Why is molybdenum critical for legumes?
Written by
Tina Carter
Reviewed by
Prof. Martin Thorne, Ph.D."Molybdenum for legumes" is key in the nitrogen fixation process, and the root nodules cannot convert nitrogen in the air into ammonia usable by plants if molybdenum is missing. I have seen green and lush soybean fields with low nitrogen tests because there was a molybdenum deficiency that no fertilizer could fix.
Deficiency Signs
- Yellowing: Older leaves show nitrogen deficiency despite soil tests
- Stunted nodules: Root nodules remain small and ineffective
- Poor pod fill: Peas develop hollow pods with few seeds
Application Protocol
- Rate: 2-4 oz/acre sodium molybdate pre-planting
- Timing: Mix with inoculants at seeding for optimal nodulation
- pH: Maintain soil pH >6.0 to enhance availability
Soil pH influences the availability of molybdenum more than any other micronutrient. In acidic soils below pH 6.0, molybdenum interacts very strongly with certain iron oxides. For example, a farmer in Wisconsin limed his soybean fields to achieve a soil pH of 6.8 and reported that his soybeans' molybdenum uptake tripled without increasing their molybdenum fertilizer rate.
Applications at the beginning of the growing season are far better than applications at mid-season. We have been coating seeds with sodium molybdate at a rate of 1 oz/acre and have had much better success with nodules forming correctly right from the start. One Nebraska grower noted a 28% increase in pea yield by using sodium molybdate as a seed coat compared to using foliar sprays at flowering.
Make sure to test soil and tissue at the same time. Molybdenum levels in soil greater than 0.2 ppm, mean very little if the root nodules have less than 5 ppm in the tissue tests. As a suggestion, paired testing at the first flowering can catch 90% of deficiencies occurring before a loss in yield begins.
Read the full article: 7 Essential Micronutrients for Plants: Complete Guide
