Soil erosion is basically the soil being deteriorated by the gradual impact of water or wind to it, being a very mechanical situation, where water or wind will hit the soil, detaching some particles and making the soil weaker. The main cause will be hits from water or wind. Deforestation will have a huge impact on erosion, since the soil where trees are taken down is more likely to receive impacts from wind or water, if the trees were left in their place, they would be used as a "shield" for the soil