Greenhouse - Iron Deficiencies

Iron deficiency is a common problem in greenhouse plants. We often see iron deficiency in fall pansies and violas and in crops such as petunia and calibrachoa in the spring season. The following is an article on the subject.

Iron deficiency symptoms generally show up as an interveinal chlorosis, normally starting at the shoot tips, but often they occur throughout the entire plant. Sometimes the leaves of iron deficient plants turn almost white. Symptoms of TMV show yellow and green mottling, upward leaf curling, leaf distortion and overall stunting. Bacopa, calibrachoa, scaevola, snapdragons, and petunia are crops susceptible to iron deficiency. Preventing iron deficiency can be accomplished by controlling pH and using an iron chelate fertilizer.

pH control. Acid pH favors the availability of iron to plants, therefore the target pH range for crops susceptible to iron deficiency is fairly low, 5.5 to 5.8. Most commercial soilless media have pHs in this range to start and the use of an acid-forming fertilizer (e.g., 20-10-20, 15-16-17, 15-15-15) may be enough to keep the pH in this range. 21-7-7 Acid Special fertilizer is effective at quickly lowering pH, but it should not be used more than once or twice to fertilize bedding plants and other annuals. If plants are irrigated with high alkalinity water then iron chelate fertilizer or acid injection should be considered. If a grower mixes his/her own sphagnum peat-based growth medium dolomitic limestone should be added at a rate of no more than 5lbs./yd. Too much limestone is an aggravating factor contributing to iron deficiency.

Iron Chelate. Fertilizing sensitive crops with iron chelate fertilizer from time to time is probably the least complicated way of preventing iron deficiency. Most greenhouse supply companies carry Sprint 330 (10% iron), Sprint 138 (6% iron), or similar iron chelate products. Sprint 138, however, is the preferred chelate because it maintains iron availability over the widest pH range. Sprint is generally applied as a soil drench at the rate of 8 oz./100 gal.(½-¾ tsp. per gallon). At this rate, iron chelate can be applied every 3 or 4 weeks if desired. Iron chelate can also be mixed as a concentrated solution for injection or low rates can be mixed and injected with other fertilizers. Note that some growers have replaced a separate iron chelate fertilizer with a commercially available, all in one, fertilizer that contains chelated iron.

Do not use fertilizers containing chelated iron on iron sensitive crops such as geraniums, marigolds and other crops that are sensitive to iron and exhibit iron toxicity symptoms. To review pH and fertility needs of spring crops see information below.

Reprinted from the New England Greenhouse Update http://www.negreenhouseupdate.info/greenhouse_update/?cat=12