The most important function of a mulch is to act as an insulator - this reduces the evaporation of water from the soil, 
(this; reduces; the evaporation of water; from the soil) Context(this reduces,List([67, 79))):(The most important function of a mulch; is; to act as an insulator)
this reduces,List([67, 79))
this The most important function of a mulch
reduces is
the evaporation of water from the soil
to act as an insulator