I think what everyone was trying to describe is the effect of osmotic pressure.
As an experiment, try to gain access to some of the membrane they use for reverse osmosis. Place it in a water proof box, about a third of the way down the length, make sure its sealed to the sides of the container so water can't go around it. On the short side add a saline solution, and on the other add pure water (distilled). Make sure you fill the to the same height, over a period of time the saline side will rise to a level higher than that of the pure water. (the smaller the saline side the more noticeable the height change, it's best when the waline is stuck in a tube.) Make sure you try it, because its hard to believe if you just hear it.
The pressure of the saline being held up then is equal to the osmotic pressure. To explain this we consider what happens at the membrane, the membrane only allows small molecules such as water to pass, whereas the larger salt ions cannot. So, the water molecules on the pure side have more opportunity to pass through the membrane, while the solution side has fewer opportunities, since there are fewer water molecules per unit volume, and the salt ions tend to obstruct them. Furthermore, the forces that exist between ions and dipoles are much stronger than those between two dipoles (water, being polar, has a dipole and is held together by dipole-dipole forces) So there is an attraction of water molecules to the salt ions. This of course fits with free energy considerations, no appreciable heat is required or produced by the process, but the lower concentration on the saline side is favored by entropy.
This explains the behaviour of your plants, the roots of a plant contain such a semi-permeable membrane, and the osmotic pressure (as well as the capillary effect in the small tubes) is used to passively distribute the water to the top of the plant. However, when the water in the soil has a salt concentration it lowers the osmotic pressure, making it more difficult for water to get to the top of the plant.
As an example of how large this pressure can be, a .15M solution of table salt (sodium chloride) in water (about 8.8 grams of salt per liter of water) at 25C when placed in a situation with pure water will produce an osmotic pressure of approximately 7.3 atm. This could hold up a saline column around 250 ft high.