Test Your Water
A plant uses water in lots of different ways. From delivering its food and keeping it cool to actually holding itself up. As the properties of water vary from region to region you need to test your supply and maybe adjust it to make sure it is at its most efficient for growing cannabis. The two main things to test are the pH of the water and the TDS in the water.

What is pH
Put simply pH is a measure of acidity. The pH scale goes from 1.0 which is highly acidic, through to 7.0 which is neutral, up to 14.0 which is highly alkaline.

Why pH is so Important
Cannabis is only able to absorb the full spectrum of nutrients it needs if its roots receive them at the right pH. If the pH is wrong no matter how many nutrients you add the plant will simply not be able to get at them. The water, the growing medium and the nutrients will all effect the final pH so first test the water on its own. Then add the nutrient and test again. Finally test the run off from watering your plants to see if it is still at acceptable levels. The final pH to aim for will depend on your nutrients and whether you are growing in soil (6.0 - 7.0) or a soiless mediums (5.2 - 6.5).

Hydro (soiless) pH Graph | Soil pH Graph

Hard and Soft Water
Pure water has a pH of 7 (neutral). Most peoples water supply however is not pure and is often referred to as HARD or SOFT water. Hard water has a higher pH and is therefore more alkaline while soft water has a lower pH so is more acid. Nutrients aimed at cannabis often come in HARD or SOFT water mixes to compensate for these regional differences. Choosing the right type of nutrient to start with will ensure fewer potential problems later on. Don't be tempted to use water softeners as they can add too much sodium to the water.

Testing pH
There are numerous kits and meters to test pH. Simple kits involve mixing a little water with a pH indicator and comparing the colour of the resulting solution on a colour chart. (Do this in daylight or under a regular bulb as the orange or blue glow of HID bulbs will make it hard to tell the exact shade). These kits are fine for modest set ups but If you are colour blind, want a more accurate reading or have a lot of water to test an electronic meter is invaluable. The best meters are waterproof and compensate for temperature fluctuations in the water.

Adjusting pH of Water and Nutrient Solutions
Phosphoric acid is good to use for lowering the pH of water in both hydroponic and bioponic systems. The phosphorus it contains is good for the all stages of growth from germination right through to flowering. 1 ml will lower 10 litres of water around 1 point on the pH scale (7.0 will go down to 6.0 approx.). To raise pH you can use potassium hydroxide but use in smaller amounts than phosphoric acid as it is even more powerful. There are other pH adjusters, these are just the two most commonly available. Whatever you use be careful as most adjusters are highly caustic.

Adjusting pH of Growing Mediums
Most rockwool has an initial pH of over 7.0 and should be treated before use. Soak it in hot water with a pH of about 5.5 for at least 24 hours before use. This will bring down the pH of the medium to acceptable levels. Draw out the water from inside the block with a syringe to test the internal pH. Soil pH can be raised using hydrated lime very sparingly (One or two tablespoons per gallon of soil). If the plants are already in the soil, mix the lime with their water and test the soil again a few days later. Do the same with gypsum to lower soil pH.

TDS (Total Dissolved Salts)
TDS meters are used to find out the concentration of nutrients in a solution (nutrients are salts). These meters won`t tell you if the nutrient solution is properly balanced but they will tell you its overall strength. CF, EC and PPM meters all measure the amount of dissolved salts in water. Although they use different scales and techniques they do near enough the same job.

2.0 ms = 20 CF = 1280 ppm (approx.)

TDS of your Water Supply
As most water already contains dissolved salts of one sort or another you should test your water with a TDS meter before adding anything to get a base reading. Halve this amount then minus it from any reading you take after adding nutrients. Don't think a higher TDS reading will mean bigger buds, stick to the directions of your nutrient. Remember nutrients are salts, if you put too much salt in the water you will kill your plants.

Distilled Water
Some people use distilled water (also called de-ionised water) for cuttings and young plants. It has no impurities so its TDS is 0 and pH is neutral (7.0). If you use it from seed to seedling stage you won't use too much and it`s available from most garages and DIY shops. Distilled water also makes a good cleaning and basic calibration solution for pH and TDS meters.

Rain Water Vs Tap
Rain water is excellent to use for outdoor plants as it is aerated and contains small amounts of nitrogen. Check the pH though because due to industrial pollution it could be too acidic. Be careful about bringing "dirty" maybe pest ridden rain water into a sterile growroom however. Tap water may not be the best but at least it should be "clean".