The main reason large rates of potassium are required on cotton soils in the San Joaquin Valley, is that most of the added potassium becomes fixed within the soil and is not available to the cotton plant. This soil condition, resulting in small amounts of exchangeable potassium and large potassium-fixing capacities, may be widely prevalent within the valley, according to recent chemical and minerological analyses of many soils—and is reflected in plant deficiencies and large potassium fertilizer requirements.