Molybdenum (Mo) is a plant micronutrient. So little is used that they used to be called trace minerals, but that doesn’t mean they are not important. Molybdenum is very important to your plants’ health.
Generally speaking, molybdenum is plentiful in alkaline soils and tends to be deficient in acidic soils. You can’t know what your soil contains without a soil test from a reputable lab. Those cute, colorful kits from the garden center don’t even test for molybdenum. Even if they did, they are not [yet] accurate enough to be useful.
How plants use molybdenum
Molybdenum is an essential ingredient to some very important enzymes. These enzymes are used in nitrogen, oxygen, and sulfur cycles. Specifically, molybdenum is used convert nitrate into nitrite and then into ammonia in order to be used to synthesize amino acids. It is also used by the bacteria responsible for converting atmospheric nitrogen into forms usable by plants. Molybdenum is also part of the process that converts inorganic phosphates into organic ones. Cruciferous plants, such as broccoli and cauliflower, and legumes, such as soybeans and clover, and citrus use a lot of molybdenum.
Plant nutrients are either mobile or immobile within a plant. Molybdenum is mobile, which means it moves around easily within a plant. This makes diagnosing deficiencies easier because they are most often seen in older leaves as plants pull nutrients to make new leaves. Molybdenum toxicity is practically unheard of, but deficiencies can be a serious problem.
Symptoms of molybdenum deficiency
Without molybdenum, leaves turn yellow and die and flowers may fail to form at all. The yellowing is often along leaf margins and downward cupping may also appear. In some cases, leaves develop a whiptail shape, rather than the leaf’s normal wider blade shape. Corn kernels may germinate on the cob prematurely in a last-ditch effort at reproduction. Legumes will have fewer or no root nodules if molybdenum is in short supply.
Again, you don’t know what your plants have access to without an inexpensive, lab-based soil test. Take my word for it, it is worth the effort.
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