Such benefits include conservation of soil water, avoiding the extremes of soil temperatures, supplying nutrients to the plants, and lastly, mulches can improve the physical condition of the soil. Mulches can aid water conservation in a garden. Covering the soil with a layer of mulch will reduce the 'waste' of water through direct evaporation from the soil surface, as the main loss of soil water occurs through solar energy evaporating water from the soil if the soil surface is wet. Evaporation occurs directly from the soil surface or indirectly as transpiration from the leaves of plants when the water lost is replaced from the soil via the roots. It has been calculated that on a hot sunny summer's day 1 m-2 of bare wet land surface will lose 4 litres of water through evaporation or 1 gallon of water per square yard.
Soils differ widely in the extent to which they are self-mulching.
These microscopic pores behave as a network of minute channels through which water can more upwards easily to replenish water lost from the surface through evaporation. This means that the amount of water lost from clay soils during the days following rain is much greater than from sandy soils. In terms of water conservation mulching provides benefits. The soil close to the surface is kept moist for a much longer period after rain. This encourages the activity of soil organisms important in recycling of nutrients.