Adjusting Plant pH

Written by Ryan Scott

What is pH?

When it comes to growing just about any type of plant, an often overlooked, but important, step involves pH. In chemistry, pH is a logarithmic scale used to specify the acidity or basicity of an aqueous solution. To do this, the pH meter measures the relative amount of free hydrogen ions in the solution, helping to determine whether it is acidic or alkaline.

It is important to note that the pH scale is logarithmic. This means that for every one point on the scale, the concentration changes by a factor of ten. As an example, an increase of pH from 8.0 to 9.0 is a tenfold increase in alkalinity. A pH of 7.0 is considered neutral, with any reading below 7.0 being acidic. A pH reading of above 7.0 results in a more alkaline solution.

Plants and pH

The chemical properties of a growing medium are important in that they regulate water and fertilizer elements. The pH level of the medium is an indication of how the plant roots will assimilate fertilizer elements. If the soil or medium’s pH level is too acidic or too alkaline, some fertilizer elements cannot be taken in by the roots, causing various nutrient deficiencies.

When adjusting plant pH, it’s important to consider the relative pH of the growing medium, as well as its water or nutrient content, as the pH of one can affect the other.

Plants have different pH preferences. For cannabis, the ideal conditions for pH balancing are within a slightly acidic range. This “range”, rather than a specific number, is due to different nutrients being made available to the plant at various, but still slightly acidic, positions within the scale.

For cannabis plants growing in soil, the optimum pH range is around 6.0 to 6.8. For cannabis plants growing in a soil-less medium or hydroponically, the optimal pH range is around 5.5 to 6.5. Growing in soil also acts as a natural buffer with regards to pH, with fluctuations in pH taking longer to affect plants grown in it. Fluctuations in pH with soil-less mediums seem to react almost instantly with roots as these mediums tend to be close to neutral on the pH scale.

pH Adjusting Tools

There are a few necessary instruments used to measure and adjust pH, including digital or analog pH meters, and chemicals for adjusting water or nutrient solution pH.The pH of the plants’ root zone is the primary concern as this is where nutrient action takes place. A useful way to gauge what’s transpiring in the soil is to test the pH of water runoff after watering, as this will alert the grower to any problems accumulating in the soil.

A few different options exist for adjusting the pH of water or nutrient solution. Alkaline products labeled “pH Up” will raise the pH of water or solution. These products include dolomite lime and sodium bicarbonate, most often in liquid form to mix well with water. Acidic products, labeled “pH Down” will lower the pH of water and nutrient solution and commonly contain food-grade phosphoric acid to achieve this.

When adjusting plant pH, it is important to limit your work with either pH Up or pH Down. Using both when overdoing one can result in unnecessary stress on cannabis plants. Adjusting pH should always be done in small steps to avoid lowering or raising pH levels too much.

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Ryan Scott

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