Some bacteria and fungi living on or near roots help plants obtain nitrogen from the atmosphere, while others release nutrients and minerals. Grassland soils are fertile, with thick surface horizons that are rich in organic matter and nutrients. The black soils of sub-humid regions are especially productive. Soils found under forests and in more moist climates in the northern part of the Plains are less suitable for agriculture. They have less organic matter, are more acidic and contain fewer nutrients.
Soils formed in grasslands are different from forest soils.
The upper part of the Dark Brown grassland soil is enriched in organic matter resulting from the decomposition of the many fine roots of the grass vegetation. This fertile soil has reserves of organic nutrients such as nitrogen, phosphorous and sulfur, and a high content of mineral nutrients such as potassium. The gray upper mineral soil has a low organic matter and nutrient content. The brownish horizon contains clay minerals that have been moved downwards by intense leaching under acidic conditions. Early advances in agriculture, such as better seeds, livestock and improved implements, were made mainly by farmers.