This morning I got an email from one of my gardening colleagues, wondering about the wisdom of watering tomato plants with salt water. He had a link to a UC Davis website which tacitly endorses spraying tomato plants with 10% salt water “to increase their nutritional value and taste.” Unreferenced “worldwide studies” are mentioned, along with the “major potential benefit of providing irrigation for crops in areas with freshwater restrictions.”
Before we deal with the impracticalities and out-and-out harm of using salt water for irrigation, let’s look at why this practice would work on tomatoes. By training I’m a plant stress physiologist (and I’m well versed in the primary literature on this topic). Watering tomato plants with a salt solution imposes a drought stress on the entire plant, as less water is taken up under these conditions. So leaves and fruits are smaller and they may produce stress-induced biochemical compounds in response. The upshot is that you have smaller tomatoes with a higher concentration of various solutes, some of which might be tastier or more beneficial to humans.
Guess what? You can do the same thing by decreasing irrigation during fruit set! Less water means smaller fruit and increased concentrations of sugars and other plant compounds, and voila! So you can skip that step of adding salt water and just cut back on irrigation to induce a mild drought stress.
So…why in the world would you dump salt water on your garden soil? The article blithely dismisses this: “Many are still concerned about salt causing soil degradation and rendering some seawater-treated tomatoes inedible, but scientists cite that plants thrive in balanced soil containing both macro- and micronutrients.” Sorry, but sodium is NOT a micronutrient for most plants and does NOT contribute to a “balanced soil” in one’s vegetable garden.
An ironic twist to this whole article is that most of the research that’s been done is relevant to arid parts of the world (the Middle East, primarily) where saline soil conditions and limited water are common. I can’t imagine what they would think about people who would deliberately contaminate good soil by adding salt water to it.