Aluminum toxicity is discussed, including general effects (symptoms and physiological effects), differential aluminum tolerance in plants, beneficial effects of aluminum, and the genetic control of aluminum tolerance. Manganese and iron toxicity are discussed in the same framework. The toxicity of other metals (Zn, Cu, Ni, Cd, and Pb) is also discussed, though not as extensively as aluminum, manganese, and iron. 476 references.

The Physiology of Metal Toxicity in Plants

Plant roots serve a multitude of functions in the plant including anchorage, provision of nutrients and water, and production of exudates with growth regulatory properties. The root–soil interface, or rhizosphere, is the site of greatest activity within the soil matrix. Within this matrix, roots affect soil structure, aeration and biological activity as they are the major source of organic inputs into the rhizosphere, and are also responsible for depletion of large supplies of inorganic compounds. Roots are very complicated morphologically and physiologically, and their metabolites are often released in large quantities into the soil rhizosphere from living root hairs or fibrous root systems. Root exudates containing root-specific metabolites have critical ecological impacts on soil macro and microbiota as well as on the whole plant itself. Through the exudation of a wide variety of compounds, roots impact the soil microbial community in their immediate vicinity, influence resistance to pests, support beneficial symbioses, alter the chemical and physical properties of the soil, and inhibit the growth of competing plant species. In this review, we outline recent research on root exudation and the role of allelochemicals in the rhizosphere by studying the case of three plants that have been shown to produce allelopathic root exudates: black walnut, wheat and sorghum

The role of root exudates and allelochemicals in the rhizosphere

This article reviews literature about the impacts of cover crops in cropping systems that affect soil and water quality and presents limited new information to help fill knowledge gaps. Cover crops grow during periods when the soil might otherwise be fallow. While actively growing, cover crops increase solar energy harvest and carbon flux into the soil, providing food for soil macro and microrganisms, while simultaneously increasing evapotranspiration from the soil. Cover crops reduce sediment production from cropland by intercepting the kinetic energy of rainfall and by reducing the amount and velocity of runoff. Cover crops increase soil quality by improving biological, chemical and physical properties including: organic carbon content, cation exchange capacity, aggregate stability, and water infiltrability. Legume cover crops contribute a nitrogen (N) to subsequent crops. Other cover crops, especially grasses and brassicas, are better at scavenging residual N before it can leach. Because growth of thes...

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Using winter cover crops to improve soil and water quality

A review of the use of composted municipal solid waste in agriculture

The chemistry and behaviour of antimony in the soil environment with comparisons to arsenic: A critical review

This study determined distribution of feces and urine from 36 lactating dairy cattle (Bos taurus) managed in a rotationally grazed 0.74-ha endophyte-free tall fescue (Festuca arundinacea Schreb.)-white clover (Trifolium repens L.) pasture. Cows were observed for 24 h five times from July 1997 to April 1998, and for 13.5 h in September 1997. During each 24-h observation period, the first grazing period (12 h) used 54% of the paddock and the second grazing period (8 h) used the entire paddock. Times and locations of all defecations and urinations from a subgroup of eight cows, observed while in the pasture, feed area, milking parlor, or in transit were recorded during the observation periods listed above and another time in May 1997. On pasture, all defecations and urinations were surveyed and mapped for all 36 cows. Feces and urine from six observation periods covered an estimated 10% of the paddock area in one year. Within 30 m of the water tank, concentrations of feces and urine from three warm-season observations were significantly greater than concentrations during three cool-season observations. Percentages of defecations and urinations on the pasture, feeding, and milking areas were highly correlated (r > 0.90) with time spent in those areas. Pasture-based systems could reduce manure handling and storage requirements proportional to the time cattle are on pastures. Manure on the pasture was evenly distributed, except around the water tank during warm-weather grazings. Results indicate that pasture-based dairy systems may require smaller, less-expensive manure management systems compared with confinement dairy farms.

Spatial and time distribution of dairy cattle excreta in an intensive pasture system.

Retention of Cadmium by Several Soils of the Southeastern United States

Soil core (0–2.5 and/or 0–10 cm) samples were taken from wheat no till, wheat-conventional till, and fallow-conventional till soybean cropping systems from July to October of 1989 and extracted with water in an autoclave. The soil extracts were analyzed for seven common phenolic acids (p-coumaric, vanillic,p-hydroxybenzoic, syringic, caffeic, ferulic, and sinapic; in order of importance) by high-performance liquid chromatography. The highest concentration observed was 4 μg/g soil forp-coumaric acid. Folin & Ciocalteu's phenol reagent was used to determine total phenolic acid content. Total phenolic acid content of 0- to 2.5-cm core samples was approximately 34% higher than that of the 0- to 10-cm core samples. Phenolic acid content of 0- to 2.5-cm core samples from wheat-no till systems was significantly higher than those from all other cropping systems. Individual phenolic acids and total phenolic acid content of soils were highly correlated. The last two observations were confirmed by principal component analysis. The concentrations were confirmed by principal component analysis, tions of individual phenolic acids extracted from soil samples were related to soil pH, water content of soil samples, total soil carbon, and total soil nitrogen. Indirect evidence suggested that phenolic acids recovered by the water-autoclave procedure used came primarily from bound forms in the soil samples.

Phenolic acid content of soils from wheat-no till, wheat-conventional till, and fallow-conventional till soybean cropping systems

Long-term swine lagoon effluent applications on coastal bermudagrass. II, Effect on nutrient accumulation in soil

Establishment of maximum cumulative metal loading rates of sludge require metals in soils be related to metal concentrations in plants grown on those soils. The relationship between plant concentrations of Cd, Cu, and Zn and soil properties on sites of long-term municipal sludge application were evaluated. Swiss chard (Beta vulgaris L.) and lettuce (Lactuca sativa L.) were planted on five fields (Typic Hapludults) with known sludge disposal history. After harvest, tobacco (Nicotina tabacum L.) following swiss chard and peanut (Arachis hypogea L.) following lettuce were planted in the same plots. Extractable Cd, Cu, and Zn (Mehlich-3, 0.05 M DTPA, 0.05 M EDTA), clay, humic matter, organic C, and pH were determined on a composite 20-cm depth soil sample and Mehlich-3 extractable metals were determined by 15-cm increments on a 60-cm depth sample from each plot. Tissue concentrations of Cd, Cu, and Zn were measured near harvest maturity (swiss chard and lettuce) or shortly after anthesis (tobacco and peanut). Simple linear regressions were poor between plant metal concentration and soil-extractable metal for all extractants. Inclusion of soil properties in the best-fit multivariate regression models improved the relationship for metal concentration in plants. Values for R 2 ranged from 0.30 to 0.96 with the poorest correlations obtained for Cu in lettuce and peanut. Best-fit models for all other crop-extractant-metal combinations had R 2 > 0.83. Metals extracted by the three extractants were correlated with each other so their inclusion in models describing plant concentrations of heavy metal in this study gave similar results.

Larry D. King

Papers

Spatial and time distribution of dairy cattle excreta in an intensive pasture system.

Retention of Cadmium by Several Soils of the Southeastern United States

Phenolic acid content of soils from wheat-no till, wheat-conventional till, and fallow-conventional till soybean cropping systems

Long-term swine lagoon effluent applications on coastal bermudagrass. II, Effect on nutrient accumulation in soil

Soil Factors Affecting Plant Concentrations of Cadmium, Copper, and Zinc on Sludge-Amended Soils