In some areas, extended periods of drought force people to use “gray” water in their gardens. Gray water is water from bathtubs and the rinse cycle of dishwashers and clothes washers. It does not include water from the toilet, which should not be used on the garden, nor does it include water from the wash cycle of dishwashers and clothes washers, which have high levels of chemicals.
Like softened water, gray water has a high sodium content from the soap used in washing. If used outdoors, this sodium is less harmful than when used on houseplants. Take these precautions to prevent harm.
Use plenty of water. The worse the quality of water you use, the more you need to leach to keep salts from accumulating in the soil. The amount of sodium in gray water won’t hurt outdoor plants by itself, but if allowed to accumulate in the soil, it can make the soil too alkaline, and can cause permeability problems. By overwatering at least 10 percent — that is, by putting on enough water so that 10 percent of the water you apply is leached below the root level of your plants — you can avoid problems with accumulation.
If this is difficult, or if problems arise in spite of leaching, wash the sodium from the soil by adding calcium in the form of gypsum (calcium sulfate). Add about 5 pounds per 100 square feet, scratch it into the surface of the soil, and irrigate well. The calcium will replace the sodium attached to the soil particles and the sodium will be leached from the soil.