The word soil has come to mean something of more value than dirt. Both words are used to describe what plants grow in. For scientists, the medium in which plants are found in nature is never dirt; it is always soil. An amateur gardener may think she needs better dirt, but a professional seeks to improve the soil. A soil science professor might say, “Soil is what plants grow in. Dirt is what you get beneath your finger nails.”
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The defining feature of soil is that it is what is found in its place. Soil develops over thousands, if not millions, of years. In its place the material is called soil. Soil is a mixture of organic and mineral materials. A certain soil has developed individual features specific to it over time because of the organisms, climate and topography that it has come in contact with.
Dirt is essentially displaced soil. It is any material that comes from the ground and is finely grained. Dirt is what gets on you from contact with the soil. A bag of potting soil could be called dirt. Dirt often is sterilized and artificial. Dirt can support plants and can become soil.
Five Soil-Forming Factors
There are five soil-forming factors: parent material, climate, topography, organisms and time. The first four all depend on the fifth for their ability to structure the way the soil looks. The type of climate, temperature and moisture will determine how chemical weathering will change the mineral content of the soil. The type of material on which the soil forms will determine many of its qualities. The kinds of plants and animals that are able to survive on the soil will determine how much and what kinds of organic materials are added to the soil. A steep mountain will develop very different soil layers than a flat plain. Dirt may have been subjected to these factors before it was displaced.
A soil's texture is determined by the percentage of sand, silt and clay in it. A soil needs to have a well-balanced texture with roughly equal percentages of sand silt and clay to hold onto nutrients. In nature, the texture of the soil depends on its parent material and other soil forming factors. Dirt can be designed to have whatever texture is required by mixing different-size particles together.