Alkali, or alkaline, soils are clay soils with high pH (> 8.5), a poor soil structure and a low infiltration capacity. Often they have a hard calcareous layer at 0.5 to 1 metre depth. Alkali soils owe their unfavorable physico-chemical properties mainly to the dominating presence of sodium carbonate, which causes the soil to swell and difficult to clarify/settle. They derive their name from the alkali metal group of elements, to which sodium belongs, and that can induce basicity. Sometimes these soils are also referred to as (alkaline) sodic soils.
Alkaline soils are basic, but not all basic soils are alkaline; see alkali for the difference between alkali and base.