Magnesium supply is vital for improving fruit yield, fruit quality and magnesium balance in citrus orchards with increasingly acidic soil

Magnesium (Mg) deficiency is becoming a limiting factor for citrus production in acid soils of subtropical and tropical zones. It is speculated that soil Mg leaching and thereby its imbalance may be a major cause of yield decline, yet Mg deficiency in citrus receives little attention. A two-year field experiment was therefore conducted to quantify soil Mg leaching in a typical citrus orchard in China fertilized with varying levels of Mg (Mg₀, no Mg fertilizer; Mg₄₅, 45 kg MgO ha^–1 yr^–1; Mg₉₀, 90 kg MgO ha^–1 yr^–1; Mg18₀, 180 kg MgO ha^–1 yr^–1). Results showed that Mg application significantly increased citrus fruit yield by 4.1–16.4% compared with where MgO was not added. The average amount of soil Mg leaching was 65.7 kg ha^–1 yr^–1 where no Mg fertilizer was added, while it reached up to 91.3 kg Mg ha^–1 yr^–1 where MgO was added at the rate of 180 kg ha^–1. Over the 4 treatments, Mg leaching accounted for 12.1–42.4% of the applied Mg fertilizer. Mg leaching and its removal through harvested fruits resulted in an orchard soil Mg balance of –69.9, –51.1, –27.4 and 10.9 kg ha^–1 in the Mg_0, Mg_45, Mg₉₀ and Mg₁₈₀, treatments, respectively. The pH values of leachate from the acid soil were alkaline and it contained higher amounts of calcium and potassium than that of Mg. Considering the high leaching of Mg from the acid soils of citrus orchards, applications of Mg fertilizer or Mg-fortified soil conditioner are vital to sustain soil Mg balance, high fruit yield and fruit quality in citrus production systems in humid subtropical regions.