my question is why is measured in Pascal's and how can you have a negative pressure ?
The osmotic pressure is related to the number of particles of solute dissoved in the water (really now solution).
To understand the current usage of "water potential", try to think of it almost as if it was diffusion of water - because if there is less solute dissolved in it [low osmotic pressure], in a way the "concentration of water" is higher, so it "diffuses" (sort of) to an area with more solute [high osmotic pressure] i.e. "lower conc-n of water".
It is measured in Pascals because of the historic term "osmotic pressure".
Hope this helps!
M (specialist biology tutor)
It might be easier to understand osmotic pressure if we take an example:-
In the human body, maintenance of the correct osmotic pressure of the blood is crucial to survival. The main solutes that confer osmotic pressure to the blood are the plasma proteins, particularly albumin. The osmotic pressure, as explained in the previous post, tends to pull water towards the "compartment" with high osmotic pressure. So, to keep blood within blood vessels as much as necessary, the blood osmotic pressure must not be too low.
In liver failure, the blood albumin level becomes too low (because the liver synthesizes most protein the body), so the osmotic pressure of the blood drops, AND FLUID LEAKS OUT OF BLOOD VESSELS giving a swollen tummy (called oedema).
The same happens if the amount of protein in the diet is reduced drastically as in extreme starvation causing a disease called kwashiorkor (the word [in West Africa] literally means "the disease the first child gets when the 2nd child is born"). [you might remember seeing babies with bulging tummies in ads asking for charity money for children].
I hope this helps further.