Calcium carbonate concretions in a soil horizon

The presence of carbonate nodules in the lower horizons of the soil profile indicates a wash (decarbonation) in the upper part and the subsequent accumulation of carbonates in the lower horizons (calcic horizons).
The picture shows a calcic horizon immediately below an argic horizon. In this cae, decarbonation occured prior to clay washing, since clay flocculates in the presence of calcium and can not be dispersed. On the other hand, the appearance of nodules in a clayeous horizon indicates an aridification after the accumulation of clay.
The relatively large size (> 1 cm) of the carbonate nodules gives an idea of ​​their age. The growth of the nodules from small previous crystals occurs thanks to a continuous supply of material during a long time.
The shape of the nodules also provides information about their age. In general, young nodules are irregularly shaped (formed from small crystals), while older nodules are spherical or polyhedral (grown by recrystallization). Nodules grow occupying pores and galleries (such as those excavated by the soil fauna or the roots of the plants), as shown in the image.