Types of soil
Soil covers the Earth's surface and is made up of different substances, such as minerals, water and gas. It can be studied according to aspects such as color, porosity, permeability and texture. Each characteristic has an influence on what that soil will be used for.
The color of the soil depends on the material of the area and the content of organic matter (live and inanimate carbon-based elements). For example, the higher the amount of organic matter, the darker the soil will be. The color indicates whether it is fertile or not. Reddish or yellowish tones are associated with iron oxide, and for this reason, such soil may represent good land for planting. Such is the case of what is called "terra roxa" (purple land) in Portuguese, which comes from the Italian word "rosso", which means "red". In Brazil, this type of soil is found mainly in the states of São Paulo, Minas Gerais and Mato Grosso do Sul.
Porosity refers to the portion of space occupied by liquids and gases in relation to the soil's mass, that is, it relates to the "holes". This characteristic results in lesser or greater amounts of water circulating through the soil. If there are more pores, there is greater penetration of water and it reaches deeper levels, decreasing the soil's humidity.
The soil's permeability is directly related to the porosity since it concerns the water's circulating condition through the soil.
Other factors that influence a soil's permeability are the size and proportion of its composing particles, that is, its texture. From the smallest to the largest particle diameter, the soil can be clayey, silty (made of rock fragments or particles arising from the destruction of other rocks), sandy or of gravel (calcareous).
Clayey soil is the least permeable, so it holds more water. In addition, it has great amounts of aluminum and iron oxide. Purple land ("terra roxa"), which favors agriculture, and massape, found in the northeast and closely related to sugar cane plantations, are examples of clayey soils.
Silt soil has small, light particles and can usually suffer erosion (land wearing or sliding due to water, wind, transportation or other agents). For this reason, it is not used for agriculture. Dirt roads that produce a lot of dust in dry periods have this type of soil.
Sandy soil, very common in the northeast, has good porosity and is very permeable. It is drier due to the penetration of water to the deepest layers. Plants and microorganisms have greater difficulty to grow under these conditions.
Finally, gravel or calcareous soil is made up of rock particles, and is not appropriate for agriculture. However, the white or yellowish dust that can be retrieved from it is used in agriculture to change the acidity of soils to be used for planting. Quite common in desert areas, it also provides raw material for lime and cement manufacturing.