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An infiltration problem occurs if the irrigation water does not enter the soil rapidly enough during a normal irrigation cycle to replenish the soil with water needed by the crop before the next irrigation. The reduced infiltration rate, if due to quality of applied water, is generally a problem within the upper few centimetres of soil but occasionally may occur at greater depths. The end result is a decrease in water supply to the crop, similar to the reduction due to salinity, but for a different reason. A water infiltration problem reduces the quantity of water put into the soil for later use by the crop while salinity reduces the availability of the water in storage. Infiltration refers to the entry of water into the soil. The rate at which water enters is referred to as the rate of infiltration. Permeability, the term used in the previous edition of Irrigation and Drainage Paper 29 (1976), more correctly refers to the percolation of infiltrated water through the soil. Since the water quality problem is primarily one related to the ease with which water enters and moves through the upper few centimetres of soil, we have chosen the term 'infiltration problem' rather than the previously used term 'permeability problem'. An infiltration rate as low as 3 mm/hour is considered low while a rate above 12 mm/hour is relatively high. This can be affected, however, by many factors other than water quality, including physical characteristics of the soil, such as soil texture and type of clay minerals, and chemical characteristics including exchangeable cations. The guidelines of Table 1 refer to infiltration problems as they relate directly to the unfavourable changes in soil chemistry caused by the quality of irrigation water applied. These problems concern both salinity and relative sodium content in the applied water. Figure 21 shows in graphic form that both salinity (ECw) and the sodium adsorption ratio (SAR) of the applied water affect the rate of infiltration of water into surface soil. Figure 21 can be used in place of the numerical evaluations in Table 1 given for infiltration problems. The infiltration rate generally increases with increasing salinity and decreases with either decreasing salinity or increasing sodium content relative to calcium and magnesium - the sodium adsorption ratio. Therefore, the two factors, salinity and SAR, must be considered together for a proper evaluation of the ultimate effect on water infiltration rate.