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Mechanism of Native Manganese Release in Salt-Treated Soils

Dep. of Soil Science, NWFP Agricultural Univ., Peshawar, Pakistan

A. L. Page

Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA 92521

Wesley M. Jarrell

Environmental Sciences and Engineering, Oregon Graduate Ctr., Beaverton, OR 97006-1999

* Corresponding author.

ABSTRACT

The release of native soil Mn was evaluated under laboratory conditions in Aiken (fine, kaolinitic, thermic Abruptic Durixeralfs) and Redding (fine, kaolinitic, thermic Abruptic Durixeralfs) clay loam soils saturated with either deionized water, or 75 and 150 mmol L^–1 NaCl-CaCl₂ solution. As the NaCl-CaCl₂ concentrations increased, soil solution Mn increased and exchangeable Mn decreased proportionately. The regression of the net increases () in soluble Mn vs. the corresponding decreases () in exchangeable Mn produced the relationship, exchangeable Mn (mg kg^–1) = 0.788 + 0.868 soluble Mn (mg kg^–1), R² = 0.63** (P < 0.01). This suggested that in these salt-treated soils Mn was released into solution by a cation exchange reaction involving Ca and Na. The increases (122 to 252%) in exchangeable Mn after 5 d incubation, as compared to untreated air-dried soils, and the favorable pH (4.44 to 6.01) and Eh (redox potential) values (+202 to 254 mV) suggested that Mn(III,IV) was reduced to Mn(II). The Mn(II) primarily accumulated on the exchange phase and was subsequently displaced from the exchange sites by added Na⁺ and Ca²⁺. At equal equivalent concentrations, almost twice as much Mn entered the soil solution with Ca²⁺-salts (Cl, ClO₄, and NO^-₃) as with Na⁺-salts. The sum of soluble + exchangeable Mn did not increase (rather decreased significantly at the highest salt treatments). It was concluded that salinity had no effect on the extent to which Mn(III,IV) were reduced to Mn(II). These results indicated that management practices that increase soil solution salinity on soils high in native Mn could elevate soil solution Mn to concentrations potentially toxic to Mn-sensitive crops.

Contribution from the Dep. of Soil and Environmental Sciences, Univ. of California, Riverside.

Received for publication February 9, 1988.