The value of wastewater as a source of nutrients for crops depends on concentrations of nutrients, type of crop grown, and soil fertility level. Nutrients are immediately available to the crop, as long as they remain dissolved in wastewater and soil solution, but may be rendered less available by several soil processes. Some processes result in permanent loss (leaching, volatilization and erosion) and others in nutrient accumulation in the soil (microbiological immobilization, adsorption, precipitation). Hence, the proportions of nutrients taken up by plants are different than the proportions of nutrients applied via wastewater or fertilizers. Because soils and wastewater seldom contain nutrients in optimum ratios, a set of guidelines is presented to optimize wastewater irrigation. Some information on nutrients in plants, nutrient sources and soils is required to be able to apply the guidelines. Part of that information may not be available and has to be found in trials. Evaluation of wastewater as a nutrient source is not possible by simple comparisons of sources of wastewater and fertilizers, because effects of N, P and K in wastewater are confounded. A plea is made for factorial designs with combinations of wastewater and straight fertilizers.