Soil Characteristics determining the soil water retention of Pumice soils from Puebla-Tlaxcala  valley, Mexico.

Segura-Castruita, M.A.¹, Gutiérrez-Castorena, M.A.², Ortiz-Solorio, C.A.², Sánchez-Guzman, P.² y Frías-Ramírez, J.E.¹

¹Instituto Tecnológico de Torreón. División de Postgrado. Carretera Torreón-San Pedro Km. 7.5. Torreón, Coahuila. México. CP 27170. email: dmilys@hotmail.com

²Colegio de Postgraduados. Montecillo, Texcoco, Edo. México. México.

Soil of Puebla-Tlaxcala valley, known for regional farmers as residual moisture soils (RMS), present mainly pumiceous material scarcely altered. These soils have generally coarse texture and greater water retention capacity than sandy soils. RMS are formed from volcanic ash and pumiceous rocks, and usually classified as Fluvisols, Regosols and Cambisols. However, their water retention capacity has not been well known. The objective of this research work was to identify the physical, chemical and mineralogical characteristics of RMS which determine the water retention capacity. The work was developed at the hillside of the Mexican volcanoes Popocatepetl and Iztaccihuatl (southern Mexico state and west Puebla state). Seven experimental sites with RMS from an altitude range from 2240 to 2350 meters were selected to determine the morphological and pedological variations. Every sampling site was characterized and a soil profile description was realized for each location. Soil samples (2 kg) were collected from each soil horizon and location. Soil chemical, physical and mineralogical characteristics were obtained to perform a soil classification. Soil clay particles were analyzed by x ray technique. Free and active oxides of iron, silica and aluminium were extracted from the soil fine fraction. Image analysis was developed to observe the presence of water inside pumice particles. Moisture parameters were obtained to establish the water retention capacity as a function of particle diameter (0.25 -11.5 mm). The results showed that RMS have pumice material up to 42%, mainly particles of 2-6 mm (lapilli size), some particles 11.5 mm (bomb size), as well as rounded basalt, andesite and riolite.  These soils belong to the order of Entisols and Inceptisols, however their taxonomic classification  as subgroup do not explain their  water retention capacity, and are classified into general classes as typical Typic Udipsamments, or classes related to humid and temperate weather (Udic ustifluvents) and only in one case of volcanic origin (Vitrandic Udifluvents) without specification of parental material. The RMS present poor chemical weathering where clay formation is low. The soil fine fraction is mainly made of non crystalline minerals as allophone and ferrihydrite (0.15 to 3.3 %).  Considering that RMS are exposed to dry periods so the non crystalline minerals are dry out and lose their water retention capacity. Presence o hematite mineral is evidence of this phenomenon. The above may explain why mineralogical and chemical factors do not determine the water retention capacity of RMS.  Inside pumice material is where water movement and moisture storage occur up to saturation content. The higher water retention capacity (57%) was obtained with particle diameters from 2.38 to 3.35 mm, holding this amount of water at tension lower than 0.0024 KPa. Pumice material is responsible for the water retention capacity of moisture residual soils, since it has interconnected capillary porous acting as natural cavities, which allow water storage and avoid moisture evaporation.