What’s pH got to do with it?

by Kathy Goff, EdD

I have grown plants for many years and have heard of pH but never really paid much attention to it.  My previous marijuana growing experiences were clandestine outdoor grows where you found a place and planted the seeds.  Then you let them grow and hoped that nobody else found them before harvest.  So tending them was not really an option. 

Now that it is legal to grow personal marijuana with a patient card, I found out that pH is totally important.  Soil pH is the measure of acidity (sourness) or alkalinity (sweetness) of a soil. The pH scale goes from 0.0 to 14.0. The most acid soil is 0.0 and the most alkaline is 14.0. Halfway along the scale, 7.0, is neutral, neither acid nor alkaline. A soil gets more acid as the pH values decrease from 7.0 to 0.0 and is more alkaline as pH values increase from 7.0 to 14.0.

Marijuana thrives in a slightly acidic growing medium in the range of 5.5-6.5. In fact most plants prefer a slightly acidic pH.  The ideal pH for growing marijuana should be considered as a range rather than a specific number. This is because different nutrients become available to the plant at slightly different pH levels. By having your pH sitting within a range rather than at a specific point, you make more nutrients available.

If the pH of your growing medium veers outside of these ranges, certain nutrients and trace elements will no longer be available to the plant. This means that the plant may begin to show symptoms of deficiency of a particular nutrient, even though that nutrient may be physically present at the plant’s roots.

If you do not monitor the pH of your growing medium, you can end up feeding a plant more and more nutrients to fix a deficiency and the plant simply cannot take them on board. If unchecked this situation can lead to a build up of salts in the growing medium that block up the plant’s roots. This is nutrient lock out. Your plants can literally die of starvation despite how much you are feeding them.

When you grow marijuana in soil in containers, pH of the water changes the pH of the soil and the whole range of nutrients become available to your plant as the soil slowly dries out again.  Soil pH affects the availability of nutrients to the plants and the beneficial bacteria and microbes that help with the uptake of nutrients.

If you are using tap water, let it sit in a bucket or reservoir for a day or two to allow it to dechlorinate, then check the pH to make sure it is within the required range.  Tap water often contains minerals and impurities that can affect pH.  Rapidly growing plants consume different nutrients at different rates, thus altering pH. Maintaining the correct pH level for your soil is an ongoing task.

If you grow marijuana in a hydroponics system, then pH management is a much more important issue. With no soil to act as a buffer changes in your pH values take effect much more quickly. With hydro grows allowing your pH to fluctuate within a prescribed range is important as it allows all of the nutrients in the solution to become available to the plant in turn. Luckily, this happens naturally as the pH of the solution in your hydroponic reservoir will drift over time. Whilst pH levels are important to all marijuana growers, hydro growers need to be more in control of pH than soil growers.

In general there are three ways to measure your pH level, either by using a pH pen/meter, pH testing liquid and pH strips. They all measure the acidity and alkalinity based on a 0 to 14 scale, 7 being neutral (water). The pH meter/pen costs the most, but provides more precise measurements to the decimal level. 

pH testing liquids allow you to raise and lower pH of your nutrient mixture and water using proprietary pH Up and pH Down liquid solutions With either the nutrient testing liquids or a pH test strip, you will just get a general idea of the pH level based on the color of the testing solution or strip.

There are several organic solutions to raising and lowering pH as well.  To raise the soil pH use lime or dolomite. The addition of Dolomite Lime to soils at about 1 – 2 tablespoons per gallon of soil is a good way to help control the pH of overly acidic soils.  To decrease the soil pH use superfine dusting or water soluble sulfur, sphagnum peat or organic mulches mixed into the soil. 

When the new growth of my plants was bright yellow, I knew something wasn’t quite right.  I did some research and found that it was a pH problem.  I had not been paying attention to the pH.  I began to monitor the pH by testing the water, with and without nutrients, that I gave to my plants.  I fed them lower pH water to bring it down which resulted in new growth that was green.  Now I monitor the pH in all of the water that use on my plants.

Monitoring pH levels must be part of your regular plant maintenance routine.  For more in depth or personal information, I suggest you contact a local grow supply store, such as Skunk Grow Supply in Tulsa or Lucky’s Grow Supply in OKC.

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