The upper portion of the soil that is used by plants for obtaining water and nutrients, often referred to as the horizon. Higher levels of organic matter in topsoil cause it to be darker and richer than subsoil and give it greater potential for crop production. The loss of topsoil is a critical problem worldwide. The net effect of this widespread topsoil erosion is a reduction in crop production equivalent to removing about 1% of the world's cropland each year. Soil material that can be purchased to add to existing surface soils is also referred to as topsoil. This kind of topsoil may or may not be from the surface of a soil. Often it is a dark-colored soil that is high in organic matter and in some cases may be more organic soil than mineral soil.