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United States Department of Agriculture

Agricultural Research Service

Title: Effect of Exchangeable Mg on Saturated Hydraulic Conductivity, Disaggregation and Clay Dispersion of Disturbed Soils

Authors
item Zhang, Xunchang
item Norton, Lloyd

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 17, 2001
Publication Date: January 1, 2002

Interpretive Summary: The deterioration of soil structure (how soil particles are arranged and attached to each other) due to improper management of soil resource is a major problem lowering our land productivity and affecting agricultural sustainability. For example, accumulation of sodium in soils causes poor soil structure (muddy when wet, rock-hard when dry) and aggregate (ped) stability, which further cause soil erosion and water entry problems at th soil surface. This study was conducted to evaluate the effect of accumulation of magnesium in soils on soil structural and aggregate stability as well as water movement through the soil. Results showed that accumulation of magnesium in soils, compared with calcium, had a deleterious effect on aggregate stability and soil structure. Poor soil structure, caused by accumulation of magnesium, slowed water movement through the soil. As a result, more water would run off from fields during rainfall, and a greater amount of runoff will cause more soil loss from the fields. This finding is of great importance for farmers and extension personnel when choosing liming materials or soil amendments. If soil magnesium content is sufficient for plant growth, liming materials containing magnesium such as dolomite should be avoided. In addition, gypsum may be applied to magnesium-rich soils to ameliorate the deleterious effect of magnesium.

Technical Abstract: ThereexistdifferentopinionsregardingthespecificeffectofMgon saturated hydraulic conductivity (KS). Opinions also differ regarding the dominant mechanisms that reduce KS. This study was conducted to evaluate the specific effect of Mg on KS and to further elucidate the major processes causing reductions in KS. Three loamy soils containing dominant smectite were packed in soil columns (150 mm long and 76 mm i.d.) and were leached in duplicate with either Ca or Mg salt at successive dilute concentrations of 250, 10, 2, 1, 0.5 and 0 mM. About 4-6 pore volumes were collected for each concentration. Critical flocculation concentration (CFC) was determined with a flocculation series test. There was a specific effect of Mg, compared with Ca, on KSR (normalized with initial KS) and clay dispersion for these soils. The steady KSR's in the Ca treatment were 2 times those in the Mg treatment for the Miami and Catlin soils, and were more than 6 times for the Fayette soil. The steady state leachate % transmittances were lesser in Mg than in Ca for all three soils at P 0.1, showing good agreement with measured CFCs. Reduction of large pores resulting from massive aggregate slaking was observed as the major cause of reduction in KSR. Clay dispersion and subsquent pore plugging supplemented the slaking process, and became increasingly important in causing the continuing reduction in KSR following the massive slaking. Slaking released clay particles and enhanced clay dispersion; however, clay dispersion hardly resulted in direct slaking even though it occurred at the peripherals of aggregates.

Last Modified: 11/23/2014
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