Wettability of soil aggregates from cultivated and uncultivated Ustolls and Usterts

Abstract

Soil organic matter can modify the interaction of clay minerals with water, limiting the rate of water intake of swelling clays and stabilising soil aggregates. Soil structural stability and organic C content usually decrease with cultivation. Faster wetting increases stresses on aggregates and decreases stability. Aggregate wettabilities of prairie soils under 3 different management systems (grassland, no-till, and conventional-till) were compared in the Northern Great Plains of the USA. Six Ustolls and 2 Usterts were selected as replications along the Missouri River. Wettability was measured as water drop penetration time (WDPT) and as rate of water intake under 30 and 300 mm tension. At low tension, aggregates from both cultivated fields and uncultivated grasslands showed similar wettability. Water intake in grass aggregates was attributed to a greater amount of stable pores relative to cultivated aggregates. In cultivated aggregates, slaking created planes of failure that allowed rapid water entry. Differences of wettability between management systems at 300 mm tension (in Ustolls, grasslands had greater wettability than cultivated soils, 0.24 v. 0.17 g water/h.g dry soil) and between soil orders (Usterts had longer WDPT than Ustolls, 2.9 v. 1.7 s) were explained by both clay and organic C contents. Simple measurements of aggregate wettability may be effectively used for soil quality characterisation. Aggregate wettability is a desirable property for agricultural soils when it is related to stable porosity, as may be found in high organic matter soils (e.g. grasslands). Wettability is excessive when fast aggregate wetting results in aggregate destruction as observed in low organic matter cultivated soils.

Extent of soil water repellency under long-term no-till soils

“…The significant development of water repellency due to adoption of no-till technology observed in the 8 soils contrasts with some previous studies reporting no differences in water repellency between no-till and plow tillage (Eynard et al, 2004) but agrees with others which reported slower aggregate wetting in no-till soil (Chan, 1992;Hallett et al, 2001;Blanco-Canqui et al, 2007). These inconsistencies in data suggest the site-specificity of no-till impacts.…”

“…A watershed study by Blanco-Canqui et al (2007) showed that soils under 44-yr NT continuous corn had WDPT about twice higher than plowed soils on a sloping silt loam. In contrast, a field study by Eynard et al (2004) showed no significant differences in WR between NT and PT fields across loam, silt loam, silty clay loam, silty clay, and clay soils.…”

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“…The results generally may be attributed to the organic carbon content of the textures as water stability of micro aggregates depend on OC binding agents a characteristics of soil independent of the land use management and the extent of tillage. Denef et al 2001 andEynard et al 2004 observed that the structural stability of aggregates decrease with OC content and cultivation. Soil structure and aggregates according to Holeplass et al 2004 andKhurshid et al 2006 are strongly influenced by cultivation related practices.…”

Water vapour adsorption on water repellent sandy soils

“…Soil water repellency (SWR) has serious effects on natural ecosystems, like preferential flow, irregular pattern of vegetation growth, enhanced surface runoff and soil erosion (Burcar et al, 1994;DeBano, 1981;Ehwald et al, 1961;Imeson et al, 1992;Shakesby et al, 1993) and it is common especially in uncultivated sandy soils (Blanco-Canqui, 2011;Dekker, 1998) although different observations are also reported for instance from olive orchards (Bughici and Wallach, 2016) or from prairie soils (Eynard et al, 2004). Water repellency phenomenon results from the decrease in attraction between soil particles and water molecules which means that large-scale environmental effects may depend on processes that vary at microscopic scales.…”