Sodicity, or high sodium (Na), affects the physical condition of the soil by dispersing aggregates. Irrigation water high in Na can cause the soil to become pasty when wet and develop a "puddling" where water remains on the surface for an extended period of time. Once dry, high sodic soil can become very hard and its permeability to water and air is reduced--reducing plant and root growth and productivity. Sodium buildup from irrigation usually occurs slowly and may not be easy to detect from one year to the next. However, regular soil testing can determine sodium accumulation. The sodium hazard associated with either a soil or an irrigation water is characterized by measuring the levels of sodium, calcium, and magnesium. These values are used to calculate the sodium adsorption ratio which is the ratio of sodium to both calcium and magnesium. With sodium dispersing clay particles in the soil, aggregation will be poor, resulting in poor water infiltration and poor root penetration with less water and nutrients available for plant growth.