The objectives of the study were to determine the chemical composition and layer charge of smectite found in calcareous till of the Interior Plains region of western Canada and to examine the effects of acidification on alteration of the smectite. Samples of acidified and non-acidified (calcareous) late-Wisconsin till were obtained from four soil pits located immediately adjacent to an elemental sulfur block located in southern Alberta. Samples of the surface material (0–10 cm depth) had been subjected to extreme acidity for 25 years due to the oxidation of elemental sulfur and displayed pH values of about 2.0. Samples of the till obtained at depth (65–75 cm) remained calcareous with pH values between 7.3 and 7.6. A combination of analytical methods was used to determine the chemical composition of the smectite found in the samples. The layer charge of the smectite was determined independently using X-ray diffraction data for n-alkylammoniurn saturated specimens. Smectite found in the non-acidified calcareous material was characteristic of montmorillonite with a low content of Fe and very little substitution of Al for Si in the tetrahedral sheet. The smectite had a structural formula of M+0.40(Si3.96Al0.04)(Al1.56Fe3+0.10Mg0.33)O10(OH)2, which compared well with a mean value for layer charge of 0.399 mol(−)/O10(OH)2 determined using X-ray diffraction data for n-alkylammonium treated specimens. Smectite remaining in the till material subjected to extreme acidity underwent incongruent dissolution with a net loss of layer charge and preferential loss of octahedral Mg.

On soils dominated by high proportions of clay and organic matter, soil acidity and poor nutrient use efficiency have a major impact on output potential. Due to the inherent chemical properties of these soils, reducing soil acidity and the prevalence of undesirable metallic cations poses challenges. As a result, these soils have a large capacity for phosphorus (P) fixation, therefore reducing plant P availability. Limestone (CaCO3 or MgCO3) is applied to agricultural soils to counteract soil acidity and reduce P fixation. The current study investigates the effects of four contrasting annual P application rates (0, 50, 100, 150 kg P/ha); split (50:50) between spring and summer, across soils with a range of soil pH values from a previous liming trial. The effect of soil pH ranges and P treatment rates on seasonal herbage growth and herbage P concentration was investigated over three years. Soil nutrient status was also investigated. Soil pH had a significant impact on the rate of mineralization and soil P concentration across each site. A soil pH of 6.2 caused a 1.8 mg/l increase in soil test P. An annual P application was necessary to maintain sufficient herbage P concentration for animal dietary requirements (0.35% DM), however there was no effect of P application or liming rate on herbage productivity across the three sites as all sites possessed sufficient soil P reserves. The current experiment has shown that despite optimal soil fertility status, ensuring sufficient plant available P is a problem on these particular soils.

Highly weathered soils of the tropics and subtropics commonly have kaolinitedominated clay fractions. Under acidic conditions prevailing in these soils kaolinite dissolution occurs, contributing to the high levels of soluble Al in these soils. This study evaluates the dissolution behaviour of kaolinites from subsurface horizons of highly weathered soils from Thailand, along with a soil kaolinite from Western Australia (WA kaolinite) and Georgia kaolinite (KGa-2). Kaolinite-dominated clay fractions were isolated from soils by sedimentation and chemically treated to remove iron oxides. The dissolution rate of kaolinites was measured in 0.01 M NaCl as a function of pH (1–4; HCl) at 25±1°C using non-stirred flow-through reactors. Kaolinite dissolution rates were calculated from the release of Al and Si at the steady state. In most of the experiments and at all pH values, the release of both Si and Al underwent a distinct transition from an initial period of rapid release to significantly slower release at the steady state. Aluminium and Si concentrations at the steady state were higher for soil kaolinites than the reference sample (KGa-2). At the steady state the dissolution of all kaolinites was stoichiometric except for the soil kaolinites from Thailand at pH 4, where the Al/Si ratio was well below the stoichiometric ratio. Log dissolution rate (RSi) of soil kaolinites ranged from –13.75 to –12.51, with the dissolution rate increasing significantly with decreasing solution pH. However, the dissolution rate was similar or pH independent between pH 2 and 3, which is the pH range of the point of zero net charge (PZNC) for both soil and reference kaolinites. The dissolution rate of soil kaolinite was significantly higher than the KGa-2 sample at pH < 3. The results obtained on kaolinite samples from highly weathered soils show that in highly acidic systems kaolinite is a source of soluble Al. Soil kaolinites with poorly ordered small crystals dissolve faster than better crystalline reference kaolinite (KGa-2) with larger crystals.

