The pH of the effluent will affect the pH of the soil, so have soil samples from the drain field tested periodically so that you can adjust the soil pH as necessary if you have planted species that are not pH adaptable. Salt levels are likely to be high no matter what the pH is, so plan to use plants that are somewhat salt tolerant in a leach field landscape. If your field is well drained, there are a number of salt tolerant species that may prove suitable, including yarrow, columbine, chrysanthemum, delphinium, daylily, peony, clematis, and rose. Use of fertilizer may be reduced for plants growing over a leach field because some of these salts are forms of the plant nutrients nitrogen, phosphorus, and potassium. Sometimes the ideal place to put a vegetable garden seems to be over the leach field, raising the question of bacterial and viral contamination from the effluent.
Soils vary a great deal in their ability to filter viruses and bacteria.
Clay soils work best, eliminating bacteria within a few inches of the drain trenches, but sandy soils may allow bacterial movement for several feet. A properly operating system will not contaminate the soil with disease-causing organisms, but it is very difficult to determine if a field is operating just as it should. If at all possible, use your septic drain field for ornamentals and plant your vegetables elsewhere. If you must plant vegetables, take the following precautions. Do not plant root crops over drain lines.