Sombric horizon: five decades without evolution

The sombric horizon is a diagnostic subsurface horizon defined in the soil classification system of the United States (Soil Taxonomy) and FAO (WRB), used to classify the soil at different categorical levels. The sombric horizon has a soil color darker than the overlying surface(s) horizon(s), and must show illuvial humus accumulation features, though they are not associated with aluminum (Al), as in the spodic horizon, nor associated with sodium (Na), as in the natric horizon. There are also criteria to distinguish it from buried A horizons. However, since the first references and proposed concept of the sombric horizon in African soils made by Sys and co-workers in the 1960s, and adopted by the Soil Taxonomy edition of 1975, few modifications have been made to its definition. Moreover, the pedogenic process involved in illuvial humus accumulation in these horizons remains inadequately clarified, making the distinction between the sombric and spodic or buried A horizon difficult and unclear. This review reports the historical evolution of the sombric horizon concept, its definition and inconsistencies under different soil classification systems, and the current hypothesis, together with its fragilities, proposed to explain the soil illuvial humus accumulation. Although it is recognized that further research is necessary, alternative criteria are proposed for the definition of the sombric horizon in the Brazilian System of Soil Classification

Soil and foliar phosphorus determination in Rwanda: procedures and interpretation

Revised reference values of available soil phosphorus by Bray I below which bean (Phaseolus vulgaris L.) and sorghum (Sorghum bicolor L.) yield response to applied P is expected, were 6 and 9 ppm, for January and July soil sampling dates, respectively. Reference values determined for interpretation of foliar test results of bean differed from those estimated previously. The critical foliar level of P concentration was estimated to be 0.40%. The importance of modifications of the Bray I procedure for soil P analysis was assessed using the modifications of three soil laboratories in Rwanda. Variations in shaking time, soil to extraction solution ratio, and soil sieve size were found to significantly affect available P values using the Bray I procedure. The modifications of one laboratory gave values which were 75% higher than those of the original Bray I procedure. Errors in preparation of standard solutions and in calculations resulted in values ranging from 2.5 to 300% of the correct value.Des valeurs de reference revisees pour des teneurs du sol en P assimilable determine par Bray I, en dessous desquelles on s'attend e une reponse positive des cultures (Phaseolus vulgaris L. et Sorghum bicolor L.) e un apport en phosphore, etaient 6 et 9 ppm pour des echantillons preleves en janvier et en juillet respectivement. Des valeurs de reference etablies pour l'analyse foliaire du haricot se distinguent des valeurs publiees avant. Une teneur de 0.40% P dans les feuilles est proposee comme niveau critique pour l'alimentation du haricot. L'importance des modifications de la methode "Bray I" pour l'analyse des teneurs du sol en phosphore assimilable a ete revelee dans trois laboratoires au Rwanda. La methode a ete modifiee de plusieurs faons differentes et les resultats provenus de differents procedes ont ete compares. Les valeurs trouvees pour le phosphore assimilable dependaient significativement de la duree d'agitation, du rapport sol/solution d'extraction et du tamisage (e 0.5 ou 2mm). Les modifications des valeurs depassaient dans un laboratoire de 75% les valeurs estimees par la methode originale Bray I. Des erreurs faites lors de la preparation des solutions etalon et calculs ont fait aboutir e des valeurs entre 2,5 et 300% de la valeur correcte

Soil and foliar phosphorus determination in Rwanda: Procedures and interpretation

Revised reference values of available soil phosphorus by Bray I below which bean (Phaseolus vulgaris L.) and sorghum (Sorghum bicolor L.) yield response to applied P is expected, were 6 and 9 ppm, for January and July soil sampling dates, respectively. Reference values determined for interpretation of foliar test results of bean differed from those estimated previously. The critical foliar level of P concentration was estimated to be 0.40%. The importance of modifications of the Bray I procedure for soil P analysis was assessed using the modifications of three soil laboratories in Rwanda. Variations in shaking time, soil to extraction solution ratio, and soil sieve size were found to significantly affect available P values using the Bray I procedure. The modifications of one laboratory gave values which were 75% higher than those of the original Bray I procedure. Errors in preparation of standard solutions and in calculations resulted in values ranging from 2.5 to 300% of the correct value

Caracterização e classificação de solos com horizontes subsuperficiais escuros derivados de rochas graníticas no Escudo sul-rio-grandense

Horizontes subsuperficiais escuros com características morfológicas similares aos solos com caráter sômbrico são encontrados nas áreas mais elevadas da porção leste do Escudo sul-rio-grandense, desenvolvidos nos horizontes A2, AB (ou AC) e BA de Argissolos, Luvissolos e Neossolos Regolíticos, respectivamente. Os mecanismos responsáveis pela sua gênese são ainda pouco compreendidos, apesar de a maioria dos autores concordar com a origem iluvial do húmus e de constituir horizonte diagnóstico. Os objetivos deste trabalho foram caracterizar e analisar o enquadramento de nove perfis de solos com horizontes subsuperficiais escuros do Escudo sul-rio-grandense nos critérios do SiBCS, WRB e Soil Taxonomy, sendo sete deles com características morfológicas de sômbrico. A observação de campo (continuidade lateral na paisagem) e a morfologia (ausência de horizonte E, cores mais escuras que os horizontes sobrejacentes e presença de cutans) apóiam o caráter sômbrico para a maioria dos perfis estudados, com a inserção do caráter sômbrico em nível de subgrupo nas classes de Argissolos Bruno-Acinzentados, Amarelos e Vermelho-Amarelos, bem como em Luvissolos e Neossolos Regolíticos, além da inclusão de eutrófico no nível de grande grupo na classe dos Argissolos Bruno-Acinzentados. Os dados sugeriram a revisão da definição de dessaturação na determinação do caráter sômbrico (distrófico) no SiBCS, com a adoção do valor V de 65 %, adequando essa à definição do Soil Taxonomy e WRB, bem como uma investigação mais aprofundada quanto à não associação do húmus com Al; e a consideração da retirada da capacidade de troca de cátions da matéria orgânica no cálculo da atividade da argila, para os horizontes subsuperficiais escuros