Soil fertility, fire regime and hydrology are thought to be most influential in determining Cerrado vegetation. Cerrado soils are always well-drained and most are oxisols with low pH and low calcium and magnesium. The amount of potassium, nitrogen and phosphorus has been found to be positively correlated with tree trunk basal area in Cerrado habitats. Much as in other grasslands and savannas, fire is important in maintaining and shaping the Cerrado's landscape; many plants in the Cerrado are fire-adapted, exhibiting characters like thick corky bark to withstand the heat.