Testing…1, 2, 3

In hydroponic systems, producers try to provide the appropriate concentrations of mineral nutrients to plants in enough quantity and frequency with the goal of growing healthy plants for maximum yields. While much of controlled environment agriculture and hydroponic crop production is automated day-in and day-out, establishing a clear relationship between water and mineral nutrients (and their uptake by plants) is not. By developing and maintaining a regular testing procedure for clear water, nutrient solutions and plant tissue, a clear and complete picture can be put together for more successful management. Regularly testing your water, nutrient solution and plant tissue and will give you the information you need to not only address problems as they arise, but to avoid them altogether by providing data for proactive measures.

The clear water used in hydroponic nutrient solutions is the backbone, serving much the same purpose a canvas does for a painting. However, unlike a new canvas, water is not usually “blank” with respect to its qualities. One of the most important qualities of water is alkalinity. The alkalinity of clear water will affect pH management for nutrient solutions when they are initially mixed and as they are subsequently maintained. Depending on your water source, alkalinity can vary over the course of the year, with higher concentrations in the winter and lower concentrations in the summer. If alkalinity is high, measuring the calcium and magnesium concentrations in clear water can help to account for these additions when formulating a fertilizer needs for nutrient solutions. There are other specific elements that can be useful to quantify in water, including, but not limited to: iron, sodium, chloride and fluoride. While there is publicly available data for some water sources (such as municipal water), regular testing will likely provide additional details. For water sources with no publicly available information, testing water is the only way to get this information.

The recirculating nature of nutrient solutions in most hydroponic systems make them dynamic in nature. Fertilizers are designed to meet the needs of hydroponic crops based on their composition and subsequent mixing. However, once nutrient solutions are first mixed, they immediately start to change in composition. As nutrient solution is provided to plants, water and mineral nutrients are taken up by the plant and out of the solution. Next, as the nutrient solution is recirculated, electrical conductivity (EC) and pH are adjusted using concentrated fertilizer and acids bases, respectively. As these changes are compounded over days, even with the periodic partial replacement of recirculated nutrient solution with some that is freshly made, the relative proportions of nutrients start to change from when the solution was initially mixed. As nutrient concentrations change in the nutrient solution, imbalances between them can occur. In some instances, an excessive amount of one nutrient can suppress the uptake of another.

One of the most common nutrient antagonisms is the relationship between calcium and magnesium, where an excess of one can impede the uptake of another. These imbalances occur even as pH and EC are rigorously controlled and maintained within target ranges, which means there is no other way to determine nutrient concentrations and detect imbalances in them besides analyzing the nutrient solution. Not only will analyzing nutrient solutions allow you to know what is actually in your nutrient solution and available for plant uptake, it will help determine how to better manage your nutrient solution, with respect to modifying replacement solution composition and partial exchange volumes and frequencies.

The last important test to perform is plant tissue analyses. So much of what we do with diagnosing plant nutritional status is done using visual diagnostics in the greenhouse. This is an easy and rapid way to assess the nutrient status of crops, but there are limitations. First, if a visual symptom of a toxicity or deficiency is visible, the problem has already begun. Second, but no less important, if a plant has a toxicity or deficiency starting but no symptoms are visible, visual diagnoses will not be able to catch it early enough for prevention. By using regular tissue analyses, growers will be afforded opportunities to improve plant culture. First, a tissue sample will aid in accurately diagnosing deficiencies or toxicities when they appear, as the tissue concentrations will complement visual symptomology. More importantly, tissue testing can determine when a deficiency or toxicity could be starting prior to visual symptoms developing — affording the opportunity to remedy the problem before any damage is done.

While water, nutrient solution and plant tissue tests have all been discussed separately, the real strength from these tests come from the synergy formed when all three tests are performed. If there is a problem in your plant tissue, being able to go back to the water and nutrient solution tests will help see the whole story of what is happening with respect to managing crop nutrition. If the test results indicate the problem is not with what is being provided to plants (as determined with water and nutrient solution tests), then environmental data from the greenhouse on light, air temperature and humidity can be used to determine if the problem is in the uptake of nutrients as affected by the growing environment. The bottom line: your diagnostic power will increase with these tests.

Does this testing come at a cost? Yes, these regular analyses incur expenses. This reality often leads to the next question, “How often should I submit tests?” As often as can be economically justified is the best answer. An ounce of prevention is worth a pound of cure, and that perspective can help ease the upfront costs of a monitoring program, as well as reduce the costs associated with solving potential problems. Additionally, not all tests need to be taken with the same frequency. Nutrient solutions and plant tissue should be sampled more frequently than water, as they are more likely to change with greater frequency than water quality. As samples are submitted and data is collected, you may modify your testing frequency based on the results you are seeing.

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