How To Mix Masterblend 4-18-38 Hydroponic Nutrient Solution

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Masterblend 4-18-38 is one part of a three-part nutrient solution specifically for hydroponic plants. Masterblend, with calcium nitrate (15.5-0-0), and magnesium sulfate (0-0-0), otherwise known as Epsom salt, gives hydroponic plants all the nutrients they need for each stage of growth.

Below is a step by step of how I prepare tap water for my two hydroponic systems:

Picture of partially buried storage totes

I am filling both othe these 55 gallon storage totes, each with 40 gallons of water

Preparing the Tank for New Solution

The first step is to unplug your pump if you are not using it to drain the system. Do this before draining your tank, otherwise you risk running your pump dry and damaging it. Even in an NFT system that is a continuous flow system, the pump can be turned off for a short time to mix a new batch of fertilizer. Then drain the nutrient reservoir. If you are starting with a new reservoir, give it a quick rinse with soap and water. Cleaning the tank between nutrient changes can minimize potential problems in your system, but since my reservoirs are partially buried I will only do this at the end of the season.


Since I am refilling the tank with fresh nutrients, I am pumping out the old solution before filling with fresh water. The old solution still has some nutrients, so I pumping it outside the greenhouse to water my in ground plants and lawn.

Note: Always plug into a GFCI (also called GFI) protected outlet! Water and electricity can be a lethal combination, and GFCI protected circuits help protect you from electrocution. All my outdoor outlets are GFCI protected.

Picture of Submersible pump

Here is the pump I use to pump out old nutrients- a Superior 1/4 HP supmersible utility pump. This has multiple uses around my house.

Pump out the old solution and use it to water outdoor plants. The pump will never completely get all the liquid out, just pump out the most you can.


Pumping out the old nutrient solution

Adding Water and Adjusting PH

After the old solution pumps out, add fresh water to the tank. These reservoirs are 55 gallon storage totes from Home Depot, and I fill them to 40 gallons. If you plan on using storage totes and do not bury them or otherwise brace the sides, they can bow heavily and will eventually fail. Plan on only using half or less of the storage capacity if using a tote above ground and unsupported.

As a side note, I don’t like the idea of a sight gauge on a nutrient reservoir. Even if I did, I buried this one so it wouldn’t do me a lot of good. So instead I created a dip stick to approximate how much water I’ve added. I made this one by dumping 5 gallons of water at a time in the tote, and scoring the dip stick at that water height (my dip stick is a scrap piece of 1/2″ pvc pipe). 40 gallons seems to be about the right amount for my tanks.

Storage tote full of water

Pretty close to 40 gallons of water

I am adding ascorbic acid to neutralize chloramine in the water. This is necessary because chloramine does not easily evaporate, or filter out via a charcoal filter like chlorine. Depending on where you live, your tap water can have chlorine, chloramine, or both. I add around 1/4 tsp of ascorbic acid to each reservoir and mix it up before anything else to neutralize the chloramine in the water. Ascorbic acid will also neutralize chlorine without having to filter or leave the water sitting to evaporate the chlorine. 

Water from my tap measures about 8.1-8.2 on the ph scale. To bring the water ph down to a more appropriate value, between 6 and 7, I use Ph Down.

Bottle of Ph Down additive

Ph Down ready for use

How much Ph Down should you use? I typically use about 50 ml of Ph Down per 40 gallons of tap water. This brings the ph of the water to around 6.5. Why don’t I take it down to 6.0? Adding the nutrients will take the ph down to around 6.2-6.3, and I will make any final ph adjustments after that if I think i need to. If I lower the ph of the water initially to 6.0, then adding nutrients will bring the ph to less than 6.0. The ideal ph for most vegetables is 6.0-6.5, so that is my goal for both nutrient tanks.

Ph meter displaying 6.6 ph.

The ph of the water after adding Ph Down hovered around 6.5-6.6 on the meter.

Adding the Fertilizer Blend

Now it’s time to start adding the nutrients. Measuring fertilizers (and most other things, even water!) by weight is always a best practice, it is more accurate than measuring by volume. Masterblend recommends the following amounts depending on the type of crop:

For lettuce and leafy greens add the following to 5 gallons of water:

  • 10 grams Masterblend 4-18-38
  • 10 grams calcium nitrate
  • 5 grams magnesium sulfate (Epsom Salts)

For tomatoes and other fruiting vegetables, add the following to 5 gallons of water:

  • 12 grams Masterblend 4-18-38
  • 12 grams calcium nitrate
  • 6 grams magnesium sulfate (Epsom Salts)

Forty gallons of water is equivalent to eight, 5 gallon buckets of water. So, I multiply the above numbers by 8 and get:

For lettuce and leafy greens add the following to 40 gallons of water to the left tank: 

  • 80 grams Masterblend 4-18-38
  • 80 grams calcium nitrate
  • 40 grams magnesium sulfate (Epsom Salts)

For tomatoes and other fruiting vegetables, add the following to 40 gallons of water to the right tank:

  • 96 grams Masterblend 4-18-38
  • 96 grams calcium nitrate
  • 48 grams magnesium sulfate (Epsom Salts)

For the left tote for the leafy greens, first the Masterblend goes in and is mixed fully:

80 grams of Masterblend

Then, 80 grams of calcium nitrate is measured out and dumped into the tote:

80 grams of calcium nitrate

mixing calcium nitrate in the fertilizer solution

Masterblend mixed in first, then calcium nitrate

Last, the 48 grams of magnesium sulfate is weighed and added to tote, and completely mixed in.

48 grams of magnesium sulfate

The same is done with the next tote, but with the increased amounts of nutrients:

96 grams of Masterblend

96 grams of calcium nitrate

48 grams of magnesium sulfate

After each part of the fertilizer is fully mixed into the water, plug the pump back in for duty. At this time I also check all the feed lines going into the buckets to make sure that they are flowing with no obstructions.

full nutrient reservior for NFT system

Fresh tank of nutrients running through the NFT system


storage tote reservoir for dutch buckets

Fresh nutrient solution for dutch bucket system. When pumping, the dutch buckets can hold a fair amount of solution, this needs to be accounted for when sizing the proper storage conatiner.

Everything is flowing like it should, so the lids are put back on the containers and the job is complete.

NFT system with nutrient totes underneath

Good to go!

Dutch buckets in greenhouse

Dutch bucket side of the greenhouse

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