Many of the water quality problems that gardeners face stem directly from what comes from the tap. Local municipalities need to provide water safe enough for human consumption. In general, humans are not adversely affected by water that has a high pH, but plants often are.
Similarly, high levels of salts in tap water can certainly be a problem for both human and plant life, but it is expensive to remove these minerals. Two aspects of the water supply—pH and total dissolved solids (TDS)—affect many aspects of plant growth and vigor.
TDS measures various salts that have been dissolved in water. These dissolved minerals cannot be removed by traditional filtering, but only through membrane, reverse osmosis or distillation. The quantity of TDS in the soil or growing medium will move in the direction of the water. If you keep applying water that is high in pH and high in TDS to your soil, the root systems of the plants will eventually have trouble taking up many of the nutrients you have applied. If the water supplied is too acidic (the pH is too low), nutrient uptake can also be adversely affected.
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