How to create nutrient solution for indoor hydroponics

Arguably, one of the most important aspects of successful indoor hydroponics is the quality of nutrient solution that you use. Since hydroponics, by definition, replaces soil with water, the water must contain the necessary nutrient molecules that are usually present in soil. Specifically, they are nitrogen, phosphorous, potassium, calcium, magnesium and sulfur, commonly called macronutrients; as well as micronutrients such as iron, manganese, copper, zinc, molybdenum, cobalt, boron and chlorine. Interested in more about how nutrients influence plant growth? Read our overview here.

In order to allow the plant to absorb these nutrients successfully, the nutrient elements need to be dissolved in water at an appropriate concentration, with the roots in contact with the solution. As a grower, your responsibility is to ensure that the needed nutrients are dissolved in and available for your plants to absorb.

There are many commercially available fertilizers that include the necessary macro and micronutrients, and all that needs to be done is to mix these with water. The tricky part is that once the fertilizer is dissolved, it is impossible to accurately judge concentration with the naked eye. For these reasons it is recommended that you invest in an EC meter to ensure that your water concentration is appropriate for the plants you are trying to grow.

Things you will need (in the order you will need them)

  • Mixing bucket (2x preferred)

  • Water source

  • Weight scale (or measuring spoon)

  • Nutrient solution (typically provided as a two-part mix)

  • Total dissolved solids EC meter

Mixing bucket

The exact type or shape here is not critical, as long as you have enough capacity. I recommend something that can hold up to 10 gallons. Note that many nutrient solution recipes call for 5 gallons, so to keep things simple that level of capacity would be helpful. It would be great to have two of these if possible. I use two SAMLA boxes from IKEA because they happen to be just the right size and are relatively low cost. Pictured below is from the IKEA website in white.

Larger tubs can work too, but it can be difficult to fit into your sink or underneath your faucet, and once you get to 10+ gallons, it can get very heavy quickly (1 gallon = 8.3 lbs!)

Water source

The primary requirement here is to ensure that your water source is relatively pure and has an appropriate pH for indoor hydroponic use. In general, you will want to ensure that the pH is kept between 5.0 and 7.5 (slightly acidic).

A quick way to cheaply test is using pH paper strips. If you find that your water source has a pH level that is too high or low, you can find special solutions that will adjust pH for you. (I have not personally tested this product).

Luckily, my source water is consistently around 6.0 - 7.0, so I have not needed to adjust my pH levels. The fertilizer nutrients I discuss below have not shown to significantly alter pH levels, but you will want to check both before and after you mix your nutrients in.

Next, find out, or measure the total dissolved solids in your water source. You can do this with a TDS EC meter (discussed below) or by making some general assumptions based on your local water utility's water quality report. Most municipalities should have TDS levels that are between 100 - 500 ppm. See the nutrients section below for why this is important.

Weight scale (or measuring cup)

When mixing nutrients, directions for how much of each nutrient to use are usually given in weight units (e.g. oz or grams). It is therefore most accurate to use a digital weight scale to determine those amounts.

In reality, however, I have found that volume is a close enough approximation, and as a cost saving measure have favored a simple teaspoon measurement method. Keep in mind the assumption here is that the density (mass per unit volume) is constant across all fertilizers, which may or may not be the case - so you may want to try some trial and error tests. In the long run, however, as long as you are consistent, you should not see any issues.

Nutrient Solution

For almost all of my hydroponics, I look for a primary nutrient solution mix with an NPK of 4-18-38. This means that there are 4 parts nitrogen, 18 parts phosphorous, and 38 parts potassium, per 100 parts. Many nutrient solution mixes will claim to be developed for a particular type of plant (e.g. tomatoes as pictured below) but I have found that many will work across different plant types. Unless I mention otherwise, all of my plants use the same formula discussed later in this post.

Be sure that the fertilizer includes additional micronutrients as well.

For the secondary nutrient mix, I use calcium nitrate and magnesium sulfate (epsom salt). This is necessary to supplement the macronutrient needs that are not covered by the primary N-P-K nutrient mix.

The products I use are all dry - i.e. they are in powder or pellet form. I don't have experience with anything in concentrated liquid form, but I haven't yet found the need to do so.

I use a ratio of 2:1:1, between primary : calcium nitrate : magnesium sulfate. I mix these in separate mixing tubs and ensure full dissolution - i.e. I don't see any of the powder floating or collecting at the bottom of the mixing tub.

If you see nutrient powder settling at the bottom, it could mean that the concentration is too high and the water cannot physically hold any more of the nutrient in solution. If this happens, simply add water so that more of the nutrient solution can dissolve. Keep in mind that it is fine to err on the side of too much nutrient concentration, as you can simply dilute it later with water. What is critical, however, is that you keep the balance between primary, calcium nitrate and epsom salt at a constant.

Once the individual nutrients are fully dissolved in their respective tubs, mix them all together in a single tub. You may want to use your hand to stir the nutrients, and allow it to sit for a couple minutes. Again, it is important that all of the nutrient mix is combined in a single tub, and mixed well. This is to ensure a balanced nutrient mix.

Total dissolved solids EC Meter

In addition to the N-P-K ratios we discussed above, we also need to know the ratio of water to nutrients. Once you begin to dissolve the nutrient mix into water, it will become invisible, and you will be left with clear liquid. The only way to be sure how much nutrient molecules are dissolved per unit of water, an electrical conductivity (EC) meter is used. These devices work by measuring how much electrical current passes through the nutrient solution, since the conductivity of a solution increases as the concentration increases.

For indoor hydroponic use, a basic EC meter can be bought online for less than 20 dollars. They are very easy to use - simply dip the sensor into the solution and the LCD will display the concentration in PPM. The photo below shows a measured concentration of 673 PPM (parts per million). This means that for every 1 million water molecules, there are 673 dissolved molecules that are not water molecules.

Can you get by without one? In theory, yes, and perhaps for beginners who want to try indoor hydroponics without a big up-front investment, you could estimate rough concentrations based on what the packaging tells you and then extrapolating from there. However, it is always good to be able to quantify and confirm that the amount of nutrients you thought you were feeding your plants is correct, and can help you become an even better grower.

The final step

Your nutrient solution is almost ready! Use the EC meter to determine what your final concentration is, and subtract the source water total dissolved solids you measured (or inferred) prior to mixing your nutrients in.

Then, determine your target PPM level based on what you are growing. In my experience, lettuce and pepper plants don't need much (450 - 900 PPM) while cabbage, mizuna and bok choy can tolerate a higher nutrient concentration (750 - 1750 PPM). I hope to have a comprehensive chart / list soon.

Hopefully you ended up exactly at, or above your target PPM. If you are above, simply add more pure source water until the nutrients are diluted enough to reach your target PPM.

That's it! It's a relatively straightforward process and will become easier after multiple attempts. As long as the water is at a reasonable temperature (roughly room temperature) your plants should be ready to absorb the nutrient solution.


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