Polymer blending as an efficient method for blending at least two~polymers is usually used to create new materials with desirable physical properties. This article focuses on the biodegradability evaluation of polypropylene (PP) fibers modified with disposable recycled poly(lactic acid) (r-PLA) plastic flakes. Biodegradable modified PP fibers containing 30% r-PLA were prepared by melt spinning process and evaluated with different methods of biodegradation assays. The soil burial test method, CO$_{2}$ evolution analysis, weight loss, mechanical properties, and average molecular weight measurements and analysis of surface morphological changes were performed for biodegradability evaluation of the modified PP fibers. Surface morphology of the blend fibers illustrated that by increasing the soil burial period cracks were formed on the surfaces of fibers. While different testing methods showed different values for the biodegradation process, the results obtained from the CO$_{2}$ evolution analysis, weight loss, and variation of mechanical properties confirmed a good agreement between different degradation methods. After incubation in soil for a long period of time, the initial moduli and tenacity of the modified fibers decreased up to 72% and 53%, respectively. In addition, the average molecular weight measurement showed a 28% decrease in average molecular weight after 80 days of soil burial.