Current status of soils and responsiveness of wheat to micronutrient applications

Abstract

Most micronutrient trials in the UK were performed when winter wheat yields were less than half those achieved now. Recent publications suggest that modern high-yielding wheat varieties have lower micronutrient concentrations in grain. It is not clear if this is due to changes in the crop or in the micronutrients available in soils.

Over 30 years, the geometric mean total concentration of copper (Cu) in soil decreased by less than 1 mg/kg and manganese (Mn) by nearly 40 mg/kg, but total zinc (Zn) increased by almost 5 mg/kg. Soils were also extracted with EDTA (ethylene diamine tetraacetic acid) and there was only a decrease of just over 1 mg/kg for Cu and Zn, which is not likely to be biologically significant. Manganese extracted by EDTA decreased by nearly 50 mg/kg, but for Mn this extract is not thought to be useful for indicating crop availability.
Fifteen field experiments were performed on soils likely to be deficie

nt in micronutrients (light, calcareous or soil high in organic matter) all sown with winter wheat variety Solstice. Each experiment was treated at the early stem elongation stage with sprays containing Cu, Mn or Zn at the rates recommended for severe deficiency. Only two statistically significant yield responses were observed. These were +0.27 t/ha in the Zn spray treatment at a site on light loamy sand at Morley (P =0.04), and +1.39 t/ha with Cu sprays on a highly organic soil at Waterbeach (P =0.058). EDTA soil extraction gave correct prediction on responsiveness in yield to Cu in one out of two cases where concentrations were classed as low, but only by adopting a critical value for Cu of 1 mg/kg. Leaf analysis in spring was predictive of the one Cu responsive site, but this appears to be less useful for Mn and Zn.

The percentages of responsive trials, although low, are in fact similar to UK recent results of winter wheat trials, selected as those performed since 2005, for Cu (7%) and Mn (0%) but not for Zn, where the percentage was slightly smaller (7% versus 16%). It seems unlikely that modern high-yielding wheat is more responsive in yield to micronutrient applications than older varieties. This may be because most UK soils contain sufficient concentrations of available micronutrients, although there is specific soil and local variation. Although our experiments were not specifically set up to examine the possibility of “growth dilution” of micronutrient concentrations in grain, there was no evidence that the lowest grain micronutrient concentrations were associated with the highest yields in our experiments.