Micronutrients fail the ‘common sense’ test

2 Oct 2013

Micronutrient treatments are often relatively cheap in terms of cost/ha but not necessarily in terms of cost/kg of the nutrient(s) in question. However, despite their relatively low price/ha, treating the whole farm with them will often cost more than a good holiday, even after tax.

There is tremendous pressure to use them but limited independent evidence of their value. The farming press has printed articles over the last few months suggesting that they are of great value but the evidence put forward is compromised by the fact that they have been applied together with extra nitrogen or extra fungicides or both. Who is to know where the claimed extra yield comes from? So it’s good news for the industry that HGCA has recently published the results of a research project on the value of three micronutrients (copper, zinc and manganese) when applied to wheat (Current status of soils and responsiveness of wheat to micronutrient applications – Project Report no 518).

Part of the continuing interest in micronutrients is the fear that despite the fact that annual uptake is low, their reserves in the soil may be falling to or below critical levels to support crop production. So, quite sensibly, this project included analysing 132 soil samples and comparing the micronutrient levels in the soil to those in a survey carried out 30 years ago. The results indicate that levels have fallen marginally but overall the changes are not likely to be biologically significant.

Another issue frequently raised when discussing micronutrients is that yields are now higher than when much of the original research on the responsiveness of wheat to their application was carried out. Hence, the research team that carried out the project did a total of 15 experiments over three years in order to measure the potential benefit of micronutrients in the context of today’s wheat management and yields. Sites were on soils that had a risk of micronutrient deficiency: light sandy soils, soils high in organic matter and calcareous soils.

The problem came when interpreting yield responses because they, both negative as well as positive, were typically below statistical significance. However, the levels of yield responses required to pay for the micronutrient application are typically way below the level of statistical significance. This is an age old issue and was a subject of my blog on 22nd March this year (Can you take the risk of not using it?).

In this situation, you have to be pragmatic and apply common sense by looking at all the results to see if there is a consistent trend. Should a very high proportion of the trials have a positive but non-significant yield response that is sufficiently high to more than pay for the micronutrient, then personally I would accept that their application is economically viable.

However, the results in this project by no means reflect this. For instance, at face value, the response to copper was at or less than 0.1 t/ha in 12 of the 15 trials with five trials indicating a negative yield response. The equivalent figures for manganese showed nine out of 15 trials had a response at or below 0.1 t/ha with seven trials indicating a negative yield response, and for zinc eight out of 15 trials and six trials respectively. This suggests that generally the small and non-statistically significant yield variations from those recorded in the untreated plots were due to random error and not to the application of micronutrients. So, with the best will in the world, the results suggest that there is no ‘common sense’ evidence that these micronutrients should be used as a matter of course, even on the soil types where yield responses are most likely to occur.

There were however the two statistically significant results. A very large response to copper application was recorded on one site. An economic response would reasonably have been expected from a soil analysis of the site. There was also a statistically significant response to zinc on one site, which was also supported by a soil analysis but, in this case, not by a leaf analysis. In fact, overall the project indicated that leaf sampling in the spring was of no value for determining the need for an application of copper or zinc.

The real surprise was the lack of statistically significant yield responses to manganese. This led the researchers to say that it “has had attention disproportionate to its yield benefits”. However, it would be a brave person not to treat a wheat crop showing deficiency symptoms which might increase susceptibility to disease and pesticide damage. On the other hand, the results provide food for thought for those who are fond of multiple applications in the absence of symptoms.

So there you have it. Even on soil types where these micronutrient deficiencies are most likely to occur the ‘common sense’ test, as well as the statistical tests, suggest that it is uneconomic to apply these micronutrients to modern high-yielding wheat as a matter of course. Soil analysis needs to be carried out to determine the probability of a response to zinc and copper and in the project critical values in the soil were identified.

This is an important project but there are, of course, other sources of trials information on the value of micronutrients. These have been reviewed for HGCA and will be subject of another blog in the near future.