Sampling nutrient solutions

Departments - Hydroponic Production Primer

Getting the nutrient solution right is an essential part of growing successfully in a hydroponic system.

August 26, 2019

The concentrations of mineral nutrients in recirculating hydroponic solutions are most frequently reflected in electrical conductivity measurements. However, comprehensive laboratory analyses that quantify concentrations of each essential nutrient are most useful.
Photo courtesy of Christopher J. Currey

In hydroponic production, the nutrient solution is one of the most important aspects of any system. Since having adequate mineral nutrients in nutrient solution is essential to hydroponic crop production, the concentrations of nutrients is frequently monitored by measuring the electrical conductivity (EC) and management decisions are made based on measurements. When below acceptable levels concentrated fertilizer is added, or when above acceptable levels clear water is added.

The EC, reflecting the total amounts of nutrients in recirculating solutions, is very useful for day-to-day management of mineral nutrients. However, it doesn’t always provide all the information needed for successfully managing mineral nutrients in recirculating hydroponic systems. The EC measures the total concentration of electrolytes in water. When fertilizer salts are added to water, the salts dissolve and the positively (cations) and negatively (anions) charged components dissociate. For example, when calcium nitrate [Ca(NO3)2] is added to water, it will separate into calcium (Ca2+) and nitrate-nitrogen (NO3-), which are both electrolytes that are measured and reflected in the EC of the nutrient solution. But the EC doesn’t measure the concentration of individual nutrients; rather, it is a measurement reflecting the total concentration of all electrolytes in solution.

Though the EC is a useful measurement for managing nutrient solutions, it doesn’t always provide the information needed for longer-term management of solutions. When concentrated fertilizer from a one- or two-bag (i.e. “A and B tank”) fertilizer is added to nutrient solutions, the ratio of individual nutrients are in fixed proportions to one another. Regardless of how well-designed any fertilizer is, the uptake and use of nutrients by plants in hydroponic systems does not occur in the same ratio in which they are added to nutrient solutions. Therefore, for longer-term nutrient solution management, nutrient solutions should be sampled and submitted to commercial laboratories for a thorough and comprehensive analysis of concentrations for each essential element.

For longer-term nutrient solution management, nutrient solutions should be sampled and submitted to commercial laboratories for a thorough and comprehensive analysis of concentrations for each essential element.

There are several useful pieces of information you can get from commercial nutrient solution analyses. By submitting nutrient solutions for complete analyses, the concentration of each essential element becomes known. There is no more guesswork about how much of each nutrient is in the solution; instead, concentrations for each essential element are measured and reported. These analyses may reveal that an excessive amount of one nutrient may be building up in solutions, potentially leading to a nutrient toxicity. Alternatively, concentrations of other nutrients may be low and insufficient for healthy plant growth and development. While EC reading reflect the total nutrient concentration of a solution, only a complete analysis can measure the concentration of specific essential elements.

While the concentrations of each essential nutrient are important, the ratio of certain nutrients to one another are important. Nutrient antagonisms, where an excess of one nutrient can inhibit the uptake of another, can occur if the concentrations of some nutrients get too high. One of the most well-known antagonistic relationships of nutrients is calcium and magnesium. In their ionic form in nutrient solutions, they compete with one another for uptake; too much calcium inhibits magnesium uptake, and vice-versa. A recommended ratio for the concentrations of these nutrients is three to five parts calcium to one part magnesium. Potassium is another nutrient that can compete with calcium and magnesium for uptake, and its concentration should be monitored to avoid excessive levels.

A common practice for recirculating hydroponic nutrient solutions is to periodically replace a fraction of the nutrient solution. This adds fresh nutrient solution that has every nutrient in better balance with one another, with one another, alleviating deficient or excessive concentrations of nutrients in the solution. However, this replacement is frequently done on a time basis, such as every week or every other week. But is this always necessary? Without measuring the concentrations of mineral nutrients, there is no way of knowing. Using a commercial test to quantify individual nutrient concentrations can help producers make more informed decisions about replacing nutrient solutions. Sufficient concentrations of nutrients in balance with one another may allow forgoing this replacement, saving labor, water and fertilizer. A commercial analysis may reveal deficient or excessive nutrient concentrations and prompt action, thereby avoiding nutrient deficiencies or declines in crop productivity.

One common question is, “How often should nutrient solutions be analyzed?” The simplest answer is “as often as you can afford it.” This really is not meant to be a flippant answer. Submitting recirculating nutrient solution samples to commercial laboratories for analyses has a cost associated with it, running from approximately $25 to $40 per sample, depending on the laboratory and the type of analyses required. Producers will need to weigh the cost of this sample with the value of the results. If careful monitoring of nutrient concentrations allows reduced inputs associated with partial or full nutrient solution replacement, this clearly can pay off. Alternatively, if the nutrient solution analyses reveal deficient or excessive concentrations of nutrients that may lead to deficiencies, toxicities or insufficient growth before they occur, this also has value.

The EC is a great measurement for day-to-day management of mineral nutrient concentrations in recirculating hydroponic solutions. However, the detail revealed in complete nutrient solution analyses can help maximize crop health and productivity, while help avoid unwarranted expenses or unwanted problems.

The author is an assistant professor in the Department of Horticulture at Iowa State University.