Soil and herbage magnesium responses to long- term applications of mineral fertiliser and slurry

Abstract

Magnesium (Mg) is vital for plant growth processes. Furthermore, if plants develop a Mg deficiency, it can negatively impact the mineral intake of livestock. The aim of this study was to analyse long-term nutrient (including two types of slurry and mineral fertiliser) effects on the concentrations of Mg in soil and herbage, the Tetany Index, and the Mg balance. Data were examined from a long-term grassland experiment established in 1970 in Northern Ireland. Nutrient treatments had significant effects on the soil extractable Mg (SEMg) (using a 1 M ammonium acetate solution). The mineral fertiliser treatment had the lowest concentration of SEMg, and temporal trends indicated there was significant mining of Mg. The concentration of SEMg was high for all the rates of cow slurry applied, which were in excess of the optimal soil Mg status. The rate of applied slurry significantly increased the SEMg values; however, there was no slurry rate effect on the concentration of herbage Mg. Nutrient treatment responses were found for the Mg offtake and Tetany Index, and this included significant seasonal effects. The Tetany Index remained below the risk threshold value up until 1985; thereafter, in most years, the high cow slurry treatment often exceeded it. This indicates a low risk of hypomagnesaemia for low rates of cow slurry, pig slurry (all rates) and the mineral fertiliser (NPK) treatment. A large positive Mg balance was determined for all the slurry treatments, while for the NPK treatment, there was a negative Mg balance, which corresponded to soil Mg deficiency