the compost absorbed moisture within the polyethylene bags in a manner similar to silica gel. In composts with high moisture content, water activity was highly correlated with moisture content. Generally, higher moisture content results in higher water activity, which aligns with our observations. In contrast, under lower moisture content conditions, the water activity of compost correlated with ADF_om. Reportedly, lower humidity results in reduced availability of free water in fibrous components such as cellulose or lignin. Consequently, composts with high fiber content exhibited lower water activity. Therefore, composts with low moisture content may inhibit the caking of bulk blending fertilizers, and among these, those with higher ADFom show lower water activity.