Terms Of Stability Ratio Research Articles

High-energy moisture characteristics of various low organic matter sandy soils in different land uses

Soil structure serves as one of the most important soil physical properties influencing on water retention, aeration, plant growth as well as environmental conditions. In the current study, the effect of four land-use types on some soil structural stability indices including high-energy moisture characteristic (HEMC) at both fast and slow wetting modes, volume of the drainable pores (VDP), structural index (SI), stability ratio (SR), fractal dimensions (D) and mean weight diameter (MWD) was investigated. Jiroft County, is located in southeastern part of Kerman Province, Iran (28ο40׳ N, 57ο44׳ E) and characterized by soils generally high in sand and low in organic matter (OM) was selected as the study area. Land-use types included disturbed and undisturbed rangelands, protected natural forest and an artificial forest plantation. The HEMC results showed that in the slow wetting mode, undisturbed rangeland (49.6%) and protected natural forest (42.6%) indicated the highest and lowest values for near-saturated water contents, respectively. However, in fast wetting modes no significant differences was observed in case of near-saturated water content between undisturbed rangeland and disturbed rangeland and artificial forest plantation land-uses. Furthermore, the near-saturated water contents in the protected natural forest did not changed for both fast and slow wetting modes. The lowest MWD (0.05 mm) and the highest fractal dimension values (2.97) were observed for protected natural forest. The lowest values for VDP, VDP ratio (VDPR), and SI were found for undisturbed rangeland and disturbed rangeland, with the highest values obtained for protected natural forest. No significant differences were found among the land-use types in terms of SR. Furthermore, soil water retention was also low in the land use types with low aggregate formation agents like clay, OM, and calcium carbonate equivalent (CCE). Among the HEMC indices, near-saturated water content and soil water retention only indicated the structural condition. While other HEMC indices such as VDP, SI, SiR, and VDPR did not show the differences between structural and textural characteristics. This study revealed that, techniques such as measuring MWD and fractal dimension may better show the status of the soil aggregates in the studied region.

SODICITY AND WATER QUALITY EFFECTS ON SLAKING OF AGGREGATES FROM SEMI-ARID SOILS 1

Aggregate slaking is one of the main mechanisms responsible for the breakdown of aggregates. Exchangeable sodium percentage (ESP) and electrolyte concentration (C) of the soil solution play a significant role in determining soil physical properties and the response of soil clays to dispersion and swelling. However, studies of sodicity and C effects on aggregate stability have yielded inconsistent results. Our objective was to evaluate the effects of sodicity and C on aggregate slaking in soils with different clay contents. Using the high-energy-moisture-characteristics (HEMC) method, we studied aggregate slaking in 56 samples of Israeli topsoils varying in clay content (80-675 g kg -1 ) and ESP levels (0-30). In this method, accurately controlled wetting of the aggregates (i.e., the driving force for slaking) was the only force exerted on the aggregates. Aggregates (0.5-1.0 mm) were placed in a funnel equipped with a fritted disk and, using a peristaltic pump, were wetted either fast (100 mm h -1 ) or slowly (2 mm h -1 ). Two salinity levels were studied: dionized water (DW, C = 0.04 mmol c L-1) and saline water (SW, C = 20 mmol c L -1 ). Thereafter, the aggregates were subjected to stepwise increases in matric potential up to 5.0 J kg -1 to obtain a moisture retention curve that served as the base for calculation of aggregate susceptibility to slaking. The latter was expressed in terms of stability ratio (SR). The SR of low ESP soils increased from 0.298 to 0.751 and from 0.508 to 0.799 for DW and SW, respectively, with the increase in soil clay content from 11.3 to 67.4%. Increase in ESP decreased SR (i.e., enhanced aggregate slaking), whereas use of SW increased SR compared with DW. A triple interaction among ESP, C, and clay content (P = 0.001) in their effect on aggregate slaking suggested that the combined effects of these variables on slaking were complex. Our results suggested that: (i) in soils having inherently low aggregate stability (clay 35%), aggregate slaking decreased upon use of SW only when ESP > 15.

On certain class of linear stable systems with negative gains

An attempt is made here to obtain expressions for the negative limiting gains of Type 0 systems, which are stable for a limited range of negative gains. A simple relation to determine the gain margin of such systems in terms of stability ratio is also obtained.