Simplify:

If all of the volcanoes on Southeast Asian islands go dormant forever, the soil in these regions may lack the nutrients and minerals that are typically replenished through volcanic activity, leading to a decrease in soil fertility. Additionally, without the regular deposition of volcanic ash on the soil, there may be an increase in erosion and a loss of topsoil, further decreasing the overall health and productivity of the soil in these areas.

If Southeast Asian island volcanoes become dormant, the soil may become nutrient-poor and less fertile. Without volcanic activity, erosion could increase, leading to a loss of topsoil and reducing soil health and productivity.