Greenhouse - Iron Deficiency

Iron deficiency is a common problem in greenhouse plants if the pH drifts too high. The following is an article on the subject from the New England Greenhouse Update site.

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. 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.

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.

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.

By Douglas Cox in the March, 2006 edition of the New England Greenhouse Update http://www.negreenhouseupdate.info/greenhouse_update/?p=2438