At high water content, oxygen diffuses very slowly into the soil volume because most of the soil pores are occupied by water. Rainfall adds water to the soil profile. Irrigation introduces dissolved salts and water into the soil root zone. Dissolved salts are solid salts that have been dissolved in water. The dissolved salts are called ions and dissolved table salt includes a positive sodium ions and negative chloride ions in the solution.
Salts can also precipitate out of soil water onto soil particles.
Water is removed from the soil by plant root uptake, evapotranspiration and drainage below the root zone. Water is typically not distributed evenly with height in the soil root zone. Due to capillary suction, soil pores that are just above the water table are typically full of water. However, soil that is well above the water table has less water. A plot of water content vs. height above the water table is shown in figure 4.