What natural substance can increase the load and simultaneously decrease the resisting force, and thus has considerable influence on landslides?

Water (Mixed with Rock and soil)

Explanation:

Although the risk of facing a landslide is dependant on the characteristics bedrock and the slope gradient, Hydrology of the soil plays a major role in prediction of such an event. Specially Deep-Seated landslides, They mostly occur in season of heavy rains. Transpiration through root system is one factor that plays a significant role in reducing the risk of landslide as the water content in the soil in reduced.

Water

Explanation:

Water natural chemical substance that exists in cloud and falls as rain to form streams, rivers, lakes and seas. This chemical substance can be formed from the combustion of hydrogen in excess oxygen. It's flow ability increase load and reduces resistivity therefore it can influence land slide.