The project is in its third and final year. In the second crop year (2013) we established 20 new field trials across Iowa to evaluate effects of spraying boron (B), copper (Cu), manganese (Mn), zinc (Zn) and their mixture (in 2012 these treatments were evaluated in 18 trials). Treatments were replicated four times. We completed chemical analysis, data processing, and analysis of variance for 2013 soil tests, tissue testing for plants at the V5/V6 growth stage before spraying, mature leaf sampled at the R2/R3 stage, and grain yield. At this time we have preliminary results for micronutrient concentrations in grain, since chemical analyses were just completed.
All the planned soil and vegetative plant-tissue analyses of the project were completed. The 2013 results showed a wide range of values across sites and (as in 2012) very poor or no correlation among most soil and plant-tissue tests. The only high and statistically significant relationships found were between soil Zn measured by DTPA or Mehlich-3 methods and between soil Mn measured by the DTPA method and the Mn concentration of small plants or leaves at the R2/R3 stage. This general lack of correlation indicates that some of the soil or tissue tests are not good estimates of the crop-availability of the micronutrients for soybean.
There was no soybean grain yield increase at any of the 20 trials, but a yield decrease from application of Cu, Zn, and the mixture in one trial. Also, an analysis of averages across all trials showed a small but statistically significant yield decrease from the mixture. Results reported before for 18 trials conducted in 2012 also showed no soybean grain yield increases from foliar application of these micronutrients at any trial. Calibrations of soil and plant-tissue tests cannot be performed given the lack of grain yield response, but results suggest that some sufficiency levels for soil and plant-tissue tests being used by some companies or states encourage unneeded micronutrient fertilization for soybean.
In contrast to the yield results, preliminary data from chemical analysis of soybean grain for the trials conducted in 2013 showed that fertilization increased the grain concentration of B and Cu in most trials and the concentration of Mn and Zn in more than one-half of the trials. Results also showed that removal increased linearly as the soybean yield level increased.
During the next few months we will complete data management and statistical analyses of all data across the two crop years of the project, and we will write the final report.
A summary with preliminary results was presented in November at the annual meeting of the American Society of Agronomy (in Tampa, Florida); North-Central Extension-Fertility Conference; and in December at the University of Minnesota Extension Crop Pest Management Short Course.