pH is a 0–14 scale of how acidic or alkaline a solution is, and cannabis can only absorb nutrients properly when the pH around its roots sits in a slightly acidic window of about 6.0 to 6.5 in soil. Drift outside that range and your plant can show deficiencies even when the nutrients are right there in the pot. Along with EC (electrical conductivity, a measure of how many nutrients are dissolved in your water), it's one of the numbers that affects, and is affected by, everything in your grow.

What Is pH in Growing Weed?

pH is a universal measure of how acidic or basic a solution is. An acidic solution has lots of hydrogen ions (H+), and a basic one has lots of hydroxide ions (OH–). A solution with these ions in balance is considered neutral. Confusingly, more available hydrogen ions means a lower pH.

A hydrogen ion combined with a hydroxide ion makes H2O, the chemical formula for water. Pure water sits right in the middle of the pH scale and counts as a neutral solution.

How is pH measured?

Measuring pH isn't complicated: the scale runs from 0 to 14, with 7 being neutral, anything under 7 acidic, and anything above 7 alkaline.

For an everyday example, the cells in the human body have a fairly neutral pH of about 7.4, like most living things. And just as we don't worry about keeping the pH inside our own bodies in check, the internal pH of a plant's cells isn't something you need to manage. Soil pH, however, is a top priority.

To get your head around soil pH, it helps to think of soil as a liquid. Many of the nutrients in soil are water-soluble and dissolve into it. That keeps nutrients mobile and gives the soil an overall pH.

Soil pH is set by the chemical compounds in the soil, whether acidic or basic. Common acid-forming elements in soil are hydrogen, aluminum and iron. Common base-forming ones are calcium, magnesium, potassium and sodium. As we always say, several of these are among the key macro- and micronutrients plants need to grow strong and healthy.

What pH Does Cannabis Need?

Cannabis needs a slightly acidic pH, around 6.0–6.5, to take up nutrients from the growing medium. That's why you should check and adjust the pH of your water in any grow. Keep in mind that a one-point change in pH means a tenfold jump in acidity or alkalinity.

Why does pH matter for cannabis?

A pH below 6.0 can trigger a calcium deficiency, which shows up as burned root tips and dark spots on the leaves. A pH above 7.0 causes iron deficiency, which leads to chlorotic (pale) leaves and yellowing between the veins. The wrong pH can seriously mess with the uptake of every primary and secondary nutrient cannabis needs to grow and flower well. The ones hit hardest are phosphorus, potassium, magnesium, calcium, iron and manganese.

How Do You Adjust the pH of Your Water?

To measure both the pH and the EC of your water, you use a dedicated meter. It's an essential tool for knowing exactly where your cannabis water stands on pH and EC, and it's what you'll go by to adjust both. To get accurate readings, factor in how you're watering and which adjustment products you're using.

Types of watering

Over the course of a grow, you'll usually alternate between plain-water waterings and waterings with the right nutrients for each stage. Each one calls for a few guidelines to get a correct pH reading. On top of those two, there's watering in hydroponic grows, which usually works a little differently.

Plain water

To check the pH of plain water, just let it sit in the reservoir for a few minutes and dip in the meter. Follow the product manufacturer's directions to bring the pH to the right level, checking with the meter as many times as you need.

Water with nutrients

If you're adding nutrients to your water, the best move is to measure the pH once they're fully dissolved, though it depends on each manufacturer's instructions.

Hydroponics

Hydro setups usually run off an automatic reservoir that holds about two weeks' worth of water. Here, it's best to use monitors that track your pH levels continuously.

Best pH for Cannabis in Veg and Flower

Best pH for the vegetative stage

Vegetative growth is one of the main phases in the plant's life cycle, and it's when it needs the most care and attention, since this is when it takes up the core macronutrients.

For maximum nutrient uptake, aim for a soil pH of around 6.5 in veg and check it every time you water, because water and fertilizer can both shift the pH of your growing medium.

During these first weeks, your veg fertilizer needs to deliver more nitrogen, potassium, phosphorus, calcium and magnesium, all essential for the growth you're after.

Some growers run veg a little lower, at 5.8 to 6, making the medium slightly more acidic for faster uptake.

Best pH for flowering

This is the phase where the goal is top-quality buds. That's why you need to keep an eye on nutrients, because they change from the previous stage.

As we mentioned, in veg all the nutrients go toward growing the plant, while in flowering all its energy goes into producing buds, which calls for more potassium and phosphorus.

For the ideal pH in flower, aim for a slightly higher 6 to 6.2, so uptake stays on point.

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