Acid weathering is a progressive desilification of the soil system. The generally fertile Mollisols of grasslands, with high proportions of silicon, weather progressively as shown in the figure. Highly weathered soils are the Ultisols and Oxisols. They are common in warm to hot, humid areas, where they are subject to intense leaching. They tend to be highly desilicified, acidic, low in. essential nutrient elements, and on account of their acidity, high in soluble aluminum. All this said, there are soils in which silicon plays a subordinate role, such as those derived from calcareous parent material, and organic soils.