Are your geraniums suicidal?

One of the major production problems facing ornamental growers is controlling root substrate pH. Proper control of substrate pH can prevent more than half of all nutritional problems.

During crop production, there are customary rises and declines in substrate pH. Rises most often stem from irrigation water alkalinity, while declines result from use of acidic fertilizers in combination with pure irrigation water (lacking alkalinity) as well as from insufficient limestone in the substrate.

Shifts in substrate pH are usually gradual and can be detected in time to be satisfactorily corrected. pH correction remedies can include a shift in fertilizer type to achieve the proper acidity or basicity level or application of acids or bases such as ferrous sulfate and flowable limestone.

Geranium, pH problems

Geraniums can exhibit a different type of substrate pH decline that involves a very large and sudden drop. Geraniums growing in a substrate with a pH of 6 or higher can suddenly find themselves in a substrate whose pH has dropped one to two units within seven to 14 days. pH values as low as 3.8 have occurred.

In organic-based soilless media the pH has a large effect on nutrient availability. At low substrate pHs crops can often be impacted by iron and manganese toxicities and calcium and magnesium deficiencies. Prolonged exposure to low pH can also suppress lateral root formation leading to the death of root tips and a reduction in nutrient uptake.

Sporadic occurrence

A distinguishing feature of sudden pH decline is its sporadic occurrence. Within the same greenhouse one geranium variety might be affected and another isn’t. A geranium crop at one growing operation may show signs of decline and at another facility no plants are affected, even though all of the crops appear to be produced similarly.

Symptoms of this disorder have been inconsistently reported and include marginal necrosis, chlorotic and/or necrotic spotting of older leaves, upward and/or downward cupping of the leaves, leaf distortion, large, purplish-black spots, light-brown or orange pigmentation, bronzing and a bronze stippling on leaves.

Most symptoms are likely the result of iron and manganese toxicity. In some cases, tissue analysis tests do not confirm these conditions. Symptoms also seem to be highly influenced by genetic variability among geranium species.

Potential disorder cause

Affected plants receive a signal that triggers an acidification process by the roots. The one documented process that could explain sudden pH decline is proton efflux pumps. These pumps are located on the surface of the roots and expel hydrogen ions (acid) into the substrate, acidifying it.

Although these pumps naturally operate during nutrient uptake, they can be stimulated to operate at abnormally high levels by three situations:

1. When plants take up ammonium-nitrogen preferentially to nitrate-nitrogen due to the fertilizer applied or environmental stresses of water, adverse temperature or light intensity.

2. When more positively-charged nutrients (ammonium, potassium, calcium and magnesium) are taken up than negatively-charged nutrients (nitrate, phosphate and sulfate).

3. When iron, phosphorus or zinc are in short supply.

The second situation could result from the pattern of fertilizer application. In a production system of infrequent fertilizer application, plants may be forced to take up nutrients not ordinarily selected. If plants are fertilized once a week or at certain intervals, preferred nutrients can be in short supply at the end of the week or that interval.

Low-iron stress can stimulate the proton efflux pump to 100 times the normal rate (at which acidity is added to the substrate) involved in nutrient uptake. Phosphorus stress is an another strong possibility because growers have relied heavily on alkaline fertilizers in an attempt to prevent pH decline, particularly in geranium. Phosphate is low or absent in alkaline fertilizers with formulations such as 13-2-13, 15-0-15, 14-0-14.

Determining disorder cause

Research is being conducted at North Carolina State University to determine the cause of sudden pH decline in geranium. A survey of commercial growers and breeders determined some susceptible and tolerant varieties.

Seed-produced zonal geraniums seem to be the most susceptible, followed by vegetative zonals. Ivy geraniums are the most tolerant.

Geranium series that tend to be more susceptible are Pinnacle, Designer, Patriot, Maverick and Orbit. Tolerant series include Fantasia, Elegance and Rocky Mountain . When growing susceptible series or cultivars, it is important to maintain the pH within the acceptable range.

The results of preliminary studies have not provided a clear indication as to the cause of sudden pH decline. Phosphorus stress continues to be a strong potential trigger since it causes geraniums and other plants to acidify the substrate and it suppresses nitrate uptake. Also, geranium growers continue to rely on alkaline fertilizers to offset pH decline and these fertilizers have low or zero phosphorus.

Adversely high light levels and temperatures should also be considered as possible triggers for the onset of the disorder. They may be part of an interaction where an unknown factor sets the stage for these environmental stresses to activate the disorder. Further research is planned to determine this unknown factor.

Preventing sudden pH decline

To prevent or control sudden pH decline, the substrate pH must be constantly monitored. The recommended pH range in soilless substrate for both ivy and zonal geraniums is 6.0-6.6. Researchers at Purdue University found optimal growth occurred at 6.4-6.5. The substrate pH should be checked weekly and maintained within this recommended range. If the pH drops below this range corrective action must be taken in order to prevent micronutrient toxicity.

Three methods to increase substrate pH are application of:

1. Alkaline fertilizers.

2. Flowable lime at 1-2 quarts per 100 gallons of water.

3. Potassium bicarbonate at 2 pounds per 100 gallons.

Plants should be rinsed off after the application of flowable lime or potassium bicarbonate to remove foliar residues to prevent chlorosis. One day after the application of potassium bicarbonate, leach the substrate with water containing a fertilizer charge to remove excess potassium and re-establish the proper nutrient balance. Allow 24 hours of reaction time for potassium bicarbonate and three days reaction time for flowable lime. Recheck the substrate pH to ensure the pH is in the adequate range and whether an additional application is necessary. Alkaline fertilizers are not used for rapid pH changes, but the pH should rise gradually and a difference should be evident after two weeks.

Minimize sudden pH decline

To minimize losses from sudden pH decline:

1. Maintain substrate pH between 6.0-6.6.

2. Check substrate pH weekly.

3. Apply alkaline fertilizers and supplement phosphorus if it is low or absent.

4. Apply flowable lime at 1-2 quarts per 100 gallons of water or potassium bicarbonate at 2 pounds per 100 gallons if pH is below the target range.

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- Matthew D. Taylor and Paul V. Nelson

Matthew Taylor is research graduate assistant and Paul Nelson is professor, North Carolina State University, Department of Horticulture Science, 227 Kilgore Hall, Box 7609, Raleigh, NC 27607; (919) 515-3178; fax (919) 515-7747; mdtaylo3@ncsu.edu.