Is excessive Ca the main factor responsible for Mg deficiency in Sulla carnosa on calcareous soils?

Purpose Magnesium deficiency in plants was described on several calcareous soils. Some authors attributed such an induced Mg deficiency to excess calcium, whereas some others attributed it to an influence of carbonate minerals on Mg bioavailability to plants. In this investigation, we attempted to d...
Ausführliche Beschreibung

Purpose Magnesium deficiency in plants was described on several calcareous soils. Some authors attributed such an induced Mg deficiency to excess calcium, whereas some others attributed it to an influence of carbonate minerals on Mg bioavailability to plants. In this investigation, we attempted to determine which factor is responsible for Mg deficiency in the Fabaceae Sulla carnosa found on several types of soils, including calcareous ones. Materials and methods S. carnosa plants were cultivated on an agricultural soil (control soil) that was either added with 20 % $ CaCO_{3} $ or watered with 100 mM $ CaCl_{2} $. After 1 month of treatment, gas exchange measurements were performed in these plants which were harvested later for dry weight determination, pigment contents, and mineral analysis. Also, some physicochemical properties (pH, EC, and mineral analysis) of the soil were studied. Results and discussion Calcareous treatment showed more detrimental effect on whole plant and shoot growth, leaf area, Mg concentrations in leaves and stems, Mg uptake efficiency, carotenoid concentration, and carotenoid/chlorophyll ratio as compared to $ CaCl_{2} $. By contrast, calcareous treatment increased chlorophyll concentrations, Mg/Ca transport selectivity, and Mg use efficiency. Soil analysis showed that $ CaCO_{3} $ had no effect on soil properties (pH, EC, extractible Mg and Ca concentrations, and Ca/Mg ratio), whereas $ CaCl_{2} $ induced substantial increases in EC (ca. +436 %), Mg (ca. +760 %), and Ca (ca. +354 %) levels and a significant decline in Ca/Mg ratio (ca. −48 %) as compared to control soil. Conclusions Ca excess was not the main factor inducing Mg deficiency in S. carnosa on calcareous soils. Indeed, $ Ca^{2+} $ concentration did not increase in the soil because of the low solubility of $ CaCO_{3} $. Soil pH and extraxtible Mg concentration were also unaffected by $ CaCO_{3} $ addition, and therefore, they cannot be directly related to Mg deficiency. Hence, one can speculate that bicarbonate ions affect root Mg transporters and/or Mg availability within plant tissues. Ausführliche Beschreibung