Molybdenum is unique among the essential nutrients for plants because the plant cells themselves do not need molybdenum. However, plants cannot process phosphorus without it, so it is still considered essential. It also plays a role in nitrogen fixing that occurs in the root nodules of legumes, so peas, beans and alfalfa have greater needs for molybdenum.

The good news is, molybdenum is needed in very small quantities by plants, and it rarely deficient in garden soil. Since the signs of molybdenum deficiency are as unique as the nutrient itself, it is also fairly easy to diagnose. Too little molybdenum lets nitrate build up in leaves, causing them to curl or cup, especially in brassicas like cauliflower. Citrus, soybeans and clover are also more commonly affected by molybdenum deficiency.

But just because it’s in the soil, doesn’t mean it’s available for uptake by plants. Another unique attribute of Molybdenum is that it becomes more available as soil gets more alkaline, which is opposite of most micronutrients. Acidic and sandy soils are more likely to be deficient in available Molybdenum, so an application of lime can help increase availability for plants.