Mulches have several effects on the soil which help to improve plant growth:
• Decreasing water loss due to evaporation
• Reducing weed growth by reducing the amount of light reaching the soil
• Preventing soil erosion
• Increasing the number of micro-organisms in the top soil
• Adding nutrients to the soil and improving soil structure
• Adding organic matter to the soil
Alternative mulching materials include black plastic sheeting or cardboard. However obviously these materials do not add nutrients to the soil or improve its structure.