Soil. Trust me I know because we had to do some wierd experiment. I did it for 14 days because thats how long it took for the sand plants to sprout while the soil plants took like three days, and they were the same type of plants. The reason why is because soil has special nutrients that help the plant to grow. If you are growing a plant you don't have to use soil, but to get the fastest and best results I recomend it!