Deficiências de magnésio em solos e culturas do norte de Portugal
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2007
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Diversos estudos realizados no norte de Portugal têm alertado para o ocorrência de carências de magnésio ou para a possibilidade de deficiente nutrição magnesiana nalgumas culturas. A partir de Julho-Agosto, pode observar-se a sintomatologia de carência de Mg em diversas zonas daquela região em culturas como o milho, em arbóreas e arbustivas, e ainda em espécies florestais. Em Trás-os Montes e Alto Douro e no Entre-Douro e Minho as deficiências de Mg nas culturas têm-se observado sobretudo em solos desenvolvidos em xistos do Silúrico e do Ordovício, em xistos do Precâmbrico e Câmbrico do complexo Xisto-grauváquico e em solos provenientes de granitos predominantemente alcalinos de duas micas. Nas formações silúricas, ordovícias e do complexo Xisto-grauváquico dominam as seguintes litologias: quartzofilitos feldspáticos, filitos, micaxistos, xistos grafitosos, ampelitos, liditos, psamitos, grauvaques, siltitos e, ainda, quartzitos xistóides e quartzitos. As unidades-solo onde, frequentemente, se tem observado baixas concentrações foliares de Mg, ou mesmo sintomas da carência de Mg são: os Cambissolos e Leptossolos úmbricos; os Cambissolos e Leptossolos dístricos; os Regossolos úmbricos; os Fluvissolos dístricos e úmbricos; e, ainda, os Antrossolos úmbricos, cumúlicos e áricos. Alguns dos dados aqui apresentados foram coligidos, não só em estudos em que o principal objectivo foi o da confirmação da deficiência de Mg, mas também em levantamentos do estado nutritivo de diversas culturas. Na maioria dos estudos utilizou-se como método de diagnóstico as concentrações foliares de Mg, mas também foram identificadas razões entre nutrientes, nomeadamente Mg/N e Mg/K, associadas à carência de Mg. Nalguns deles determinou-se o limiar a partir do qual se observaram as deficiências de Mg. Em quase todos os trabalhos se acharam correlações entre as concentrações foliares de Mg e diversos parâmetros químicos do solo, como os teores de Mg na solução do solo e o Mg de troca, com as razões Mg/K, Mg/Ca, Mg/Ca+Mg, Mg/Ca+Mg+K, Mg/CTC, Mg/NH4 e Mg/Al+H, ou com o efeito combinado destas razões. A carência de Mg parece ser clássica em solos ácidos provenientes de material originário pobre em Mg, mas também pode ser despoletada por factores locais, ou por práticas culturais desajustadas.
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 quartzphyllites, 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 Mg, exchangeable Mg, ratios between exchangeable cations Mg/K, Mg/Ca, Mg/Ca+Mg, Mg/Ca+Mg+K, Mg/ CEC, Mg/Al+H and Mg/NH4, 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.
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 quartzphyllites, 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 Mg, exchangeable Mg, ratios between exchangeable cations Mg/K, Mg/Ca, Mg/Ca+Mg, Mg/Ca+Mg+K, Mg/ CEC, Mg/Al+H and Mg/NH4, 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.
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Magnésio , Solos ácidos , Norte Portugal , culturas