Effects of Manure Amendment on Soil Chemical Properties and Hydrous Oxides

Abstract

AbstractThree Virginia soils with low, moderate, and high amounts of Fe and Mn oxides were amended with swine (Sus scrofa L.) manure at a rate of 100 mt ha−1 in an incubation experiment conducted in the laboratory over 29 days. Changes in pH, redox potential (Eh), and forms of Fe and Mn were monitored in order to assess the effect of high rates of manure loading on the chemical properties of these soils. Manure additions caused immediate reductions of 175 to 325 mv in soil Eh by day 4, although Eh recovered to near untreated soil levels by the end of the study. Minimum Eh levels were related to initial Fe and Mn contents of the soils. Soil pH increased to 7 to 8 in all soils, and organic matter contents, initially 25 g kg−1 higher than control soils, decreased as decomposition proceeded. Soil Mn was rapidly solubilized under the reducing conditions present early in the incubation, as shown by increases in soluble, exchangeable, and adsorbed forms of Mn determined by sequential extraction. Reprecipitation of MnO2 occurred in the high‐Mn soil by day 8, as Eh began to recover. Iron was reduced to more soluble forms only in limited amounts in the soils with the high and medium Fe oxide contents, while significant reduction of amorphous Fe oxide was observed in the soil with low oxide content. These results suggest that toxicities of Fe and especially Mn, along with other trace metals associated with these oxides in soils, may result from high rates of organic waste addition to soils where reducing conditions may persist for some time.