While many workers have utilized various reagents for sequential extraction of soil trace metals, few studies have examined the order of extraction for key steps in the sequential procedure. In this study, several sequences involving both adsorbed and structural (occluded) metal extractants were evaluated to determine the most appropriate sequential methodology for extracting different forms of Cu, Fe, and Mn. For “specifically adsorbed” metals, Pb(NO3)2 and CH3COOH were used. The results showed that the former reagent extracted less Cu, Mn, and Fe and was probably more specific in replacing metals covalently bound to adsorption sites. Lead nitrate was therefore placed before CH3COOH extraction in the sequence. Chao's NH2OH·HCl reagent and K4P2O7, used for Mn oxide and organic metal removal, respectively, were found to solubilize significantly different amounts of Cu and Mn depending on sequence, with K4P2O7 extracting more metal when used first. As NH2OH·HCl has little effect on organic metals, it should be used before K4P2O7. Noncrystalline and crystalline Fe compounds are solubilized next, using a variety of reagents, and residual (silicate lattice) metals are dissolved in the final step. A nine‐step sequential method is proposed to characterize trace metals in agricultural, polluted, and waste‐amended soils.

This laboratory and greenhouse investigation was under‐taken to study the distribution and plant availability of zinc in different soil fractions. Total Zn (TZn) in 19 soils, which varied widely in chemical and physical properties, was fractionated into water‐soluble plus exchangeable (CA‐Zn), specifically adsorbed (AC‐Zn), organically bound (PYRO‐Zn), Mn‐oxide bound (HAH‐Zn), Al‐ and Fe‐oxide bound (AMOX‐Zn), and residual (RES‐Zn) forms. There was a wide variation in the magnitude of these fractions among soils. Most of the TZn, on an average, was present in the AMOX‐Zn (∼25%) and RES‐Zn (∼70%) fractions. The CA‐Zn, AC‐Zn, PYRO‐Zn, and HAH‐Zn fractions averaged 0.4, 3.3, 2.5, and 2.0% of the TZn, respectively. The CA‐Zn in the 19 soils increased with a decrease in soil pH, whereas the AC‐Zn increased with an increase in soil pH. The PYRO‐Zn in the soils varied directly with organic C and soil pH, HAH‐Zn increased with an increase in both soil pH and free Mn, AMOX‐Zn correlated more closely with free Al than with free Fe, and RES‐Zn varied positively with soil clay. A seven‐variable regression model, which included the six Zn fractions and soil pH, accounted for 94% of the variability in Zn uptake by corn plants (Zea mays L.) from these soils. This model separated soils that supplied inadequate amounts of Zn from those that supplied adequate amounts.

It is known that PO4 is retained by soils through ligand exchange, i.e., inner sphere complexation, but the mechanism for SO4 adsorption at the mineral‐water interface has been in debate. By studying the effects of ionic strength on ion adsorption, it is possible to distinguish between inner and outer sphere ion surface complexes. This study was conducted to evaluate ionic strength effects on SO4 and PO4 adsorption on γ‐Al2O3 and kaolinite at varying solution pH (3–11), and to infer SO4 and PO4 adsorption mechanisms at the mineral‐water interface. The adsorption of SO4 on γ‐Al2O3 and kaolinite decreased monotonically with increasing solution pH and was markedly reduced by increasing the concentration of background electrolyte. On the other hand, PO4 adsorption on γ‐Al2O3 and kaolinite increased from pH 3 to 4 and decreased from pH 6 to 11, with an adsorption plateau between pH 4 and 6. Effects of change in ionic strength on PO4 adsorption varied with pH. At low pH, PO4 adsorption demonstrated a slight decrease with increasing ionic strength, whereas at high pH, PO4 adsorption increased slightly with increasing ionic strength, resulting in a crossover point where there was no ionic strength effect. The triple‐layer model (TLM) was applied to model the adsorption of SO4 and PO4 with both inner and outer sphere complexes using the FITEQL 3.1 computer program. Sulfate adsorption was better modeled by assuming outer sphere complex formation, while PO4 adsorption was better modeled by assuming inner sphere complex formation.

Surface soil samples to 15 cm depth were taken from replicated plots in an ongoing long-term field experiment involving application of animal manure on three soils in Virginia. The sampled plots had received either no manure or the equivalent of 289,000 kg ha-1 of manure as dry weight. The manure was applied annually at the beginning of each spring for 15 years from 1978 through 1992. The plots were cropped similarly since 1978. Soil textures were a fine sandy loam at Holland in the Atlantic Coastal Plain region, a silt loam at Blacksburg in the Appalachian region, and a clay loam at Orange in the Piedmont region of Virginia. The following measurements were made on subsamples: liquid and plastic limits, wet aggregate stability, aggregate size distribution, dispersible clay percentage, water retention at 0. 03, 0.1, 0. 3, 0. 5, 1.0, and 1.5 MPa tension, and modulus of rupture of moulded briquettes at a water content corresponding to 0.1 MPa tension. Organic matter content by the Walkley-Black method was significantly higher in the manure-treated soils at all three locations. Increases were 3% for the sandy loam and 25% for the silt loam and clay loam. From these values it was estimated that at least 95% of the total applied manure had been degraded over the 15 years. Results showed that the liquid and plastic limits for all three soils were higher (p< 0.05) for the manure -treated samples. However, the differences in the limits were only 2 to 3%. The modulus of rupture values were lowered by addition of the animal manure. Decreases (p < 0.05) occurred for the silt loam and clay loam samples. The wet aggregate stability increased and the dispersible clay decreased in the manure-treated soils. Increases (p < 0. 05) in wet aggregate stability occurred for the sandy loam and silt loam samples, Decreases (p < 0.05) in dispersible clay were measured for the sandy loam and clay loam samples. Water retention was consistently, but only slightly, increased by manure addition. The increases, in the order of sample texture, were clay loam > sandy loam ~ silt loam. Ihcreases tended to be higher at the lower values of tension. Manure addition consistently increased the weight percentages of aggregates passing a given mesh size. Increases, in order of sample texture, were silt loam > clay loam > sandy loam. In their entirety, these results show that the manure produced measurable changes in the soil physical properties. The magnitude of the changes, in most cases, were small and depended on the soil texture. Given the high total amount of manure applied, the results indicate that manure-induced physical changes in the soil were small and evidently did not accumulate over time. Rapid microbial degradation of the manure could be responsible for the lack of marked changes in the soil physical properties.

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