For lower attenuation it would better to go with 433 MHz, even better to 169 MHz that is also an ISM band. The big problem with compost, if I remember well how is processed, it is its high water content. There are several studies that can be found about "soil" and wireless sensors networks, search for WUSN (wirelss underground sensor network) and UG2AG (underground 2 aboveground). Usually are studies made for agriculture, forestry, soil, pipelines, ... sensors deployment.