Alkaline soil

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 which 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.

The causes of soil alkalinity can be natural or man-made:

Alkaline soils are difficult to take into agricultural production. Due to the low infiltration capacity, rain water stagnates on the soil easily and, in dry periods, cultivation is hardly possible without copious irrigated water and good drainage. Agriculture is limited to crops tolerant to surface waterlogging (e.g. rice, grasses) and the productivity is low.

Soil alkalinity is associated with the presence of sodium carbonate (Na₂CO₃) or sodium bicarbonate (NaHCO₃) in the soil, either as a result of natural weathering of the soil particles or brought in by irrigation and/or flood water.

Sodium carbonate in water can produce carbon dioxide (CO₂), escaping as a gas or absorbed by Algae, and sodium hydroxide (Na⁺OH^–), which is alkaline (or rather basic) and gives high pH values (pH>8.5).

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Wikipedia