What is the right amount of molybdenum in poinsettias?

Even after 35 years of observing poinsettia nutrient disorders, diagnosing a molybdenum (Mo) deficiency is still a challenge. Problems only occur sporadically, but when they do, they can severely impact the crop.

Molybdenum is an essential element, but it is only required in small quantities for most plants. Molybdenum’s major function is in nitrogen (N) utilization and N fixation. Poinsettias and some cole crops, such as broccoli and cauliflower, are more prone to developing Mo deficiency symptoms. In trying to induce Mo deficiencies in the NC State University Nutrient Disorder system, much to our surprise, ivy geraniums developed symptoms too.

Molybdenum acts differently from the other micronutrients when it comes to availability over a range of pHs. Availability is the inverse of the other micronutrients. Molybdenum is more available at high pHs and less available at low pHs. So, growers who have elevated alkalinity in their water supply may have the advantage of more Mo availability with their higher substrate pH! Also, a research report from the Netherlands found that Mo deficiencies developed in lettuce grown in an unamended, low pH, peat substrate without Mo supplied. This would suggest that poinsettia growers who battle acidic substrate pHs may be more prone to the development of Mo deficiencies.

What is the right amount of Mo to supply poinsettias? Poinsettias are reported to have a high requirement for Mo. Typically, recommendations suggest supplying Mo fertilizer at 0.1 ppm. Research at Purdue University (Hammer and Bailey, 1987) found that rates up to 60 ppm Mo did not result in phytotoxicity. Molybdenum toxicity is possible with other greenhouse crops, and some plants are sensitive at >0.5 ppm. Also, avoid feeding animals any plants that have received excessive Mo.