To get a head start on the growing season, try organic seed-starting mix. Horticultural-grade sphagnum peat moss helps to improve garden soil because it holds moisture well. Vermiculite also helps soil retain moisture and it's a great additive for heavy clay or sandy soil, making it more workable. For potted plants in the garden, you need organic potting soil that includes peat moss, compost, vermiculite, perlite and time-release fertilizer. In warm climates, cacti thrive in the right soil. Cactus potting soil includes the aforementioned potting soil components as well as washed sand.