Molybdenum (Mo)

Function:

Molybdenums (Mo) major function is in N utilization and N fixation.

Deficiency:

Deficiency symptoms are expressed as chlorosis (yellowing) of the young mature leaves, upward rolling of the leaves, and leaf edge burn. The leaf chlorosis of Mo deficiency somewhat resembles N deficiency, due in part to Mo role in N utilization. Distorted leaves can result under deficient conditions due to the failure of the interveinal areas to expand normally. It is important to correct Mo deficiency when symptoms first appear because leaf edge burn or distorted leaves can not be reversed. Mo deficiency symptoms will first appear on the upper, young mature leaves.

Mo deficiency symptoms on poinsettias.
Click on image for larger version. (image ©Bill Fonteno, NCSU)

Mo deficiency symptoms on the upper leaves of poinsettias.
Click on image for larger version. (image ©Paul Nelson, NCSU)

Excess:

Poinsettias have a high requirement for Mo and excessive rates up to 60 ppm are reported to cause little or no plant damage. Mo toxicity is possible with other greenhouse crops and some plants are sensitive at >0.5 ppm.

Confirm your actual Mo levels by conducting a plant tissue analysis. Root substrate (medium) tests are less reliable for determining Mo levels.

Monitoring and Management Strategy for Molybdenum (Mo)

Fertilization Rate:

0.1 ppm constant liquid fertilization

Interactions:

High levels of sulfates (SO₄) can have an antagonistic effect on Mo uptake by the plant. High levels of phosphates (PO₄) enhance Mo uptake by the plant. Mo is less soluble in soilless substrates or when the pH is below 6.0.

Tissue Concentration:

Normal range for poinsettias is 1.0 to 5.0 ppm. Levels below 0.5 ppm are considered deficient. An indirect method of assessing Mo deficiency can be done by determining the tissue nitrate-nitrogen (NO₃^-N) level. NO₃^-N levels >3,000 ppm indicate Mo deficiency. The normal Mo range is lower for most other greenhouse crops.

Options:

Preplant:

Incorporation of a micro-nutrient mix into your soilless substrate.

Continual Fertilization:

1. Most commercial fertilizers that provide micronutrients including Mo. Remember to add supplemental Mo applications if using calcium nitrate (Ca(NO₃)₂) + potassium nitrate (KNO₃).
2. Because poinsettias have a high Mo requirement, the addition of Mo at 0.1 ppm on a constant liquid fertilization program is recommended. This can be done by:

   For Stock Solution: Mix 1 oz (28.4 g) sodium molybdate or ammonium molybdate in 40 fl oz (1200 ml) [or 24 grams per liter] of water.

   For Application: Add 0.15 fl. oz. of the liquid Mo concentrate per 100 gallons of fertilizer stock solution [or 12 ml per 1,000 liters].

Corrective Fertilization:

1. Supplemental applications of sodium molybdate or ammonium molybdate at 77 ppm of Mo (see mixing rates below) for soilless media. Visible improvements (darker green leaves) with the use of a corrective Mo fertilization should evident within 1 to 2 weeks. It is important to correct Mo deficiency when symptoms first appear because leaf edge burn or distorted leaves can not be reversed.

   For Corrective Application to Soilless Substrate (Medium): Add 2.7 oz. (77 g) of sodium molybdate (38% Mo) per 100 gallons of water or 1.9 oz. (54 g) of ammonium molybdate (54% Mo) per 100 gallons of water.

Corrective Foliar Spray:

Foliar sprays have a greater risk of plant injury. Early morning applications are preferred. Avoid applications during the midday heat. Plant uptake is enhanced by the increased drying time which occurs during the moist conditions in the morning. Nutrient uptake through the leaves is also greater in the light periods than at night, thus making morning applications more desirable than evening sprays.

To apply a foliar spray of Mo: mix 2 oz. (56.7 g) of sodium molybdate (38% Mo) per 100 gallons of water to supply 57 ppm Mo or mix 2 oz. (56.7 g) of ammonium molybdate (54% Mo) per 100 gallons of water to supply 81 ppm Mo. Test on a few plants first. Do not over apply.

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