Clay minerals and soil fertility loss on Petric Calcisol under a semiarid Mediterranean environment

The authors study clay mineral and main nutrient element losses in runoff water generated by rainfall events in 1990-91 from five field plots on a Petric Calcisol or Xerollic Paleorthid. Illite is the main layer silicate in the clay Ap horizon, with a smaller proportion of kaolinite and a minor amount of chlorite and smectite. Soil samples taken in different seasons show that plots under shrub vegetation are very much alike and, with the exception of available Fe, Cu, Mn and Zn, they are markedly different from arable soil plots, being much richer in organic C, total N and available K, but poorer in available P. Annual and seasonal losses of the main nutrient elements are small, with the highest amounts in the plots cultivated for barley and with cut shrub; losses are extremely low for the plot with shrub cover.