Low Substrate pH-Induced Micronutrient Toxicity of Geranium and Other Crops
Greenhouse growers have spent weeks producing crops for spring sales. However, with COVID-19 halting sales in some states, growers are stuck holding and maintaining crops. During this time, challenges can arise, such as substrate pH drift. Growers should monitor and adjust (raise or lower) substrate pH to a crop-specific optimal range thereby preventing the development of nutritional disorders. For example, zonal geraniums prefer a substrate pH range of 5.8 to 6.5. Nutritional disorders can develop if substrate pH drifts above or below this range. One nutritional disorder we often see beginning in late-March to early-April is pH drop in geraniums. When substrate pH drops below 5.8, iron (Fe) and manganese (Mn) availability and uptake increases. Geranium plants accumulate these immobile nutrients to toxic concentrations in lower leaf tissue which we often refer to as Fe and Mn toxicity. Many other greenhouse crops are susceptible to low substrate pH-induced Fe and/or Mn toxicity and were previously reported in e-GRO Alerts or nutritional factsheets including calibrachoa, dahlia, fuchsia, gerbera, Heuchera, marigold, New Guinea impatiens, pansy, and petunia. For more extensive information and photographic guides of other bedding plants including potted flowering plants, herbs, and vegetable transplants, visit fertdirtandsquirt.com.
Symptoms of Fe and Mn toxicity in geranium plants begin in the lower leaves where the initial symptoms can be described as chlorosis or yellowing. These initial symptoms can intensify and leaves will exhibit chlorotic spotting and interveinal chlorosis just like in the photo above. Other symptoms include leaf bronzing, dark brown to black spotting along leaf margins, and marginal leaf necrosis or death. Necrosis often progresses inward between veins and leaves become scaberous (rough) due to tissue death. Leaf drop often occurs and growers must remove leaves from plants prior to shipping. For a pictorial progression guide, refer to Geranium: Diagnosing Lower Leaf Reddish-Brown Spot.
To prevent substrate pH drop and thus, Fe/Mn toxicity, growers should monitor geranium and other crop nutrition. Growers are encouraged to determine substrate pH and soluble salts [referred to as electrical conductivity (EC)] by easily performing in-house 1:2 Dilution, Saturated Media Extraction (SME), or PourThru procedures. Each method will determine substrate pH similarly, however EC values vary with each method. For more information on how to conduct a nutritional monitoring procedure, refer to Nutritional Monitoring: Geraniums. Also, be sure that your pH and EC meter is working correctly and calibrated regularly because pH and EC values are only as accurate as your last calibration! For a “how to video,” watch “How to Calibrate a pH and EC Meter.” For more helpful information or “how to sample leaf tissue for diagnosis”, refer to Diagnosing Low Substrate pH Disorders: Steps for pH and Tissue Testing and Target Leaf Tissue Sampling for Precise Nutrient Diagnosis.
Once you have determined substrate pH and it is indeed low, a corrective action is needed. Corrective procedures for low substrate pH for geraniums should begin within the range of 5.4 to 5.6. To quickly adjust (raise) substrate pH to the optimal pH range, growers can perform any one procedure such as flowable lime, hydrate lime, or potassium bicarbonate application. For a step-by-step outline, refer to Corrective Procedures for High and Low Substrate pH and Electrical Conductivity.
For more nutritional monitoring of greenhouse crops, visit e-GRO fertdirtandsquirt.com.
W. Garrett OwenGreenhouse Outreach Specialist, Michigan State University
W. Garrett Owen is an Assistant Professor and Extension Specialist of floriculture, greenhouse and controlled-environment crop production in the Department of Horticulture at the University of Kentucky. He has an appointment in research, teaching and Extension. His area of expertise is controlled environment specialty crop production; plant nutrition; production problem diagnostics; and young herbaceous perennial plant propagation.