pH for Cannabis: How to Measure and Adjust Your Water and Soil

pH measures how acidic or alkaline your water and growing medium are, and it decides whether your cannabis can actually absorb the nutrients you feed it. In soil, weed takes up nutrients best between roughly 6.0 and 7.0; in hydro and other inert media, the sweet spot is lower, around 5.8 to 6.3. Air, water, sun and soil all shape how your plants develop and how good the final product is, and if any of them is even slightly off, you get weak plants that are more likely to catch diseases or get hit by pests.

That's why this article zeroes in on pH in cannabis growing, specifically the pH of your irrigation water.

What is pH, and why does it matter for plants?

Measuring pH tells you how acidic or alkaline a liquid, or a solid dissolved in liquid, is. The pH scale runs from 0.0 to 14.0: anything below 7.0 is acidic, anything above 7.0 is alkaline and anything around 7.0 is considered neutral.

Why you need to check the pH of your water

Every plant needs fertilizer to grow healthy. It needs the macronutrients nitrogen, phosphorus and potassium, plus micronutrients and minerals like calcium, magnesium, iron, copper and many more. Without access to these nutrients, plants develop deficiencies and other serious health problems.

The catch with cannabis is that it can only absorb nutrients within a narrow pH range, between 6 and 7 when grown in soil. When pH readings fall below or above that, the plant can't take up nutrients even if they're right there in the medium, which leads to deficiencies caused by "nutrient lockout". So it pays to know the pH of your irrigation water to make sure your plants can actually absorb everything you give them.

Even if you learn to adjust the pH every time you water, deficiencies and lockouts can still happen, but the odds are much lower, and if they do show up, it's way easier to get things back on track and have your plants looking green and healthy again.

What's the ideal pH for cannabis plants?

You don't always have to water at the exact same pH. The ideal pH of your irrigation water shifts slightly depending on the strain, the stage of growth (germination, veg, pre-flower and flushing), the growing medium and whether you're running an organic or a mineral (synthetic nutrient) grow.

Generally speaking, you can grow a cannabis plant at a constant pH, as long as it stays between 5.5 and 7.0.

That'll get you decent results, but you won't unlock the strain's full potential or get the most out of your nutrients. You may also run into nutrient problems along the way.

Ideal pH for cannabis in hydroponics, aeroponics or inert media:

• First weeks of growth: pH 5.8-5.9
• Pre-flower stage: pH 6.0-6.2
• Full flowering stage: pH 6.0-6.3

Ideal pH for cannabis in peat-based mixes or soil:

• First weeks of growth: pH 5.5-6.0
• Pre-flower stage: pH 6.0-6.2
• Full flowering stage: pH 6.2-6.5

How do you measure soil pH?

One of the most common ways to measure soil pH goes like this:

• Take a soil sample and dry it at about 40 °C (104 °F) for several hours.
• Sift the sample through a 2 mm sieve.
• Take part of it and weigh out 20 g of soil.
• Add it to a beaker with 50 ml of distilled water.
• Stir the mix for 2 minutes.
• Let it sit for 15 minutes.
• Measure the pH of the supernatant, meaning the clear liquid on top. You can use test strips or a digital pH meter.

How to measure the pH of your water

• pH paper. The classic litmus paper test relies on a plant-based
  dye that changes color. Other dye indicators are more sensitive to pH.
• pH meters. A two-electrode system: a glass electrode filled with an
  acidic solution, plus a reference electrode.
  When you dip it in water, an electrical potential forms between the acid and the
  water, and the meter measures it. That reading is your pH.
• pH test kits. Analytical colorimeters based on an indicator dye that changes color according to pH.

How do you adjust the pH of your water?

You'll need a pH meter, plus pH Up and pH Down solutions to raise or lower it.

pH Up and pH Down solutions

Products for raising or lowering water pH are made from acidic or alkaline compounds, which can be organic or mineral. Depending on the brand, you'll also find separate pH adjusters for veg and for flower.

What's in mineral pH adjusters

• Nitric acid: lowers pH, and it's ideal for the vegetative stage because of its high nitrogen content.
• Phosphoric acid: the go-to pH Down for the flowering stage because of its high phosphorus content, though you can also use it in veg.
• Potassium hydroxide: raises water pH, and thanks to its high potassium content you can use it in both veg and flower.

What's in organic pH adjusters

• Humic acids: raise pH and can be used throughout the grow, though it's best to stick to the vegetative stage, since some growers believe humic acids can cut THC production.
• Citric acid: lowers pH and can be used safely throughout the whole grow.

The main difference between organic and mineral pH adjusters is that mineral ones harm the soil's microlife, so you'll need to rebuild it after each use.

Organic adjusters, on the other hand, don't cause any trouble for microbial colonies, although you'll need a bit more product than with mineral adjusters to hit your target levels.