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Grow Hack: Control And Monitor Soil Water Content
May 05, 2016

Growing cannabis in containers, whether indoors, outdoors or in a greenhouse, allows gardeners to take full control of the soil and the water content of their plant’s roots. Plants growing in waterlogged soils will wilt and yellow much like plants growing in dry soils, so don’t make the mistake of adding more water…

When plant roots find themselves in over water soils, so many thing start to go wrong. Plant roots need oxygen to survive (unlike their aboveground counterparts, the leaves) and without gills, they can’t breathe underwater. Swimming in water anaerobic bacteria take over and begin to consume the roots and other organic matter, while absorbing fertilizer in the process as well. Waterlogged roots can’t absorb water, nitrogen and don’t perform any of the functions they’re supposed to serve. First you’ll notice the soil smells rotten, the plant wilts as if it had no water, leaves begin to turn yellow from a lack of nitrogen, and new growth begins to die. Don’t let this happen to your weed; start by mixing up some well draining soil and never water too often.

It’s impossible to make an exact recommendation for watering frequency that will work for everyone. The temperature, humidity, airflow and light of the environment your potted plants grow in all influence the water demand on the soil. The higher the demand, the more you need to water. Few two growers keep their watering schedules the same due to the amount of variables that go into water demand, so here’s a rundown with a few of the main influencers.

High temperatures in the growroom means water will be quickly evaporating off every surface of the plant, and the roots will suck water from the soil faster than they would at low temperatures. The reverse holds for low temperatures.

Cannabis plants like a humidity range of 55 – 65% relative humidity, but many experienced growers will make sure humidity stays between 50 – 55% during bloom to prevent bud rot. Simply enough, high humidity environments put less demand on water from the soil, while plants in low humidity transpire more rapidly and demand water from their roots. Vent fans control humidity, but growers like to use circulating fans for airflow to strengthen stalks enough so they can hold up pounds of buds, but this airflow also increases foliar transpiration, and water demand from the roots.

When watering, you need to saturate the soil enough so about a fifth of the volume you added drains from the bottom. At saturation most well draining soils should hold around 45 – 50% water by volume. If water takes too long to start draining, you probably didn’t add enough perlite or vermiculite, and this problem probably won’t go away until you repot with more adequate soil.

Many growers have recommended to water when their soil is dry up to the first knuckle, but make sure you listen to your plants as well. Try to figure out how many days your plants can go without water before they start wilting, this will indicate exactly how dry too dry really is. Once you get to this point you can either dip your finger in the soil to see how far down the soil is dry, pick up the pot to see how much it weighs, use a soil water meter to measure exactly the volumetric content of water in the soil, or all of the above.

If you wait too long to water your plants the soil could get excessively dry. When soil water content falls below 10%, you might need to mix in a “wetting agent” in with the water to make sure water actually penetrates the soil. Wetting agents, like unscented dish soap (ideally one that does not contain any biocides like methylisothiazolinone), helps water re-wet all of the soil because it lowers water’s surface tension. Without one, water will simply take the easiest path straight down to the bottom of the pot without making contact with any of the roots.


Watering enough so it drains out of the pot prevent salt build up, which can cause nutrient lock out and pH swings. Even watering this much still means you need to flush the pots with enough water so a whole volume of the pot drains from the bottom every two weeks. Even when using meters, probes, hygrometers and thermometers, nothing beats simple listening to your plants. Numbers and values can help greatly in the growing process, but they are certainly no replacement for attention and close care.


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