Measuring, Analyzing, and Adjusting Hydroponic Nutrient Solutions

In an industry where precision is key, it is important to know how to interpret data accurately and to be able to use it constructively to drive decision-making.

One gray area that continues to come up is nutrient monitoring. This includes how to measure your nutrient solution, what to do with the data, and how to adjust the levels.

Because of the multiple measuring techniques and devices, as well as different units of measure that exist in the gardening space, you will have to know how to convert one unit to another fairly often.

Many nutrient companies list their feed schedules per their own standard unit of measure, so if it doesn’t line up with the units you are used to or your device does not measure in those units, some amount of conversion will be necessary. Read on to learn how to measure, analyze, and adjust your nutrient solution no matter which units are thrown at you.

Electrical Conductivity

Perhaps the most universal and favored method of ion measurement, electrical conductivity (EC), is the go-to method for many scientists and researchers, especially in the US. Although not solely designated for nutrient monitoring, EC is a measure of electrically charged nutrient ions in a solution reported in the unit Siemens (S).

There is a positive correlation between the ion concentration (or salt content) of a solution to the EC; as one goes up, so does the other. Deionized water and reverse osmosis water (RO) theoretically should have ECs of zero, as both undergo a process to eliminate the contents of the solution that produce a charge. In hydroponics, the EC of most nutrient solutions ranges from 0.0-1.6, depending on stage of growth.

Certain recirculating hydroponics applications facilitate lower ECs, depending on other factors including dissolved oxygen content (DO) and the frequency of nutrient solution delivery and changeout.

Soil, coco coir, and other drain-to-waste applications typically require higher ECs than hydroponics. This is due to the fact that the plant roots in drain-to-waste applications aren’t continuously exposed to nutrient solution, so the nutrient strength at delivery must be slightly elevated to ensure proper absorption potential.

Total Dissolved Solids / Parts Per Million

Despite the fancy lingo, all of this essentially means the same thing. Total dissolved solids (TDS) refers directly to the amount of solute (i.e. salt/solids) that is dissolved in a solvent (i.e. water/liquid). It is measured in parts per million (ppm), which comes from another unit of measure, milligrams per liter (mg/L).

When scientists produce a known solution with a calculated chemical analysis, they typically measure out a certain mass of solute in milligrams and dissolve it into a known volume of liquid in liters. One liter of water weighs 1,000 grams, or 1,000,000 milligrams.

Therefore, one milligram of solute dissolved into one liter is one part per million. So, technically, mg/L and ppm are one in the same when deciphering a conversion calculation, and the number can be considered directly transferable and equivalent (i.e one mg/L = one ppm).

Most nutrient solutions you see in hydroponics measure in between zero and 1,000 ppm, primarily dependent on stage of growth. (As a quick and interesting reference, seawater typically measures in at 35,000 ppm, give or take depending on region.)

It is important to note which units your meter displays, as popular brands offer the ability to switch between ppm 500, ppm 700, EC, and others. Converting from ppm to EC and vice versa is very easy; you simply divide your total ppms by the scale you are using to get EC.

You can also multiply EC by the scale of your ppm measurement device to get TDS. For example, if you have an EC meter, but you are growing with nutrients from a manufacturer that lists its feed schedule on the ppm 500 scale, you can simply take your EC reading and multiply by 500.

So, if your nutrient solution measures in at 1.2 S on your EC meter, you would multiply that by 500 to get 600 ppm. From there, you can compare your results with that of your feed schedule to determine whether to dilute, intensify, or leave your solution as is.

American-based manufacturers typically use the ppm 500 scale in their literature, whereas European manufacturers tend to use the ppm 700 scale. When in doubt, it is best to verify before calculating and subsequently dosing nutrients.

Molarity

Technically speaking, molarity (mmol) indicates the number of moles of solute per liter of solution. Again, seemingly redundant language aside, it really is just another way to measure the concentration of a solution. First, moles are calculated as a fraction of actual mass of the solvent over the atomic mass of that molecule.

So, for example, if you have 100 grams of solute that you know is made up of magnesium sulfate (MgSO4), you would divide 100 by 120 to get 0.83 moles. The figure 120 comes from the molecular mass of magnesium sulfate, which is calculated from the atomic mass of magnesium at 24, sulfur at 32, and four atoms of oxygen at 16 apiece, for a total of 120 g/mol. From there, molarity is calculated from moles divided by volume in liters and displayed in mmol.

Dosing

If you are dosing your nutrients the old-fashioned way by hand, you will need your nutrient monitoring device on deck continuously monitoring the solution. When starting with a fresh batch of reservoir water, fill it up to just below the total fill line, leaving five to 10 percent of the capacity as room to add more water should your solution become too concentrated.

Typically, nutrient manufacturers will suggest a dose rate to theoretically achieve the proper strength when in solution. If you have multiple parts to add, dose them out in a pipette, garden syringe, or volumetric flask at the suggested rate on your feed chart.

Add each part one at a time, being careful not to mix the separate parts in undiluted form, as a precipitate will form starting at the micro level, which represents the early stages of nutrient lockout.

Eventually, this would become a visible mass of rock that is obviously unabsorbable to plants. Once you add the recommended doses, take a reading with your meter. If the target strength is within acceptable range, you’re all set. If the reading comes in too hot, this is when you would add more water slowly while observing the reading on your meter.

Once reached, your solution is now ready. On the flipside, if your solution measures too low, you will want to slowly add more nutrient until the target strength is achieved.

Understanding how to read your meter, take a measurement, and adjust the levels in unison is important to boost productivity. Maximizing potential lies in the nitty gritty, and a prerequisite is simply understanding the fundamentals.

Find a way that you understand, stick with it, and be able to convert it inside and out so you can start to get to know your plants on a different level.