Assessment and Description of Magnesium Deficiencies in Chestnut Groves

Abstract

In order to diagnose the symptoms of conspicuous magnesium (Mg) deficiency in sweet chestnuts (Castanea sativa Mill.) and to determine its effect on yield, two types of studies were carried out. In study 1, twenty six chestnut groves were selected to assess both foliar and soil nutrient status. In study 2, a twenty year‐old chestnut grove with a conspicuous Mg deficiency was selected and trees were classified according to foliar chlorosis, into three categories, in order to determine the effect of Mg deficiency on several tree growth parameters and yield. Study 1, showed an average foliar Mg concentration of 0.5 g kg−1 in trees with evident leaf chlorosis. When Mg concentration was 0.3 g kg−1 or less premature leaf fall was observed. The symptom free trees had an average Mg concentration of 1.6 g kg−1 or higher. The foliar K/Mg ≥ 9 ratio seemed to be the threshold value above which leaf chlorosis was observed. The ratio N/Mg ≥ 23 also indicated the boundary above which visible Mg deficiencies might be observed. Most soil fertility parameters were more favorable in the vigorous groves and no significant differences were detected in the soil within the chlorotic groves regardless of some Mg deficiency. Calcium (Ca) and Mg are well correlated both in the leaves and in the soil. Very low values of exchangeable Mg and Ca and a high percentage of exchangeable acidity in Mg deficient chestnut stands were observed. Multiregression analysis reveals that the Mg/NH4, Mg/Al + H and Mg/K ratios in the soil explain 53% of the foliar Mg variation. Study 2, showed a negative correlation between severity of chlorosis and foliar Mg concentration, tree growth parameters and nut yield. Tree height, leaf area index and nut production were reduced to half in trees with acute leaf chlorosis. Nut calibre (fruits/kg) increased 50%. The threshold value below which the foliar Mg deficiency could be detected was 1.2 g kg−1. Soil exchangeable Mg was, in general, very low and few differences in soil fertility parameters were observed beneath the three categories of leaf chlorosis, excepting the high levels of inorganic N underneath the chlorotic trees.