Calcarious soils are rich in calcium and are many times saline in nature .They have montimorillonite type of clay .In these soils phosphorous gets fixed as calcium phosphate and is rendered unavailable to the plants for removal easily. Consequently the plants suffer from P deficiency needing supplemental application of P for proper growth and development of plants on such soils.
Simple immobilisation on account formation of calcium phosphate ,in addition to adsorption of phosphates on calcium carbonate particles of calcareous soils...
Because of higher amount of Ca in calcareous soils resulting in fixation of phosphorus with calcium forming calcium phosphate which plant can't uptake that's why deficiency of phosphorus in question
Perhaps you may also like to know that (Australian) research shows that the availability of P fertilisers can be increased by applying them in solution.