Effect of crop rotation on the changes of potassium forms and clay minerals under Mediterranean climatic condition

ABSTRACTThe influence of intensive crop rotation on the distribution of potassium forms and clay mineralogy was assessed under a Mediterranean condition in the Piranshar region, northwest of Iran. For this purpose, surface soil samples in relation to six soil subgroups from crop rotationfiled (sugar beet, wheat, pea, and barley) over five decades and the adjacent uncultivated lands were described and sampled. Soil analyses were concernedwiththe determination of physicochemical characteristics, clay mineralogy, the forms and adsorption properties of K. XRDpatterns revealed that soils were similar in clay mineral compositions, including illite, smectite, chlorite, and kaolinite for both cultivated and uncultivated soils, but some changes occurred in the peak intensity and position of the minerals mainly chlorite with cropping. Consistent with this, the sharp peak of chlorite (d001, 14.2 Ao) withthe intensity of 1700 to 1800 Cps in the uncultivated lands shifted toward peaks with intensity of about 1000 Cps in the adjacent cultivated soils along with the appearance of a disordered chloritevermiculite mineral.A pronounced decline in the solubleK from 0.001 to 0.53 mmol l1 (a drop from 17 to 87%), exchangeable K from 6 to 115 mg kg1 (a drop from 5 to 53%), and nonexchangeable K from 9 to 244 mg kg1 (a drop from 1.5 to 29%) wereobserved for the majority of the studied soils as a result of crop rotation. Under cultivation, K adsorption effectively increased in the Chromic Calcixererts, TypicCalcixerolls, TypicCalcixererts, and TypicHaploxerepts where exchangeable and available K decreased.