18 december 2018
This month 'Focus on nutritional elements' focuses on molybdenum. Molybdenum is an essential trace element for some enzymes that largely determine the nitrogen balance of the plant.
Molybdenum (Mo) is absorbed by the plant as MoO43--ion. The trace element has an important role in the nitrogen system of the plant. A large number of enzymes in plants do not function without molybdenum. This is especially the case for the enzyme active at the uptake of nitrate (nitrate reductase). This enzyme reduces nitrate (NO3-) to ammonium (NH4+) in the plant. Ammonium can be built in amino acids. Whereas plants do take up nitrogen directly as ammonium they are less vulnerable for molybdenum deficiency. The molybdenum content in plants is on average 0.0002% of the dry matter.
Molybdenum is added to most potting soil base fertilizers. The uptake of molybdenum by plants is strongly dependent on the pH of the root environment. At a low pH (around 4 or 5) molybdenum is present as H2Mo4. This cannot be taken up by plants. Molybdenum can also be adsorbed on metal oxides. Humic and fulvic acids however can also complexate molybdenum thus improving solubility at a low pH. This can also play a role in peaty substrates. At a high pH, a luxurious uptake can occur.
The nitrogen present in the plant will not be sufficiently tied due to the poorly functioning enzymes. Also the leaf meristem could be damaged. This is the case with roses for example. In the young leaves necrosis develops from the leaf margin. Deciduous tree crops show molybdenum deficiency by the upwards curling of the leaf margin, mainly at the young leaves. Necrosis symptoms are often found on the leaf margins. With cauliflower molybdenum deficiency causes whiptail and with lettuce soft leaves. The chance on molybdenum deficiency increases at a low pH. Molybdenum toxicity is not known in practice.
Images of molybdenum treatments in cabbage after 18 days.
Image of cabbage with 0 and 0.06% molybdenum dose at pH 5.5.
Images of molybdenum treatments in lettuce after 18 days.
Molybdenum is added to most potting soil base fertilizers. Among others, the molybdenum level of RHP certified products is checked before application in a substrate. The substrate producer and the grower decide together what the level of molybdenum needs to be, suitable to the culture. In substrates it is advised to use a potting soil base fertilizer with a molybdenum content of at least 0,2%. As such a good base is created for an optimal provision of molybdenum in the plant.