Re: What is inside salt that harms most plant and what does salt contain?

Dear Kate:

Salt alone is not harmful to plants. However, how much salt a plant gets can affect it. Plants die when you water them with salty water because they lose water and shrivel up. Cells in general gain and lose water by the process called osmosis. I'm sure you've had the experience of getting wrinkly skin from spending too much time in the bath. This is because your skin cells absorb water and swell up, so your skin wrinkles. Try to imagine what's inside a cell in your skin. In the cytoplasm, or 'cell juice', there are lots of proteins, small molecules, and dissolved substances swimming around a watery medium. Compared to your bath water, which is mostly water with only a small amount of soap dissolved in it, the concentration of stuff in your cells is much higher. We say that your cytoplasm has a lower water potential compared to bath water. That means the bath water is more watery than the inside of your cells. Water tends to move from more watery to less watery places. Therefore, water moves into your cells and they swell, causing your skin to wrinkle.

The same thing happens with plant cells. When you water a wilted plant, it absorbs water very quickly. This is because the plant has lost water, and the insides of its cells are much less watery than the water you're pouring on it. Therefore, water moves into the cells until it swells up and becomes turgid. As you may know, plant cells are enclosed by cell walls made of cellulose, which is the stuff that makes up paper, so they don't swell until they burst, like animal cells might. You can imagine a turgid cell as something like a balloon, that's been blown up until it's stretched and firm. That's why well-watered plants are upright and firm, while dried-out plants are soft and bent-over.

So now we get to your question. When we water plants with salty water, the salty water is less watery than the stuff inside the cells. The cells are at a higher water potential compared to the salty water, where earlier they were at a lower water potential compared to the fresh water. This means that the cytoplasm ("cell juice" or "cell sap") is more watery than the salty water you've given the plants. Therefore, water moves out of the plant cell, and the plant loses water. When the plant loses water, the cells are no longer turgid, they are no longer firm. Without enough water, the plant will die.

You can try a simple experiment. Take a bunch of raisins, and soak them in water for a long while. They'll swell up. If you take these swollen raisins and put them in very salty water, they'll shrink again if you wait long enough (tip: use fewer raisins and more salty water, and stir from time to time). It doesn't happen just with salt. If you use very sugary water, or honey, they'll shrink too. That's how people make pickles, by leaving cucumbers in very salty water, so they'll shrivel up and become crispy.

So you see salt is not a poison or a toxin, but because it affects how much water a plant has, it can affect its health. As for your question on what is salt, I'll direct you to some other webpages and answers from the MadSci archives....

Hope this helps to answer your question! If you have more to ask, do feel free to send them over to the Madsci Network. =)

Do browse through the links I've included below.