If the formation of carbonates is of pedogenic origin due to rise in temperature and decline in rainfall then the soils will be moderately to highly alkaline and would lose the productivity even when their CEC is remarkably high.

Dear Layth Saleem Salman Al-Shihmani

It is known that CaCO3 is a chemically active in adjusting the soil acidity by removing the H+ ions from the cation exchange sites ,producing water and carbon dioxide and thus higher the soil pH value by means of the following reaction:

Exchangeable acidity soil (H+) +CaCO3= exchangeable (Ca+2) soil+H2O+CO3

When the level of Ca+2 exceeds the capacity of the soil to absorb it, it interacts with other elements like phosphorous, boron and iron and forming compounds that are not easy to absorb by the plants. Please burin in mind that calcium carbonate is not highly soluble to Ca+2 and its solid various forms percents added to the soils does not represent the concentration of Ca+2 that going to be in this soil. It depend on the surface to volume ratio and apparent density since these parameters restricts the solubility rate and so the concentration of Ca+2. I hope its clear that the cation-exchange capacity (CEC) decreases when the maximum quantity of total holding Ca+2 cations is reached at a given pH value.

Best regards

The chief characteristic of sodic soils from an agricultural standpoint is that they contain sufficient exchangeable sodium to adversely affect the growth of most crop plants. For a definition, sodic soils are those which have an exchangeable sodium percentage (ESP) of more than 15. Excess exchangeable sodium hurts the physical and nutritional properties of the soil, with a consequent reduction in crop growth, significantly or entirely. The soils lack appreciable quantities of neutral soluble salts but contain measurable to appreciable quantities of salts capable of alkaline hydrolysis, e.g. sodium carbonate. The electrical conductivity of saturation soil extracts are, therefore, likely to be variable but are often less than 4 dS/m at 25 °C. The pH of saturated soil pastes is 8.2 or more and in extreme cases may be above 10.5. Dispersed and dissolved organic matter present in the soil solution of highly sodic soils may be deposited on the soil surface by evaporation causing a dark surface which is why these soils have also been termed black sodic soils.

Under field conditions after irrigation or rainfall, sodic soils typically have convex surfaces. The soil a few centimetres below the surface may be saturated with water while at the same time the surface is dry and hard. Upon dehydration cracks 1-2 cm across and several centimetres deep form and close when wet. The cracks, generally, appear at the same place on the surface each time the soil dries unless it has been disturbed mechanically. The Physico-chemical characteristics of two soil profiles are presented in Table 20.

The principal cause of the alkaline reaction of soils is the hydrolysis of either the exchangeable cations or of such salts as CaCO3, MgCO3, Na3CO3, etc. Hydrolysis of the exchangeable cations takes place according to the following reactions