A chemical and biological approach towards the definition of calcareous soils

Summary

Several agricultural problems are associated with the presence of certain levels of CaCO₃ in soils. The level of CaCO₃ at which the phosphate fixation becomes an apparent agricultural problem, is thought to be an appropriate margine at which the soil can be considered calcareous. Thus, a set of soil mixtures, ranging in CaCO₃ content from 1 to 96% was prepared and used in a column study to determine the level at which the CaCO₃ fraction becomes a dominant factor controlling. P³² movement and distribution.

Increasing the percentage of oolitic sand, in the soil mixture, from 1 to 10% caused a sharp drop in P³² movement with soil solution and any increase in CaCO₃ content above 10% did not show any further drop in P³² movement. The amount of P³² removed with the soil solution was generally low compared to that retained in soil columns. Studying the distribution of P³² in soil columns, after five displacements, has indicated that the migration of P³² from the top soil increased by increasing CaCO₃ from 1 and 2 to 6%. The amount of P³² removed was however retained in lower sections. A very sharp decrease in P³² migration from the top soil was observed when CaCO₃ content was raised from 8 to 10%.

A similar picture was shown when the CaCO₃ material used was in clay size fraction. However the sharp increase in phosphate retention in top soil sections took place at CaCO₃ content of 8% rather than at 10%. A limit of 8 to 10% CaCO₃ was proposed as an appropriate margine for defining calcareous soils.