Effects of Magnesium Fertilizer on the Forage Crude Protein Content Depend upon Available Soil Nitrogen

Magnesium (Mg) is important for both plant photosynthesis and protein synthesis. Nevertheless, latent Mg deficiencies are common, and Mg addition has shown an improved yield. Might such an increasing yield cause "hidden" hunger for microelements and protein, and if so, what is the mechanism? We conducted two greenhouse experiments using low-Mg soil to investigate (i) effects of five levels of Mg fertilizer (20-400 mg kg^-1) on eight elements and crude protein concentrations in annual ryegrass and white clover and (ii) if any protein effects of the Mg fertilizer depend upon soil nitrogen (N). Mg addition significantly increased the yield in both species, simultaneously decreasing concentrations of crude protein, calcium (Ca), sodium, manganese, and potassium/Mg and Ca/Mg ratios caused by increased biomass dilution effects and increased [Mg]. Other mineral dilution effects of the Mg fertilizer depended upon species: the concentration of phosphorus decreased only in ryegrass, and the concentration of zinc decreased only in white clover. Mg addition in soil rich with available N (from N fertilizer in ryegrass or biological fixation in white clover) showed an increased crude protein content as well as increased yield in the forage of both species. These results suggest that the Mg fertilizer can affect the protein content positively or negatively depending upon available N in soil and that sufficiently available N must be ensured along with Mg addition in low Mg soils.