Soil Slurry: Testing the pH of Your Soil for Cannabis Plants

A soil slurry is soil that has been mixed with water to create a liquid. Soil slurry is used in soil testing labs because soil retains its pH when it’s wet, unlike soil samples dried out of the oven. This gives you an accurate reading of what your soil should be at. The ideal range for soil pH varies depending on the type of medium you are using to grow plants in, but generally speaking anything between 6-7 is optimal and this holds true for cannabis too. If you notice your soil pH falls outside this range, there are steps you can take to correct the problem.

What does soil pH mean?

The soil pH is a measure of the soil’s acidity or alkalinity on a scale from 1-14. Soil with a low pH means it is acidic, and soil with a high pH means it is alkaline. There are different soil types that have different optimal ranges for soil pH, but in general, most plants prefer neutral or near-neutral soils. The soil slurry test will give you the exact measurements for your soil so that you can adjust it to be in an optimal range.

Why is soil pH important for cannabis plants?

The soil’s pH can affect the amount of nutrients that are available to your plants, and therefore it affects how well they grow. Soil with a low soil pH has too much acid in it for most plants, while soil with a high soil pH has too much alkali in it. Different plants prefer different soil pH levels, so it is important to test your soil and adjust it to be in the optimal range for the plants you are growing.

What equipment do I need to conduct a soil slurry for cannabis plants?

A soil slurry is a great way to test the pH of your soil for your grow or garden. It is relatively easy and most of the equipment should be available at your local garden centers. To conduct a soil slurry you will need:

1. pH pen or pH strips: you get what you pay for. We highly recommend splurging on a pen as the cheaper pens found on Amazon tend to break easily and aren’t as accurate
2. A cup to put the mixture in: red solo cups are fine but if you have a beaker available it is nice to know how much of everything goes into the solution
3. Soil from your garden or grow: only a few spoonfuls are necessary
4. A spoon or something to stir the mixture
5. The water you use to water your plants: distilled or reverse (RO) osmosis water is ideal, but if you use tap water that is fine too as long as you pH beforehand

How do I test soil pH with a soil slurry for my cannabis plants?

To test the soil’s pH using a soil slurry, take an actual site soil sample from your garden using a soil probe, shovel, spoon or even simply scraping the cup in the dirt. Put the soil in a cup, beaker, or container. If you are using a red solo cup, 4-5 tablespoons of soil are all you need which is about a quarter of the cup. If you are using a beaker, this is roughly 4 oz. Add distilled or RO water to the soil until you have added roughly the same amount as the soil or up to two times as much of the soil which will create an aqueous solution. Then stir it with a spoon or something similar to get all parts of the soil wet and mixed together. Let the mixture rest for a few minutes. Repeat stirring and let it rest 4 or 5 times. When finished, let the mixture rest for 15 minutes allowing the organic matter to settle at the bottom and finally take the pH of the solution.

Please note that pH strips are not as accurate which is why we urge you to get a quality pH pen. If you have a pH pen, please calibrate before every use to ensure the readings are correct.

Check out our video on how to test the pH of your soil using a soil slurry.

What is the optimal range of pH for different mediums for cannabis plants?

Different mediums require different optimal pH ranges for cannabis plants. It is important to get your water within these pH ranges so your plants can thrive. If you are not within these pH ranges then it could cause a nutrient lockout, where your plant is unable to uptake certain nutrients through its root system. Let’s take a look at the different optimal ranges below.

• Soil
  • 6.0 – 7.0 pH range
  • 6.5 pH is optimal
• Coco Coir
  • 5.5 – 6.0 pH range
  • 5.8 pH is optimal
• Hydroponics
  • 5.5 – 6.0 pH range
  • 5.8 pH is optimal

 

What happens to my cannabis plants if the pH is not in the optimal range?

If the soil pH is not in the optimal range for your cannabis plants, it can cause deficiencies in your plants. Some signs that you have a pH imbalance are:

• Yellowing leaves
• Brown spots on leaves
• Wilting or drooping leaves
• Slow growth rate for your plants

These can be caused by a nutrient lockout, which is caused by soil pH not being within the optimal range. Nutrient lockout occurs when a cannabis plant isn’t able to uptake nutrients from its root system due to soil pH being too high or low. This can happen if there is an abundance of certain minerals in your soil, causing other essential elements to become locked out and unable to be absorbed through the roots and taken up by the plants.

How do I fix my soil pH being too high for cannabis plants?

You can adjust soil pH by adding soil sulfur, which will help lower the soil’s pH and turn it into a more balanced state. We recommend using organic soil sulfur in your soil mixture to avoid burning plants with chemical soil sulfur, also called sulfate. Start by adding ¼ teaspoon of soil sulfur per gallon of water and continue to add more until you reach the desired pH level. Be sure to mix it in well with the soil and then wait a few weeks for the plants to show signs that they have taken up the nutrient before making any further adjustments.

You can always adjust your water by adding pH up or pH down, which adjusts its pH to the desired levels. Always make sure to pH your runoff to make sure that the water and soil together are in the optimal range.

How do I fix my soil pH being too low for cannabis plants?

If the soil pH is too low, you can add limestone to your soil in order to bring up the soil’s pH levels. Start by sprinkling a light dusting of ground limestone over the surface of the soil and then water it in well. Be sure not to use hydrated lime as this will raise the soil pH too high. As stated above, you can also adjust your water using pH up or pH down while measuring the pH of the runoff to ensure it is in the optimal range.

When should I flush my cannabis plants?

You should also flush your plants if you notice a nutrient lockout as this helps with soil detoxification. This will help remove any built-up salts or organic compounds that may be present in the soil or medium and will also help to keep your cannabis plants healthy.

Signs that you need to flush your plants are:

• Leaves that are crispy or brown
• Plants that are wilting

You can flush your plants by watering them with pH’d water and running two to three times the normal amount through the medium. There should be plenty of runoff, which is the excess water that comes out of your pots, and be sure to continue to measure the soil’s pH from the runoff and adjust accordingly.

How do I keep my soil PH in the optimal range for cannabis plants?

Now that we have discussed soil pH and how to keep it in the optimal range, let’s talk about some ways you can help ensure your soil will stay within this range.

The easiest way of soil pH testing is by using a soil slurry test (go figure!). The slurry method is a simple test that measures your pH levels by creating the slurry solutions detailed above so there isn’t any guesswork involved.

Another way to help keep your soil pH in check is by using a soil meter which is another soil pH tester. Soil meters are easy-to-use devices that measure the moisture and pH levels of your soil. They are compact, easy to use, and can be used by both newbies and veterans.

Summary

pH is the measurement used to determine if your medium is acidic or basic. If you are unsure of the pH of your soil, a soil slurry is a great way to measure it. If your soil pH is too high, you can adjust it by adding soil sulfur, which will help lower the soil’s pH and turn it into a more balanced state.

To raise your soil’s pH levels, you can always add limestone to it. You can also adjust your water using pH up or pH down while measuring the pH of the runoff to ensure it is in the optimal range too. If you flush your plants, you will help remove any built-up salts that may be present in the soil or medium and will also help to keep your cannabis plants healthy. You can help to ensure your soil will stay within the optimal pH range by using a soil slurry test kit or soil meter.

Thank you for reading our article, should you have any further questions we recommend reading our articles on how to pH your water, pH meters, soil, coco coir, and hydroponics.

To learn more about growing cannabis at home consider checking out our guides on: Getting Started, Troubleshooting, and Optimizing your Grow or search our Knowledge Base.

We strive to help your plants thrive.