Black-coloured soils are characteristically rich in organic matter (humus) derived from decayed plant and animal material. High humus content gives soils their characteristic dark colour, improves water retention, nutrient availability, and soil structure. Examples include chernozems (black earth) of the Ukrainian steppes and regur (black cotton soil) of the Deccan Plateau, India. Iron minerals produce reddish-brown (oxidised) to grey-green (reduced) colours.