Quality of water required for horticulture

Of what quality should water for horticulture be ?
And similiarly for plants in nursery ?

We find that the quality of water does make a difference in growth. Every location seems to have water which is different in taste, Ph, has different minerals etc. Hence for good growth in the nursery & better quality of produce, the issue of quality of water has definitely got to be addressed.
We often hear "the water here is sweet"....."the water here makes me more hungry"... "the water here makes me feel heavy" .."the growth here is so nice". So water is different at different places. Everything being the same, good water will give better results, productivity will rise, in turn bringing greater returns to the farmer.
As the followup: If the water is hard - will the cost of softening be economical & viable.

Sreekant Mehta