Palm kernel shell (PKS) was incorporated into natural rubber composites at loadings ranging from 0 to 20 phr using a laboratory-size two-roll mill. A soil burial study of the PKS-filled NR composites as a function of filler loading and silane coupling agent incorporation was then conducted over a period of six months. The degraded composites were evaluated by means of tensile testing. To study the degradation of the composites after soil burial, their morphological properties were observed by scanning electron microscopy. Prior to the soil burial tests, as the PKS filler loading increased, the tensile strength and elongation at break of the composites both with and without the silane coupling agent decreased, while the tensile moduli (M100 and M300) increased. At identical filler loading values, the NR/PKS composites with the incorporated silane coupling agent exhibited better tensile properties than the NR/PKS composites without the silane. Following the soil burial tests, the tensile strength, elongation at break, and the tensile moduli (M100 and M300) all decreased because of microbial attacks on the composites. The addition of silane enhanced the retention of the tensile properties of the NR/PKS composites after soil burial because of the improved adhesion at the filler-rubber matrix interface.