Low molybdenum levels in sub clover limiting nitrogen fixation

A survey of soil constraints across the South West beef farms has found that half appear to be low in molybdenum. The trace element is critical for the development of nitrogen-fixing nodules on legume roots. The survey also scored nodulation in the South West, finding that nodules were scarce or rare in 42 percent of sub clover plants sampled.

The project, run by South West Catchments Council in collaboration with Western Beef, surveyed 24 farms in the high rainfall zone roughly west of a line between Brunswick and Pemberton. Molybdenum was assessed by tissue testing sub clover plants, which are typically more sensitive to nutrient deficiencies compared to ryegrass, and comparing results to a critical value of 0.1-0.2 parts per million (ppm). Of the 21 farms with sub clover in the pasture sward, six had at least one site with less than 0.1 ppm of molybdenum and a further five had between 0.1 and 0.2 ppm. A total of 37 sites were tested across the farms with 14 below 0.2 mg/kg (38%).

Molybdenum is often applied in fertiliser with two other micronutrients, copper and zinc. However, they both appear to be well-maintained with copper found to be low in just one site while zinc was not low in any site. Phosphorus (22% of sites) and potassium (11%) were the only other nutrients found to be low in sub clover according to CSBP’s NuLogic analysis.

The soil constraints project recently held an event in Donnybrook where Murdoch University’s Professor Richard Bell spoke about micronutrient deficiencies in South West WA.

“Historically, Western Australia had millions of hectares deficient in copper, zinc and molybdenum, and until they were added, a lot of land couldn’t be used for agriculture,” Professor Bell said. “Now we tend to consider micronutrient deficiency to be the exception rather than the norm, because once you’ve added a reasonable amount, they last in the soil for quite a long time. However, we still need to keep track to make sure they aren’t running out.”

Professor Bell said that changes in farming over the past few decades does raise questions about the current micronutrient status. For example, soils have acidified with lower soil pH compared to the 1960’s when much of the research into micronutrients was done, which can affect the availability of micronutrients. In the case of this survey, 34% of all sites had low soil pH below 4.5 in the topsoil.

“For most micronutrients, availability is higher at low soil pH. As you increase pH, the availability of elements like copper, zinc and manganese drops. Growers need to be aware of this when they lime because high rates of lime can tip these elements into a deficiency if they are already marginal. However, molybdenum is the exception to this rule. It is least available at low pH.”

Professor Bell said that the removal of micronutrients each year is relatively small, so levels can take years to run down. For copper and zinc, it may take 15 to 20 years, while molybdenum is not as long (5-10 years). He also pointed out that too much molybdenum can affect copper metabolism in animals and cause copper deficiency.

“The key with copper is that the residual value is extremely long. Some trials in the wheatbelt have shown that additions 30 years ago are still correcting copper deficiency.

“In the case of zinc, high phosphorus levels can reduce the availability of zinc, and if pH is seven or above you can tip into zinc deficiency.

“Molybdenum is mostly there to make sure nodules develop and work properly to fix nitrogen. So, one of the first indications of a molybdenum deficiency is nitrogen deficiency in the grasses growing with legumes or in the legumes themselves. Low pH may be the reason for molybdenum deficiency so liming may be the solution on acid soils.”

Professor Bell said that tissue testing every five years in spring will help to track micronutrient levels. On the question of whether marginal levels need to be treated, he said that the range for “marginal” is quite large, so action depends on whether you are at the top or bottom end of the range. He suggested concentrating on addressing “low” results.

In the case of molybdenum, critical values are less certain compared to other nutrients, so it is often not given a rating by laboratories. For this reason, molybdenum isn’t normally included in standard tests and must be asked for specifically. Professor Bell suggested using a critical value for molybdenum in sub clover shoots of around 0.1 to 0.2 ppm.

Tissue testing is considered a better test for micronutrient deficiency compared to soil tests because plant availability depends on factors related to the physical, chemical and biological conditions in the rhizosphere. However, regardless of what test was used, he warned that contamination of plant and soil samples can cause misleading results.

“Growers sampling soil or plant tissue need to keep good cleanliness because galvanised surfaces on farms and copper, brass and bronze can get on your hands and then onto the plants or soil you are sending away for analysis, affecting results.”

Another way to monitor levels is to consider how long it has been since a decent dose of a particular element was added and comparing that to the number of years it typically resides in the soil. This can even be calculated on the back of an envelope if you know the amounts of each nutrient being exported off farm. For modern high productivity dairies, the removal of micronutrients is higher than in the past and that needs to be considered in the program for re-application of micronutrients.

Testing for or estimating molybdenum levels based on application history and addressing deficiencies will go some way towards improving nitrogen fixation in legumes. However, note that molybdenum should be applied conservatively to avoid reducing the availability of dietary copper in the rumen.

This project is supported by South West Catchments Council and Western Beef Association Inc, with funding through the Australian Government’s National Landcare Program.