by Tamara Scully
One of the challenges of growing floriculture crops in a controlled environment is ensuring proper nutrition throughout their growth cycles. Because every species of plant has individualized nutritional needs to obtain optimal growth, a one-size-fits-all approach to fertility doesn’t work unless you are growing just one crop, like poinsettias or zonal geraniums. Even then, cultivar specific nutrition could be a factor.
W. Garret Owen, assistant professor of floriculture and specialty crop production in greenhouses and controlled environments at the University of Kentucky, recently addressed factors which can interfere with providing floriculture crops the optimal nutrition they need to thrive. The bottom line, said Owen during the American Floral Endowment’s Grow Pro Webinar Series, is understanding “‘fert, dirt and squirt,’ or what we call the three pillars of nutritional monitoring. As greenhouse growers, you want to maximize growth potential of your floriculture crops.
Growers need to provide fertilizers which meet the crop’s needs; obtain an analysis of water quality – not for pH, but for alkalinity; and adjust the pH of the substrate to ensure optimal pH levels are maintained throughout the crop cycle.
pH & Growth
The ideal pH range for floriculture crops is 5.8 – 6.2. Outside of that narrow range, optimal growth can’t happen.
“Most all nutrients are available to the greenhouse crops being grown in soilless media” in the ideal pH range, Owen said. When the pH is outside of that range – either high or low – growth is stunted. In experiments where plants were grown in with pH levels from 3.9 to 6.3, the lower pH caused overall suppression of growth.
Higher than optimal pH causes an increase in interveinal chlorosis of newly matured leaves. Ongoing pH elevation will cause this to progress to a bleached appearance in all leaves. Leaf necrosis and even flower bud abortion can occur in severe cases. These changes are indicative of high pH iron-induced deficiency. When pH is high, micronutrients (particularly iron) are not available.
With low pH levels, micronutrient toxicity can occur as elements become more available and are readily taken up by plants. Typical signs of low pH manifest as bronzing of the leaves and chlorosis of the basal leaves (commonly seen in zonal geraniums). Necrosis can then occur. Black spotting is also an indication. Symptoms vary depending on the crop.
There is a “species-dependent pH effect,” with changes in plant size and variations in dry mass. In some plants, an increase in pH decreases dry mass and plant growth, while in others it causes increases. Dry mass, plant size and root mass reductions all are indicative of pH outside of the crop’s optimal range.
The Three Pillars
Water pH is not a concern; water alkalinity is. Water alkalinity and substrate pH work together with fertilizer to determine the actual pH impacting the plant. That pH can significantly influence nutrient availability. Alkaline water is similar to limestone, as both will raise the pH of the growing environment.
“Water alkalinity is the number one driving force of increased substrate pH over time,” Owen said. “It’s not the water pH. It doesn’t matter about the pH of the water. It’s that high alkalinity that’s going to cause substrate pH rise over time.”
Carbonates and bicarbonates in the water react with fertilizer and substrate acidity to form carbonic acid, and to consume hydrogen ions, causing pH rise over time. Growers will need to know the alkalinity of their water, and establish ways to manage it, so pH rise does not occur.
The pH of soilless substrates used in container growing is typically 5.5 – 6.5. As fertilizer or water interact with the substrate, pH drift can occur.
Fertilizer selection contributes to the actual pH of the substrate. Interactions between fertilizer and the alkalinity in irrigation water will cause pH changes. The CCE (calcium carbonate equivalent) per ton of the fertilizer will help growers determine the potential acidity, and determine if the fertilizer is a good match for the quality of the water to avoid or minimize pH drift.
“Make sure you are matching your fertility program to your water source,” Owen cautioned.
The fertilizer’s guaranteed analysis will provide the basic nutrients and the levels at which they are available to the crop. These are only accurate when the fertilizer is applied per proper mix instructions.
“Just taking the time to understand the fertilizer bag will go a long way to ensuring that you are providing the correct fertility to the plant,” he said. “By understanding the fertility need of your crops, you can mitigate the fertility waste and perhaps eliminate the need for growth inhibitors.”
Substrate pH is going to vary, with coconut coir and wood being acidic, and peat and pine bark being alkaline. The majority of media used to form substrates are “quite acidic,” Owen said. Additives such as limestone or calcium sulfate are used to adjust the substrate pH. Different types of limestone additives have differing impacts, however. Fine mesh calcitic limestone dissolves quickly and does not have a long-term residual effect; dolomitic limestone will impact pH more slowly, but its impact is longer lasting.
“Over time, we need to make sure that pH stays in the optimal range,” he said. The goal is to provide “enough pH adjustment to get through the crop cycle.”
Too much fertility can cause symptoms very similar to those seen when pH levels are not optimal. The electrical conductivity, or soluble salts, causes issues in growth and development, including stunted growth, lower leaf yellowing and purpling, leaf necrosis and root necrosis. These changes will occur when EC is high or low and mimic concerns seen due to pH imbalances.
Nitrogen immobilization can cause fertility issues too and will vary by substrate. Wood substrates are likely culprits. Fertility will need to be adjusted to combat this concern.
“Without implementing a nutritional monitoring program, you have no idea if you are actually observing a nutritional response or are you seeing something more related to a biological, or a different, abiotic factor,” Owen said.
Nutritional monitoring involves an analysis of the substrate, plant tissue analysis and “pour through” sampling. Not all crops will need substrate analysis, but if there are ongoing problems or if you have a very high value crop, substrate analysis provides detailed information on every nutrient. Tissue analysis of the most recently matured leaves provides nutrient values in the leaf tissues. If symptomatic leaves are sampled separately from those exhibiting normal characteristics, the results can help to determine why the symptoms are occurring.
The pour through test allows growers a quick, easy and accurate in-house test to determine if the crop’s nutritional needs are being met. This requires pH and EC meters, some cups and saucers, distilled water and calibration solutions. After irrigating the crop to container capacity, wait 30 minutes, place saucers under a few random samples and water the containers’ substrate evenly with distilled water. The leachate is collected in the saucers. The pH and EC of the leachate is then tested. The probe should be rinsed with distilled water between samples.
More information on how to do this test, as well as how to interpret the data, can be found at the “Fert, Dirt and Squirt” website: fertdirtandsquirt.com.
An understanding of your crop’s fertility needs, working knowledge of how substrate composition, fertilizer program and water alkalinity impact pH and the ability to monitor and identify nutritional issues before they become severe are the key to eliminating nutritional issues and producing healthy, vibrant floriculture crops.