Critical Evaluation of Diagnostic Criteria for Gleyic Properties in the World Reference Base.
Cornelius W. Van Huyssteen, University of the Free State, Nelson Mandela drive, Bloemfontein, South Africa
Gley soils typically occur in wetlands where they are ecologically important due to the water regulating and filtering action that it fulfils in these ecosystems. Accurate classification of these soils is therefore important to aid in land use planning and conservation. This paper evaluates the criteria for gleyic properties, defined in the World Reference Base for soil resources. Twenty eight soils in the Weatherley catchment, in the northern Eastern Cape Province of South Africa were studied to determine their chemical, physical and hydrological properties. Geology was mainly sandstone and mudstone. Average annual rainfall was 1 064 mm and reference evapotranspiration (ETo) 1 328 mm. Soil water, when present, was extracted weekly for three years from piezometer pipes installed in each horizon, and analysed. Average annual duration of water saturation above 70 % of porosity (ADs>0.7) was calculated from daily soil water content data calculated (using the soil water balance equation) from weekly neutron water meter measurements taken over six years (1997 to 2002). Thirty eight horizons with reductomorphic or oximorphic properties were selected for this study. These horizons had high bulk densities (1.72 Mg kg-1), relatively high clay content (25.8 %) and sum of exchangeable basic cations (5.63 cmolc kg-1). ADs>0.7 in these horizons varied between 83 and 365 d a-1. Average redox potential in the extracted soil water decreased from 178 mV to 77 mV as the ADs>0.7 increased from 83 d a-1 to 365 d a-1. Above 350 d a-1 the redox potential was variable, ranging between 77 mV and 220 mV. This variability was also evident for the other determined soil properties. pH increased from 6.30 to 7.23 as ADs>0.7 increased from 83d a-1 to 312 d a-1. Similar to the redox potential, the pH was highly variable above ADs>0.7 of 323 d a-1, ranging between 6.08 and 7.69. Fe2+ (0.37 to 2.42 mg kg-1), Mn2+ (0.04 to 0.17 mg kg-1), Ca2+ (3.77 to 10.71 mg kg-1) and Mg2+ (0.50 to 2.40 mg kg-1) content in the extracted soil water increased as ADs>0.7 increased from 83 d a-1 to 323 d a-1. Similar to the pattern observed for redox potential and pH, the Fe2+, Mn2+, Ca2+ and Mg2+ content were variable above an ADs>0.7 of 323 d a-1. Above an ADs>0.7 of 323 d a-1 the Fe content ranged between 0.14 and 20.27 mg kg-1; Mn2+ between 0.06 and 1.02 mg kg-1; Ca2+ between 3.39 and 52.42 mg kg-1; and Mg2+ between 0.58 and 15.91 mg kg-1. K+ (0.86 to 7.78 mg kg-1) and Na+ (1.90 to 17.23 mg kg-1) had a more pronounced increase with an increase in ADs>0.7 from 83 d a-1 to 312 d a-1. K+ (0.44 to 9.29 mg kg-1) and Na+ (2.20 to 25.82 mg kg-1) were also variable above an ADs>0.7 of 323 d a-1. Calculated average rH values did not correlate with ADs>0.7, due to the decreasing redox potential and increasing pH with an increasing ADs>0.7. rH was therefore not a good indicator of ADs>0.7, as 34 of the 38 horizons had average rH values of 19 or less. Reductomorphic and/or oximorphic properties were found to be better indicators of long periods of ADs>0.7. It was concluded that low rH values were not good indicators of duration of saturated conditions. Saturated conditions were better correlated to reductomorphic and/or oximorphic properties. These included the presence of grey (7.5YR7/2, dry) silans, and black (10YR2/1, dry), red (2.5YR5/8, dry), and yellow (10YR6/6, dry) manganese and iron oxide mottles. Specification of the timing and duration of rH measurements might improve the use of this criterion as an indicator of duration of water saturation.