Experienced growers scrutinize their crops for any sign of trouble. They do this because problems are much easier to address in their early stages. But it is difficult to address problems that aren’t so obvious, such as changes in pH and electrical conductivity (EC), without a preventative monitoring program in place.
“The bottom line is you don’t have any control if you are not measuring the impact of your management actions, and you can’t make informed decisions,” says Paul Fisher, associate professor and Extension specialist in environmental horticulture at The University of Florida. “The first step in monitoring is to use your experience as a grower to visually monitor plant performance. It is also useful to have some backup data to help you determine if the basic aspects of plant nutrition are in line. If you monitor the substrate for changing trends in pH and EC, check root health and have a complete fertilizer program in place, you can diagnose 95% of nutritional problems in the greenhouse.”
Management of pH is similar to balancing a glass of water on each side of a scale. The addition or subtraction of water from one side or the other will tip the scale. In other words, healthy plants result from keeping pH levels in the optimal range for good growth, and out-of-range pH will adversely affect plant health over time.
For a majority of floriculture crops, 5.8 to 6.2 is the substrate-pH range where most micronutrients are available for plant uptake. However, some plants prefer more or less acidity, depending upon how proficient they are at absorbing micronutrients. New Guinea impatiens (Impatiens hawkeri) and zonal geraniums are good examples of greenhouse crops that grow well in high pH conditions (6.0 to 6.6).
Substrate-pH dictates how nutrients will react in the soil. Solubility increases at low pH levels, resulting in problems with nutrient toxicities. High pH levels have the opposite effect. “Iron deficiencies are common when pH is too high,” says Fisher. “A good biological indicator crop is petunia. Marigolds and seed or zonal geraniums are indicator crops for low pH problems where you run into iron/manganese toxicities.”
Soil pH also contributes to the severity and occurrence of plant diseases, because when pH is out of range, stressed plants are vulnerable to pathogen attacks. Thielaviopsis root rot is an example of a pathogen that favors high pH conditions.
The best approach to pH management is a preventative one. Monitoring helps growers detect changes in pH trends over time and make small course corrections to keep pH at optimal levels. Although there are times when a grower must quickly lower or raise pH, this step should be reserved for when crops would be unsaleable without intervention.
Electrical Conductivity (EC)
Electrical conductivity (EC) is a measure of the concentration of dissolved salts in the substrate from fertilizers, irrigation water and substrate components. Overfertilization, low leaching rate and poor water quality all cause EC to increase. EC measurements do not indicate which type of salt is present, nor do they specify which nutrients are available for plant consumption. They do help growers decide if fertilizer applications are excessive or insufficient.
There is a direct relationship between EC and plant growth performance. “High substrate-EC levels in can be a sign of too much sodium and chloride in the water, or it can result from overfertilization,” Fisher says. “When substrate-EC is high, a soil test by a laboratory is needed to diagnose what specific ions are causing the issue (e.g., sodium from water versus nitrogen from fertilizer). Root damage often occurs at high EC levels, susceptibility to diseases such as Pythium and increases.” High levels also can be problematic when germinating seeds or rooting cuttings. Low EC levels cause stunting and chlorosis (yellowing of leaves).
“An EC meter is also useful for checking the fertilizer injectors,” Fisher says. “Information on the fertilizer bag tells you the relationship between EC and parts per million of nitrogen. Simple math tells you that if you measure the EC of the clear irrigation water and add that to the EC of the injected water, the total will equal the EC coming out of the hose. If that total is not what you expect, adjustments need to be made.” (See Checking EC For Common Blended Fertilizer and Fertilizer Salts, Table 1)
Monitoring pH And EC
“Every grower should purchase good quality pH and EC meters for in-house monitoring,” Fisher says. “Secondly, they should work with an Extension agent or fertilizer company to get a basic fertilizer program in place that covers all of the essential nutrients.”
Fisher also recommends a complete water test from a laboratory at least once a year, as well as weekly in-house tests of clear irrigation water and injected water. Growers need to test major crops about every two weeks so they can take small corrective actions before plants experience too much stress.
“Information obtained from pH and EC tests is important, but visual observations are also part of the monitoring process,” Fisher says. “Observing how the crop is growing and checking the roots regularly should not be overlooked. Good growers don’t become so obsessed with lab tests and managing individual nutrients that they lose track of the bigger plant health picture.”