Hi all,That would be my thought as well. It does both.
TDS
The ppm TDS value (really electrically conductivity) is a just measure of all the salts (ions) in solutions. What we call "water" isn't pure H2O, but a
<"dilute solution with water as a solvent">.
Pure H2O is to all intents and purposes an electrical insulator, it doesn't conduct electricity. If you test DI water with a conductivity meter it will read less than 5 microS (3 ppm TDS), conversely sea water has a lot of salts (mainly salt NaCl, about 33 ppt.) and it is an efficient conductor of electricity, (it would have a conductivity of ~50,000 microS).
A lot of salts are highly soluble in water, including the ones we use a fertilisers. All plants, even terrestrial ones, can only take up nutrients as ions from solution.
Hardness
The assumption is that the hardness/dHG/dKH are the same, because they all relate to how much calcium carbonate (CaCO3) is dissolved in the water. One unit of dGH, and dKH, are both equal to 17.86 ppm CaCO3.
Limestone (calcium carbonate) isn't very soluble (technically it is only soluble in weak acids), but because rain water absorbs some CO2, it is a weak acid until it has dissolved some bases, almost always CaCO3 in the UK. The remaining, undissolved limestone, acts as a "buffer" that can go into solution and ensures that new rain water arriving becomes harder. This dissolution of limestone by rain-water is how you get caves, tufa springs and limestone pavement etc formed.
When water collects in limestone geology ("aquifers") it will become fully saturated with Ca and HCO3- ions, giving it a pH of pH8, a hardness of about 18dKH/dGH and a conductivity of 650 - 800 microS. If these values are higher you have other stronger bases present, if the aquifer values are lower there is some calcium carbonate present, but you might have very high rainfall, a very hard limestone (like the Carboniferous age limestone) or a sand-stone aquifer with only small amounts of CaCO3 present.
If your tap water doesn't have any carbonate buffering (dGH/dKH), and a low TDS value you can say that it has not encountered any limestone in its journey from rain-water to your tap. Because limestones are porous, upland reservoirs are almost always in non-calcareous rocks and these are the principle source of soft water in the UK.
cheers Darrel