How to Optimize the Strength of Hydroponic Nutrient Solutions

Light is the major environmental variable that drives photosynthesis in plants. However, if temperature, relative humidity, CO₂ concentration, mineral nutrients, or media-moisture levels are outside of the optimal range for the plant species you are growing, photosynthesis (and subsequent plant growth) will be limited.

There is a principal of limiting factors that must be considered when cultivating plants, and understanding how to properly measure and control each variable is critical to the success of any controlled environment plant growth facility.

Therefore, taking a holistic approach to managing environmental variables is recommended to optimize growing conditions in a controlled environment. Measuring and adjusting the strength of your nutrient solution will be the main topic of this article, but we will also discuss how environmental variables will impact water and nutrient uptake.

EC vs. TDS Meters

It goes without saying that providing adequate mineral nutrients to your root zone is critical to produce high yielding crops. Nutrient deficiencies or toxicities are a problem that most growers have dealt with at least once, and these nutrient imbalances can significantly impact the yield and quality of a crop.

Optimizing the strength of your nutrient solution based on the stage of crop growth and your environment is an important step to avoid nutrient problems. There are two critical steps you can implement to optimize the strength of your nutrient solution. The first step is to accurately measure the concentration of your nutrient solution being applied to your crops, and the second is to monitor the amount of soluble salts present in your root zone.

Most fertilizers used for cultivation are composed of mineral salts that conduct electricity when dissolved in a solution, and this conductive property allows growers to accurately measure the strength of a nutrient solution. The higher the conductivity, the higher the concentration of dissolved salts present in a solution.

Electrical conductivity (EC) or total dissolved solid (TDS) meters are the two most commonly used pieces of equipment to measure the concentration of nutrients in a solution. However, TDS meters (commonly referred to as parts per million meters) are really EC meters in disguise. TDS meters measure the EC of a nutrient solution then use a conversion factor of either: 500, 640, or 700 to estimate the ppm of a solution (depending on the brand of the meter).

Whether you are using an EC or TDS meter, the steps used to measure your nutrient solution are the same. You will first measure the water that you will be mixing your nutrients with to see if your source water contributes to the overall conductivity of the solution.

You will then mix your nutrients into the solution and measure the overall conductivity of water and nutrients combined (if you had any charge in your source water, you will need to subtract that value from the reading if you are trying to achieve a desired EC or ppm).

Depending on your irrigation methods (hand watering, drip, ebb and flood, etc.) and irrigation frequency, the strength of your solution will need to be adjusted to maintain the optimal level in the root zone for the crop you’re growing. While it is very important to know the strength of the nutrient solution applied to your crop, it is equally (if not more) important to monitor the amount of soluble salts in your root zone.

A simple method used to measure the conductivity of your media is called the pour-thru method. The first step of this method is to irrigate your crop with your nutrient solution until the media is completely saturated.

Wait 30 minutes, then place a saucer below the container and pour distilled water over the surface of the media until you collect a sample large enough (about 50 milliliters) to be able to submerge the tip of an EC or TDS probe.

This reading will indicate whether you need to increase or reduce your nutrient solution concentration, or if you need to flush the root zone with pure water to remove soluble salts.

Environmental Variables that can Affect Hydroponic Nutrients

Several environmental variables will impact how fast water and mineral nutrients are taken up by a plant, including but not limited to: light intensity, temperature, and relative humidity. As light intensity (µmol/m²/s) is increased, the rate of growth also increases and results in a greater demand for nutrients.

However, increased light intensity also increases the rate of transpiration, which will cause you to irrigate your crop more frequently. If you are feeding a high rate of fertilizer at each irrigation, this can cause soluble salts to build up in the root zone, resulting in increased osmotic pressure that will make it difficult to for your plants to uptake water and nutrients.

Any time you have increased osmotic pressure in the root zone, your growth rate will decrease and yield will be sacrificed. Increasing the air temperature or reducing the relative humidity in your controlled environment will also increase transpiration rates, which will influence your irrigation frequency.

The bottom line is this: anytime you are changing environmental variables, it is extremely important to monitor the EC of your root zone to ensure you do not run into issues.

There are hundreds of different fertilizer brands on the market, and whichever one you decide to use, make sure it provides balanced levels of macronutrients and micronutrients necessary for the crop you are growing.

EC and TDS meters are great tools used to check the strength of your nutrient solution, however, if you want to know the composition of specific nutrient salts that are contributing to your solution or media you will need to have a chemical lab analysis done.

Different fertilizer salts contribute to the overall EC differently, and certain nutrient components like urea will not contribute to the overall EC until its converted into nitrate (NO₃).

Plant nutrition is just one important variable to consider when making adjustments to optimize your controlled environment.

To learn more about optimizing environmental conditions, check out the free Fluence Bioengineering High PPFD Cultivation Guide.