Is Molybdenum to blame?

Have you ever walked a paddock of lucerne or clover and observed underperforming plants? A paddock full of yellow, stunted plants rather than green and vibrant? Though a deficiency in nitrogen (N) is a common culprit, for legumes, a lack of Molybdenum (Mo) could be behind the N deficiency.

N and Mo are two essential elements for plant growth. However, their importance should not be gauged by the quantity plants require for healthy growth. For example, if a pasture producing
10 tonnes of dry matter per hectare per year uptakes approximately 400 kg of N, the same pasture will only uptake 3 g of Mo. Though these elements are taken up in the largest and smallest amounts respectively, they are intrinsically linked. So, even a small reduction in Mo availability can significantly impact legume ability to use N for growth.

What role does Molybdenum play?
» Mo turns nitrate, absorbed by the plant, into amino acids and proteins.
» It completes the nitrogen fixation process by the nodules on the plant’s roots (in legumes).

Many of New Zealand’s soils are Mo deficient. Sedimentary soils, especially those with a low pH, are more likely to have deficiencies. Yet, its importance can be easily overlooked or farmers may be cautious about its use (because high levels of Mo in herbage can induce copper deficiency in animals). However, this issue occurred before knowing how to use Mo was fully understood. Application rates, based on correct testing regimes and recommended industry guidelines, prevent over application.

A Mo/N deficiency must be confirmed using a legume only herbage test, as there is no reliable soil test for Mo. Once confirmed, this can be easily rectified using solid or foliar fertilisers. Soils
with low pH can restrict the soils ability to provide Mo making it unavailable to plants even with optimal levels of Mo in the soil. Liming soils, keeping the pH within the optimum 5.8 to 6.2 range,
makes Mo in the soil more available to the plants.

Managing N deficiency in December
» If crops are showing signs of nutrient deficiency, get the herbage tested. Depending on the deficiency, this can be corrected with a solid or foliar fertiliser.
» If water is limiting (drought) and plants are not growing, then N fertiliser will be of little benefit until the soil is wet again.
» If weather conditions are suitable, early summer nitrogen can help build covers going into the dry part of the summer.

To learn more about Mo deficiencies and improving your clover or lucerne crops, contact your PGG Wrightson Technical Field Representative today.

WRITTEN BY JAY HOWES, PHD SOIL SCIENCE
PGG WRIGHTSON TECHNICAL SPECIALIST – SOIL SCIENCE