black are commonly used to describe racial groups, brown is rarely used, because it crosses all racial lines. In Brazil, the Portuguese word pardo, which can mean different shades of brown, is used to refer to multiracial people. The Brazilian Institute of Geography and Statistics (IBGE) asks people to identify themselves as branco (white), pardo (brown), negro (black), or amarelo (yellow). In 2008 43.8 percent of the population identified themselves as pardo. (See human skin color). Soil The thin top layer of the Earth's crust on land is largely made up of soil colored different shades of brown. Good soil is composed of about forty-five percent minerals, twenty-five percent water, twenty-five percent air, and five percent organic material, living and dead. Half the color of soil comes from minerals it contains; soils containing iron turn yellowish or reddish as the iron oxidizes. Manganese, nitrogen and sulfur turn brownish or blackish as they decay naturally. Rich and fertile soils tend to be darker in color; the deeper brown color of fertile soil comes from the decomposing of the organic matter. Dead leaves and roots become black or brown as they decay. Poorer soils are usually paler brown in color, and contain less water or organic matter. Mollisols are the soil type found under grassland in the Great Plains of America, the Pampas in Argentina and the Russian Steppes. The soil is 60–80 centimeters deep and is rich in nutrients and organic matter. Loess is a type of pale yellow or buff soil, which originated as wind-blown silt. It is very fertile, but is easily eroded by wind or water. Peat is an accumulation of partially decayed vegetation, whose decomposition is slowed by water. Despite its dark brown color, it is infertile, but is useful as a fuel. Mammals and birds A large number of mammals and predatory birds have a brown coloration. This sometimes changes seasonally, and sometimes remains the same year-round. This color is likely related to camouflage, since the backdrop of some environments, such as the forest floor, is often brown, and especially in the spring and summertime when animals like the snowshoe hare get brown fur. Most mammals are dichromats and so do not easily distinguish brown fur from green grass. The brown rat or Norwegian rat (Rattus norvegicus) is one of the best known and most common rats. The brown bear (Ursus arctos) is a large bear