Hard water and soft water
Globally, there is debate about the precise definition of hard and soft water and what considerations should be taken into account when feeding plants. The hardness of water depends on the concentration of positive metallic ions dissolved in the water, typically calcium and magnesium.
What’s the difference?
The impact can be increased with the presence of dissolved carbon dioxide (CO2) in the water as carbonates or bicarbonates. ‘Soft water’ can be defined as 60mg/l, and water with over 120mg/l is considered to be ‘hard’, while water in between is defined as moderately hard. However, different agencies have their own definitions.
Hard water, soft water, bad water, what is the difference? Anything dissolved in water can and will react with other elements that are added to the water, or with anything that the water comes into contact with. Hard water is an issue for cleaning and equipment and it increases the chemical reactiveness of the water especially as far as pH is concerned; it is often considered healthier. Typically, hard water comes from ground water that has been exposed for longer periods to mineral-bearing rocks. Well water is a prime example.
Soft water on the other hand allows soap to foam up and work better. It affects equipment less, and provides more of a ‘blank slate’ for chemical reactions. Studies have shown a correlation between soft water and health issues including cardiac disease. Soft water is typically sourced from surface water − rivers, streams, and lakes − and has not been exposed for long periods to mineral bearing rock formations. It can also come from treated water, where most ions have been removed or replaced with single valance atoms such as sodium by water softening equipment.
TDS, EC and water hardness
Poor water is unsuitable – because it usually contains high levels of salt or other undesirable chemicals. It can be found almost anywhere, especially in industrial areas, regions where there is intense agricultural activity, or close to bodies of salt water. It has no bearing on whether it is hard or soft.
It’s important not to confuse ion or salt concentration with the hardness or softness of water. Hardness is a function of multivalent ions such as Ca2+ and Mg2+, not monovalent ions like Na+ or Cl+. Monovalent ions also show up in the total dissolved solids (TDS) of a solution, so it is possible to have a TDS of 450 mg/L (1 ppm = 1 mg/L), derived from adding table salt to distilled water, but this water can still be soft.
There is no direct correlation between TDS or electrical conductivity (EC) and water hardness. You can only be sure that the water is hard if you can know that all the electrical conductivity derives exclusively from Ca, Mg or other multivalent metallic ions. For example, sugar solution will conduct electricity, but is not (necessarily) hard. Water softeners work by replacing the problem ions − calcium and magnesium − with sodium ions. The EC stays the same or increases but the water goes from hard to soft; not a good thing for plants.