Showing papers in "Advances in Agronomy in 1992"

Use of surface complexation models in soil chemical systems

Abstract: Publisher Summary This chapter presents five common surface complexation models of the mineral–solution interface and their use in describing soil chemical systems. Common model characteristics and adjustable parameters are discussed. For each model, surface species, chemical reactions, equilibrium constant expressions, and surface activity coefficients are described. Applications of the model to ion adsorption on soil minerals and soils are presented. Incorporation of surface complexation models into computer codes is also discussed. All surface complexation models are based on a balance of surface charge expression. They contain at least one coulombic correction factor to account for the effect of surface charge on surface complexation and explicitly define equilibrium constant expressions for surface complexes. They contain mass balance equations for each type of surface site and charge balance equations for each surface plane of adsorption. Thus, all models contain adjustable parameters—the equilibrium constants, the capacitance density for the i th surface plane, and the total number of reactive surface hydroxyl groups.

Transport of Chemicals Through Soil: Mechanisms, Models, and Field Applications

Abstract: Publisher Summary The chapter presents and evaluates current experimental information and theoretical approaches used to represent chemical transport and transformations in unsaturated soil It focuses on the field regime and discusses the current approaches used for modeling chemical transport in natural media The field evidence regarding preferential flow is quite consistent in one respect Preferential flow can occur under a variety of circumstances and is not restricted to clay-rich soils with significant structural voids Fluid transport through well-defined structural voids is not predictable unless the distributions of voids, aperture sizes and shapes, depths of penetration, and interconnectivity are known Progress is being made slowly in characterizing transport through rock fractures but there the geometry is much more stable in time than it is in the soil regime Laboratory studies have demonstrated clearly that soil structure is almost certain to introduce mass transfer limitations to equilibrium between the dissolved and sorbed phases in soil The chapter concludes that there are fundamental differences between the transport characteristics of the laboratory and field environments

Fingerprinting Crop Varieties

Abstract: Publisher Summary Descriptive names given to landraces or cultivated varieties illustrate the practical necessity for farmers to identify sources of seed of proved performance. In many cases, processors and end-use consumers also want to know varietal identity. Usage of protein and DNA descriptors has propelled the science of both human and cultivated plant identification into the mainstream of peer-reviewed and widely accepted science. Morphological descriptions can provide unique identification of cultivated varieties. Morphologies reflect not only the genetic constitution of the cultivar, but also the interaction of the genotype with the environment (G x E) within which it is expressed. Proteins and DNA can be used to provide varietal profiles. They are in popular usage because the variation for these markers is ubiquitous and this variation can be understood in genetic terms. Most advances in varietal descriptions will involve the use of DNA as a descriptor. Some technical developments coming from other genome mapping and sequencing projects will also come to bear on cultivar fingerprinting in the future. Intense impetus to these efforts comes from the desirability to use genetic marker data in applied plant-breeding programs.

Pearl Millet for Food, Feed, and Forage

Abstract: The origin, breeding, biotechnology, consumer quality and grain characteristics of Pennisetum glaucum are discussed. Food products including porridges, flat breads and couscous and traditional processing of P. glaucum are described and grain as feed for poultry, cattle, sheep and pigs is considered. P. glaucum cultivars and hybrids suitable for forage production are suggested and the problems of disease management during forage production, crop management, grazing by livestock, nitrate toxicity to cattle and silage production are discussed

Recurrent Selection for Population, Variety, and Hybrid Improvement in Tropical Maize

Abstract: Publisher Summary The objectives of this chapter is to discuss tropical maize germplasm and environments in which maize is grown, provide information on recurrent selection systems, and how they are applied by the breeder; review maize population improvement work in national programs in tropical regions, highlight recurrent selection work of the international center for maize, and wheat improvement; and relate population improvement work to hybrid programs for the tropics. It is recognized that maize production practices in the tropics are generally poor. Improved production practices are bound to contribute more to increased yields initially, but as production practices improve, greater breeding effort is required to increase yields and adapt materials to improved practices. Yield potential of tropical maize should be increased by improving grain production efficiency, measured in time and space. Selection for desirable morphological traits, higher harvest index, reduced ASI, increased efficiency of nutrient uptake and utilization, improved stress tolerances, and superior quality traits should all be emphasized in the tropics.