Persistence of 1.12, 2.24, and 3.36 kg/ha of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] applied preemergence to corn (Zea mays L. ‘Funk's 4761′) was determined in three soils, each providing a range of pH from 5 to 7. Samples of surface soil from each plot were collected at intervals after atrazine application until there was no evidence of phytotoxicity to oat (Avena sativa L. ‘Florida 501′) in bioassay. Persistence (Y50) was defined as the number of days following atrazine application until bioassay indicated 50% relative growth of oats (no atrazine = 100% growth). This period ranged from 24 to 178 days, depending upon soil type, pH, and atrazine rate. Persistence of atrazine increased with increasing soil pH. The effect of soil pH was essentially the same at each atrazine rate and did not vary appreciably with years in a given soil. In McLaurin sandy loam, atrazine persisted 8 to 9 days longer with each unit increase in soil pH. In Hartsells fine sandy loam and Decatur silt loam, atrazine persistence increased 9 to 13 days and 29 days, respectively, with each unit pH.

Broadcast applications of hexazinone [3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4(1H,3H)-dione] pellets and foliar sprays were tested at four rates for hardwood control and safety to loblolly pine (Pinus taeda L.) at each of eight study locations differing in soil characteristics. Reduction in the number of hardwoods in the stand (hardwood density reduction) was greater with the pellet on soils with more than 60% sand, while the liquid formulation was most efficacious for finely textured soils. Hardwood density reduction with the pellet was negatively correlated with percent silt, clay, soil organic matter, and cation exchange capacity, and positively correlated with percent sand. With foliar sprays, hardwood density reduction was positively correlated with hexazinone rate and negatively correlated with soil pH. Pine mortality was positively correlated to percent sand with the pellet and negatively correlated to soil pH with broadcast sprays. Regression models incorporating pine height, herbicide rate, soil texture, cation exchange capacity, soil organic matter, and acidity could explain up to 78% of the variation in hardwood density change and 77% of the variation in pine mortality. Selective control of hardwoods in young loblolly pine stands is a function of hexazinone rate, formulation, and various soil factors.

Soil management and cropping systems have long-term effects on agronomic and environmental functions. This study examined the influence of contrasting management practices on selected soil chemical properties in eight long-term cropping system studies throughout the Great Plains and the western Corn Belt. For each study, soil organic C (SOC), total N (TN), particulate organic matter (POM), inorganic N, electrical conductivity (EC), and soil pH were evaluated at 0–7.5, 7.5–15, and 15–30 cm within conventional (CON) and alternative (ALT) cropping systems for 4 years (1999–2002). Treatment effects were primarily limited to the surface 7.5 cm of soil. No-tillage (NT) and/or elimination of fallow in ALT cropping systems resulted in significantly (P<0.05) greater SOC and TN at 0–7.5 cm within five of the eight study sites [Akron, Colorado (CO); Bushland, Texas (TX); Fargo, North Dakota (ND); Mandan, ND; and Swift Current, Saskatchewan (SK), Canada]. The same pattern was observed with POM, where POM was significantly (P<0.05) greater at four of the eight study sites [Bushland, TX, Mandan, ND, Sidney, Montana (MT), and Swift Current, SK]. No consistent pattern was observed with soil EC and pH due to management, although soil EC explained almost 60% of the variability in soil NO3-N at 0–7.5 cm across all locations and sampling times. In general, chemical soil properties measured in this study consistently exhibited values more conducive to crop production and environmental quality in ALT cropping systems relative to CON cropping systems.