Hydroponics Primer: The right mix

The etymology of the word ‘hydroponic’ is hydro (‘water’) and ponos (‘to work’). The role of water in hydroponic production systems should not be underestimated. When we combine our water source with fertilizer, we create the nutrient solution — the life blood of hydroponic production systems. In water-culture systems such as the nutrient-film technique (NFT) or deep-flow technique (DFT), the nutrient solution is the substrate for roots. (For more on these systems, see the Hydroponic Production Primer article in the April 2016 issue.) With slabs or Dutch buckets, because there is a small volume of soilless substrate such as rockwool, coconut coir or perlite, plants are irrigated so frequently that the nutrient solution has a dominant role in the root zone. This is the next installment in the Hydroponic Production Primer article series, in which we are going to focus on water, fertilizers and the nutrient solution.

The importance of water

One of the most exciting advantages to hydroponic production is the ability to capture and recirculate water, leading to a nearly 95 percent reduction in water use per acre compared to traditional field production. However, the recirculating nature of water in many hydroponic systems is one of the primary reasons why water quality can be so important for effective and efficient hydroponic production.

When we talk about water quality, one of the first things that come to mind is pH. The pH is the concentration of protons (H+) in the water that can affect mineral nutrient availability. However, alkalinity can have an even greater effect on nutrient availability because when the alkalinity is high, it buffers the ability to reduce pH and can make it harder to stay between 5.5 and 6.0, which is a pH range that is suitable for many crops.

In addition to the pH and alkalinity of water, it is important to know the individual nutrients and ions present in your water source. For some nutrients, it is a matter of knowing what concentrations are in the water so that additions from fertilizers may be reduced. Hard water serves as a great example. Compared to other water sources, hard water contains elevated concentrations of calcium and magnesium. These concentrations can count towards the nutrients that are provided to plants and, therefore, concentrations of calcium and magnesium in fertilizers can be reduced when being added to hard water.

While there can be some elements in water that do not necessarily negatively impact crop growth and development, some things that can be found in water can hinder crop growth and development.

For example, elevated concentrations of sodium and chlorine are undesirable for hydroponic production. With recirculating water culture, as water and nutrients are taken up and replenished in the system, undesirable elements are not taken up and their concentrations rise to the point that nutrient solutions must be partially or completely replaced.

Understanding mineral nutrients

Just like any horticultural crop, mineral nutrition impacts growth and quality of hydroponic food crops. Nutrient deficiencies can reduce growth and limit yields, while luxurious nutrients can also enhance vegetative growth and diminish yields of fruiting crops. To provide mineral nutrients to crops in hydroponic systems, there are three main methods of fertilization: 1) single-bag fertilizers, 2) two-bag mixes, or 3) individual compounds. Single-bag fertilizers have one major advantage compared to other fertilizers; they are very simple to use. Whether mixing fertilizer solutions to their final concentrations or making a stock solution for injectors, you only need to make additions from one bag. However, one drawback for the single-bag mixes is that the ratio of all the different elements is fixed; increasing or decreasing the fertilizer concentration affects all nutrients similarly.

Two-bag fertilizers are made into stock solutions in two different tanks, commonly referred to “A” and “B” tanks (Fig. 1). The reason to keep concentrated stock nutrient solutions in separate tanks is to avoid precipitation of nutrients. In one tank is a concentrated calcium nitrate solution while in another tank are the remaining nutrients, including phosphate and sulfates. If the calcium were to be placed in the same tank as the phosphates and sulfates, these nutrients would precipitate, fall out of solution, and potentially clog fertilization and irrigation equipment.

The last method to mixing fertilizers is to avoid pre-mixed bags altogether and mix individual compounds to make your own fertilizer. This is the method that provides the most flexibility for producers, allowing them to adjust concentrations of virtually every nutrient in solution. However, while this may sound like a great opportunity to have a high degree of control over mineral nutrition, it may require more effort, skill and/or interest than you’d like to invest in; it may be something you’d prefer to forego.

One of the most exciting advantages to hydroponic production is the ability to capture and recirculate water, leading to a nearly 95 percent reduction in water use per acre compared to traditional field production.

Nutrient solution management

There are two main objectives for managing nutrient solution for plant growth: 1) managing the electrical conductivity (EC) and 2) managing the pH. Unlike growing crops in soil or larger volumes of soilless substrate, we do not have a buffer for mineral nutrients and pH and, therefore, the management of these nutrient solution chemical properties is very important.

The EC of nutrient solutions reflects the total concentration of electrolytes, those compounds that conduct electricity. In a hydroponic nutrient solution, the majority of the electrolytes in solution are the different ions from dissolved fertilizer salts. While other nonessential elements in solution contribute to the EC reading, it primarily reflects the amount of fertilizer in the water. To ensure that your plants are always receiving the correct amount of nutrients, the nutrient solution EC must be checked and adjusted regularly. For small systems, this may be done by hand a few times a day. However, for larger systems this process is automated and a probe is continually measuring the EC (Fig. 2). When it drops below the target concentration, fertilizer concentration is injected into the nutrient solution to raise the EC until the desired concentration is reached.

Over time, plants will take up nutrients in different amounts than they are present in the fertilizer, so it is important to submit a water sample to a laboratory to identify the concentrations of specific nutrients. The results will allow you to see the concentrations of individual nutrients and decide when to partially or completely replace your nutrient solution due to unacceptable nutrient concentrations and ratios with other nutrients.

Keep pH in check

The pH of your nutrient solution is going to change over time in response to several factors. However, in order to keep the pH in the appropriate range, adjustments will need to be made. Acid can be injected into the solution if the pH needs to be reduced.

There are several types of acids that may be suitable for use, including citric, nitric, phosphoric acid, but sulfuric acid is the most widely used. When using acids, keep in mind that mineral nutrient may be contributed to the nutrient solution after the acid is added. For example, nitrogen, phosphorous or sulfur available for plant uptake is made available when nitric, phosphoric or sulfuric acids are used in production. Therefore, concentrations of these nutrients may be reduced in your fertilizer because they are being added by acids for adjusting pH. The same logic stands for raising pH. If the pH drops below your target range, a base, such as potassium hydroxide, may be added to increase the pH, which will also add potassium to your nutrient solution.

Take-home message

Hydroponic systems rely on nutrient solutions to act as a substrate while providing water and mineral nutrients. By starting with high-quality water and the appropriate fertilizers, you can extend the life of your nutrient solution and keep the productivity of your crop on track. While daily EC and pH adjustments will keep your nutrient solution within your target ranges, regular commercial testing of nutrient solutions will provide insight into when nutrient solutions need replacement.

Christopher (ccurrey@iastate.edu) is an assistant professor of horticulture in the Department of Horticulture at Iowa State University.