Fact Sheets

Greenhouse Management / Engineering

How to Prevent Iron Deficiency in Spring Greenhouse Crops

Iron deficiency problems are appearing more frequently among bedding plants and other spring crops. The main symptom of iron (Fe) deficiency is chlorosis, sometimes starting at the shoot tips, but more often occurring throughout the entire plant. The leaves of some plants turn almost white in extreme cases. However, in all but the most extreme cases, Fe deficiency can be easily mistaken for nitrogen or magnesium deficiency, so a soil or tissue test is necessary to confirm a suspected case of deficiency.

Susceptible crops

The fact that reports of Fe deficiency are more common is in part explained by the growing popularity of some crops susceptible to the disorder. In particular, some of the plants in Ecke's "The Flower Fields®" series and the Pleasant View "Proven Winners®" are known to be susceptible to Fe deficiency. Here is a list of crops that may develop an Fe problem:

The Flower Fields®
Vegetative petunias (Liricashowers, Cascadias, Doubloon, Marco Polo, and Petitunia)
Outback Plants (Brachycome Daisies, Scaevola Fan Flower, and Paper Daisies)
Argyranthemum 'Summer Daisy'

Proven Winners®
Calibrachoa 'Million Bells'
Scaevola 'New Wonder'
'Supertunia' and 'Surfinia' trailing petunias

Other plants
Ivy geranium
Piggyback Plant (Tolmiea menziesii)
Certain bedding plants including pansy, petunia, snapdragon and vinca.

Prevent Deficiency

There are three practices involved in preventing Fe deficiency - growth medium pH control, low phosphorus fertilization, and use of an iron chelate treatment.

pH control. Acid pH favors the availability of Fe to plants, so the target pH range for these crops is fairly low: 5.5 to 6.0. Most commercial soilless media have pHs falling in this range and in most cases the use of an acid-forming fertilizer with a balance of ammonium and nitrate will be enough to keep the pH in this range. A major exception would be if the irrigation water is alkaline and then acid injection might be needed. If a grower mixes his/her own sphagnum peat-based growth medium, dolomitic limestone should be added at a rate of no more than 5 lbs./yd. Too much limestone is an aggravating factor contributing to Fe deficiency. For most growers keeping pH in the 5.5-6.0 range will be enough to prevent Fe deficiency.

Low phosphorus fertilization. In certain circumstances phosphorus (P) and Fe can react together to form insoluble Fe phosphates. The Fe in Fe phosphate is not available to plants. The chemistry of this reaction is well understood in field soil, but has not been studied in soilless media. However, to be on the safe side excess P should be avoided for the Fe sensitive crops. This means that no superphosphate should be mixed in the growth medium, an acid other than phosphoric acid should be used if acid injection is practiced, and, if possible, a water-soluble fertilizer supplying no more than 10% P (I suggest 5%) should be the main fertilizer. Keeping P low will help prevent Fe deficiency, but the key is maintaining acid growth medium pH.

Iron chelate. Fertilizing Fe sensitive crops with Fe from time to time is probably the least complicated and most proactive way of preventing Fe deficiency. Most greenhouse supply companies in this area carry a product called Sprint 330™ or perhaps an older one called Sequestrene 330.™ Both are 10% Fe chelates and are basically the same product (Sequestrene 330 is now marketed for agronomic crops only).

The Fe chelates are applied as a soil drench or as a foliar spray at the same rate of 8 oz./100 gal. (½-¾ tsp. gal.). The chelate is also soluble enough to make a concentrated solution for injection, but it must be mixed and injected by itself. Soil drench is the safest method of application; foliar sprays should be tried experimentally first to look for injury or residues. At the recommended rate, Fe chelate can be applied every 3 or 4 weeks if desired.

Final Words

It's important to apply the information in this article only to those crops known to have a special requirement for Fe. Over the past decade and even today we still struggle with some spring crops which are susceptible to Fe toxicity rather than deficiency. These crops include marigolds, zonal geraniums, and seed geraniums and their nutrition is managed to minimize plant-available Fe. This is done by keeping pH in a higher range and avoiding fertilizing with too much Fe. Applying the treatments I have described in this article to plants susceptible to Fe toxicity would be disastrous!

Prepared by:
Dr. Douglas A. Cox
Plant and Soil Sciences
University of Massachusetts
Amherst

October 2000.

Links to Further Resources on the Web

• North Carolina State University has a good new article on Identifying Nutrient Deficiencies of Bedding Plants. This is a basic 4-page factsheet with a table naming sixteen essential elements for plant growth, their deficiency symptoms and corrective procedures. In PDF format, available from NCSU's Commercial Floriculture Information Center front page.
• New Zealand's "Hortnet" also has a general article on Fertiliser Recommendations for Horticultural Crops Nursery Stock, Bedding Plants and Pot Plants in Peat and Bark.
• GreenBeam.com has a "Cyberconference" on the topic of Plant Nutrition and Fertilizers for horticultural crops in general, with a section on Fertilizer and Irrigation Tips for Container Growing.