Winter Soil Testing, Prep & Replenishment Guide for Your Grow

You’re about to read a handy guide on soil replenishment, which includes:

  • Testing your soil
  • Altering your pH for high nutrient absorption 
  • Adding nutrients

If you’re more interested in composting best practices, you can jump to  Winter Soil Preparation Methods for Your Grow or Garden. Otherwise, keep reading. 

How to Test Your Soil for Nutrient Uptake Problems

One of the most important things you can do for your soil grow is check the pH. A pH problem at the root level is usually the culprit whenever there are nutrient deficiencies in the plants. 

A pH-induced nutrient deficiency doesn’t mean the SOIL is missing nutrients. It means the PLANTS are. That’s because pH affects uptake, which is the ability of plant roots to absorb nutrients. 

Cannabis, and many other types of plants, prefers slightly acidic environments. A pH of 7 is considered neutral. Anything lower than 7 is acidic. 

The ideal pH for your soil grow is between 6 and 7. Some growers will tell you not to go over 6.5. For hydroponic and other soil-free grows, the ideal pH is slightly lower. 6 is usually ideal. 

You gotta pre-empt nutrient deficiencies by testing and adjusting your pH, and here’s how:

A great and affordable “all-in-one” solution is the  3-Way Meter by Active Air. It costs far less than $20, and it measures pH, moisture, and light levels. It will accurately measure pH levels that are between 3.5 and 8. If you just need the pH and/or moisture reading(s), you can go with  Active Air’s 2-Way Meter for about $11. 

How do you use it? You stick it into your soil at various points, and it gives you the pH levels. 

IMPORTANT: Nutrients affect pH, so you want to test your pH only after you have the desired nutrient levels in the soil. 

How to Adjust Your Soil’s pH


PH Down Concentrate by X Nutrients is a powerful formula for lowering the pH of your soil or hydroponic grow. You have to be careful not to use too much. You typically only need to mix 1mL per gallon of water to lower your pH from, say, 7.5 to 6.5. You can get a whole quart (almost 1,000mL) of it for $17 from Grow Supply Shop. 


Its counterpart for reducing acidity is called  pH Up. As of January 8th, it’s out of stock. Most pH Up formulations are weaker than pH Downs. To raise your soil pH from 5.5 to 6.5, for example, you may need to add 2mL or 4mL per gallon of water. 

An alternative to pH UP is garden lime, also known as limestone. You can normally find that locally at a gardening store. 

Go Easy on that pH-Altering Stuff

You may need to experiment by adding a little bit of this enhanced water to your soil at a time and then testing it. Be conservative if you’re new at this. You’ll get the hang of it in time. 

And don’t forget to mix your concentrates into your water evenly by stirring or shaking. 

You’ll learn how to balance your soil nutrients below. Always remember: pH tests are invalid before you do this. 

An Awesome, But Less Precise, Alternative

Another way to balance pH is with humic acid. It’s a biostimulant that helps plants absorb minerals. It also lowers or raises the pH by neutralizing the soil. A super potent humic acid solution is NPK Raw Humic Acid. It comes in two sizes at Grow Supply Shop. 

How to Test Your Soil for Nutrient Imbalances

You never want your soil to have too little of one important nutrient. That can sink your entire grow. 

You also don’t want too much of certain nutrients. That can affect the end user’s experience when they consume your crop. A lot of “flushing” before harvest is only necessary because the growers overfortified the soil. 

The major “macronutrients” your soil needs are usually represented by the acronym NPK. That stands for nitrogen, phosphorus, and potassium. Your soil needs these in large amounts. 

There are lots of micronutrients as well. Many of them are necessary, even if you need them in lower quantities. 

To test for nutrients, you don’t have many options. Whichever one you use, you need a REPRESENTATIVE sample of your entire grow. That means collecting soil from various depths, up to about 6 inches, and collecting soil from a lot of different places. You must also remove debris, like twigs and wood chips. 

Eventually, you want to collect about one or two cups of representative soil for your sample. Make sure your collection tools and buckets are clean so they won’t contaminate the results. 

What are your nutrient soil testing choices?

  • Use a home testing kit
  • Send a sample to a lab

Home testing kits will likely only test for levels of the Big 3: nitrogen, phosphorus, and potassium. They will also test pH levels. 

A lab may give you more comprehensive information about your soil and provide tips for getting just the right nutrient balance. Your best bet for super-affordable options would be to search in a search engine for a cooperative extension office near you. There is probably one nearby. If not, you can ship your sample. 

Don’t Let “Nutrient Levels” Fool You…

Whichever path you take, one thing to keep in mind is that nitrogen levels can be misleading. Nitrogen is only freely available to the extent that microorganisms are “freeing” it for the plants to use. Without them, your plants won’t use it. Make sure to have plenty of organic matter in your soil to encourage microorganisms. 

If you want to “cheat” and stimulate microbial activity in your soil, you can try Microbe Brew® by Bush Doctor®. It’s known for boosting microbial colonization at the root level. 

One great thing about sending a sample to a lab is that they will analyze organic matter and microbial activity for you. On top of that, their employees will be a lot smarter than a home NPK + pH testing kit. They may even give you customized info on how to remove nutrients as necessary. 

Nutrient Formulas from Grow Supply Shop

NPK Industries provides the highest concentrations of N, P, and K on the market. 

Humboldt County’s Own makes Deep Fusion Micro/Soil, which supplies all the micronutrients you need plus some nitrogen. 

FoxFarm makes one of the best nutrient kits on the market. It’s called Dirty Dozen® Starter Kit (for Soil & Hydroponics). You not only get all the major micro and macronutrients. You also get a special “brew” that encourages microbe growth and nutrient absorption. It’s currently on sale for just under $140. They don’t skimp with tiny sample bottles. You get a lot of nutrients and bud-boosters in this kit. 

Ask a Growing Expert

Do you have questions about your soil or any others about your current or future grow? We aren’t just retailers. We’re growers. Call or text us. 
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