Marijuana Water Usage, Management & Disposal
Depending on your geographic region, water usage and management will be a bigger or smaller issue. Either way, it’s always a good idea to minimize waste, save labor, and avoid poisoning the water table with fertilizer and pesticide runoff.
Let’s start with analysis. The contents of tap water can vary wildly from one region to another, and you need to know what you are starting with so you can know how to compensate for it in your feeding regimen. Send a tap water sample to the local lab, or look up the analysis online from your local municipality. If your Total Dissolved Solids (TDS) are over 200 ppms, filtration is recommended, and it will have to be Reverse Osmosis to reduce that reading. Cheaper forms of filtration remove particulate matter, but not ionic compounds (which is what TDS meters read). Reverse Osmosis comes along with waste and waiting time, but the waste can be used for washing containers and for toilets, and the waiting time can be handled with storage tanks and float valves.
Jennifer Martin's favorite ppm and pH meter (by far) is the BlueLab Guardian.
A typical analysis will show some amount of Calcium Bicarbonate, Magnesium and maybe some other less significant minerals. In some areas, high levels of Sodium might be present. If you are getting less than 200ppm out of the tap, and it only contains Calcium and Magnesium as its primary components, an inexpensive KDF filter might be all you need.
As for water management, the final phase after the plants have been watered is your runoff, or waste water. Getting to know your waste water is important for a few reasons. For one, its concentration is an important indicator of how much (or little) nutrient buildup is happening in the plant medium. If you’ve got 1000ppm, 6.0 pH going into the plants, and your runoff is 3000ppm, 5.4 pH, that tells you that you are overfertilizing the plants and need to back off, or maybe even leach/flush them, depending on how they look. In the reverse, if your runoff is 300ppm, 6.3 pH, that means you might not be adding enough food for them to get their basic needs met. Generally speaking, higher ppm's come along with lower pH, and vice versa, but this is not always true due to the complexity of soil and nutrient chemistry.
Common TDS and pH meters are very important but only tell part of the story. TDS is a total reading of dissolved solids. It gives you no indication of the actual contents of the water. If you are testing water that reads 1000ppm, it could be any ratio of the 13 nutrients and even includes non-nutritive dissolved solids that are virtually impossible to identify. This is one reason why some marijuana growers are attracted to hydroponic nutrients and hydroponic media- you always know what’s in water, whereas soil contains all kinds of soluble, semi-soluble, and insoluble amendments that can complicate the scientific identification process.
For the sake of environmental responsibility, and from the perspective of regulators, waste water is an issue because of nitrates and phosphates, in particular. These two nutrients are mostly, if not entirely responsible for algae blooms and eutrophication, which is slowly sickening out natural water systems by depleting oxygen. Soil tests and more advanced meters will read these particular ionic concentrations in water runoff and help marijuana growers minimize the use of these elements in order to both improve plant health and avoid fines and environmental damage.
From all the soil analyses I've done, excess Phosphorus is the most common result. This is because Phosphorus is barely a macronutrient. It only needs to be present in quantities similar to Magnesium, which is considered a secondary nutrient. The ethos amongst the vast majority of marijuana growers is that cannabis plants need lots of phosphorus during the flowering process, but scientific research does not support this theory, and neither do my soil test results on all the commercial marijuana grows I've analyzed. A little extra Phosphorus might help the ripening process, but certainly is not necessary during the first 4 weeks of flowering. Whereas ppm levels for N, K, and Ca are in the 100-200ppm range for a full-strength watering, Phosphorus is more like 20-40 ppm. Any more than that, and you will be harming the plants and the ground water when you discard the soil or runoff.