Top soil is the second major substance to grow vegetable plants. Sunlight is the first major factor. The soil depth required for roots on vegetable plants in raised beds or ground-level home gardens.Farmlands that have been exhausted may have no viable topsoil left. An undisturbed forest area will have a thick layer of topsoil that has built up over the years. Topsoil has two distinct layers. The visible layer will be organic materials at different levels of decomposition. Just below this layer will be completely decomposed materials called humus. When you make garden compost, you are creating a type of humus.