The difference between soil and compost can be a difficult concept to grasp for beginning gardeners. Both help plants to grow. It is recommended that these two mediums be used in conjunction for the most effective plant development. While both are organic in nature, the composition of each is sometimes misunderstood. Further misdirection is found in the concept of potting soil, which is not organic in the same sense.
The University of Minnesota states that the composition of soil contains air, mineral particles, organic material, water and organisms. The bulk of the solid matter in soil is mineral particles, but half of the overall make-up is pockets of air and water. Compost is composed of mainly organic material with a mixture of "green" and "brown" matter; food waste, plant remains and manure from herbivores is labelled green matter, while woody by-products such as sawdust, paper, dried leaves and twigs are called brown matter.
Soil takes years to form from the material that composes it. The range can vary from thousands of years for rock or solid formations to hundreds of years for organic matter such as trees. Compost can be created within weeks.
Soil is used as a food storage source for plant life. Soil holds nutrients, water and oxygen until the plant life removes it. Compost helps introduce these elements into the soil when properly prepared. Soil requires amending to increase or decrease aeration and water retention. Compost helps loosen soil, adds nutrients and reduces soil erosion.
Potting soil is distinctive as it contains no minerals or humus, making it unlike natural soil and compost. Potting soil, according to the Museum of Science Art and Human Perception, is a mixture of peat moss, bark and perlite. Potting soil is steamed to sterilise it; sterilisation is needed to kill potential diseases and dangerous organisms. Unlike compost or natural soil, potting soil requires fertiliser and water to have any nutritional value to plant life.