Alkaline soil occurs when there is too much calcium, which prevents plants from obtaining phosphorous. Phosphorous is an essential plant nutrient and without it, plants cannot carry out critical life processes such as cell division.
Phosphorous is generally found in the environment as phosphate, a salt product of phosphoric acid. Phosphate can be found dissolved in organic material, such as manure, as well as from run-off from weathered rock. When phosphate reaches soil, other chemicals will also react with it, turning phosphate into solids. In normal soil conditions, this is not a problem because there is still enough soluble phosphate for plants to absorb.
In alkaline soil, excessive calcium leads to calcium becoming the main positive ion that reacts with phosphate. The resulting solids decreases the solubility of phosphate in the soil, thus reducing its availability to plants. This can lower growth in plants and lead to plant death if the soil is extremely alkaline.