Magnesium deficiencies in soils and crops in Northern Portugal

Abstract

Several studies carried out in Northern Portugal have drawn attention to the occurrence of magnesium deficiencies or deficient Mg nutrition in different crops. From July-August onwards symptoms of Mg deficiency can be observed in certain areas of this region in maize, fruit crops, fruit-trees and forest species.

In the Trás-os-Montes and Alto Douro region and in the Entre-Douro and Minho region, Mg deficiencies in crops have been detected predominantly in soils located on schists of the Siluric and Ordovician, on schists of Precambrian and Cambrian of the Schist and Greywacke complex, and soils derived from two-mica alkaline granite. The following lithology is found in the schists and in the Schist and Greywacke complex: feldspar quartz-phyllites, phyllites, micaschists, graphytic slates, siltstones, ampelites, lydites, psamites, greywackes, phyllito-quartzites and quartzites. The soil units where low foliar Mg concentrations or visible Mg deficiencies in crops more often observed are: Dystric and Umbric Leptosols, Umbric Regosols, Dystric and Umbric Cambisols, Dystric and Umbric Fluvisols, Umbric and Aric Anthrosols and Cumulic Anthrosols.

Some of the data presented here were gathered from studies where the main purpose was the confirmation of Mg deficiencies and surveys to assess the nutritional status of various crops. The majority of the studies used, as a diagnostic tool, both the foliar Mg concentrations and, in some cases, the nutrient ratios Mg/N and Mg/K. In some studies, the threshold values for the occurrence of Mg deficiencies were determined. Most studies showed correlations between foliar Mg concentrations and several soil chemical parameters, such as soil solution Mg²⁺, exchangeable Mg²⁺, ratios between exchangeable cations Mg²⁺/K⁺, Mg²⁺/Ca²⁺, Mg²⁺/Ca²⁺ + Mg²⁺, Mg²⁺/Ca²⁺+ Mg²⁺+K⁺, Mg²⁺/CTC, Mg²⁺/NH4⁺ and Mg²⁺/Al³⁺+H⁺ or the combination of some of these ratios.

Mg deficiency seems to be a classic problem in acid soils derived from parent material poor in Mg, but it can also be triggered by local factors or by inappropriate management practices.