Forms, Reactions, and Availability of Nickel in Soils

Abstract: Publisher Summary The forms of Ni in soil are diverse and range from those of high activity (water-soluble and exchangeable forms) to ones that have no reactivity in the short term (the life of a plant). The reactions of Ni are dominated by sorption reactions on soil colloids whose nature, relative proportions, and concentrations are important. Soil pH plays a major role in these reactions and thus influences the availability of Ni to plants quite markedly; few other factors appear to be as significant as pH. Deficiency of Ni in plants is unlikely to be a practical problem, but toxicity is more likely. Interest in the occurrence of Ni toxicity is dominated by research on serpentine soils (ancient Ni) and soils to which large quantities of Ni have been added in wastes (modern Ni), particularly in the forms of sewage and sewage sludge. Much of the understanding of the forms and reactions of Ni in soils comes from experiments dealing with the chemistry and biology of these two extreme situations. Needless to say, caution is essential when attempting to determine what is normal. Future research on Ni in soil could fruitfully pursue, for example, the role of microbes in influencing the behavior of Ni, the fractionation of Ni in studies of weathering and reversion of active forms of Ni to less active ones, and the speciation of Ni in the soil solution and the extent and cause of temporal changes.

The Effects of Acidic Deposition on Forested Soils

Abstract: Publisher Summary This chapter provides an overview of soil acidification that is one of the effects of acidic deposition on forest ecosystems. It also provides a brief synopsis of the dominant physical and chemical processes in forest ecosystems that interact with, modify, and respond to acidic inputs. The degree of interaction between acidic deposition and the forest canopy is illustrated. The chapter presents several case histories of soil change in regions where acid deposition may be causal and in regions where it is not. A set of recommendations is outlined, concerning areas for future research that are needed to be characterized properly and models for the effects of acidic deposition on forest soils are developed. Several hypotheses are proposed as the basis for future research on potential atmosphere–rhizosphere interactions that include above-ground physiology, nutrient uptake, heavy metal uptake, aluminum uptake, or microbes essential for nutrient absorption and changes in the forest ecosystem due to atmospheric deposition that alters rhizosphere regulation of root pathogens and soil saprophyte ecology.

Modeling the Transport and Retention of Inorganics in Soils

Abstract: Publisher Summary The interactions of dissolved chemicals and their transport in the soil profile play a significant role in their leaching losses beyond the root zone, availability for uptake by plants, and the potential contamination of groundwater supplies. To predict the transport of reactive solutes in the soil, models that include retention and release reactions of solutes with the soil matrix are needed. This chapter discusses the major features of retention models that govern retention reactions of solutes in the soil. Single-reaction models of the equilibrium type and kinetic type are discussed. Retention models of the multiple-reaction type, including the two-site equilibrium-kinetic models, the concurrent- and consecutive-multireaction models, and the second-order approach are presented. This is followed by multicomponent or competitive-type models wherein ion exchange is considered the dominant retention mechanism. Selected experimental data sets are described for the purpose of model evaluation and validation and necessary (input) parameters are also discussed. The chapter presents the formulations of the transport equations that govern the transport of solutes in a water-saturated and water-unsaturated porous medium. Retention reactions of the reversible and irreversible types are incorporated into the transport formulation. Boundary and initial conditions commonly encountered under field conditions are also presented.

Evolution of Corn

Abstract: Publisher Summary This chapter presents the key trait differences between teosinte and corn in isogeneic backgrounds that enables to determine the minimum number of genetic changes that are essential to convert teosinte into corn, to determine the inheritance and chromosomal location of these genes, to determine the modifying effects of background genes in shaping the expression of the key trait genes, and to determine from the past and current direction of corn's evolution as to where it may be going from here and which loci and genes will be useful in directing the future extension of this evolution. Only four or five inherited key trait units separate teosinte from corn and is based on the recovery rate of parental types in F 2 segregations. One viewpoint holds that these inherited units are clusters of linked genes that eventually evolved their partial isolating mechanisms through tight linkage, cross-over suppressors—such as cryptic rearrangements—, and chromosome knobs or by close linkage with gametophyte genes.

History and Status of Host Plant Resistance in Cotton to Insects in the United States1

Abstract: Publisher Summary Prior to 1892, only the cotton leafworm, Alabama argillacea , the bollworm, principally Helicoverpa zea , and the aphid or plant louse, principally Aphis gossypii were considered pest of cotton, and then only occasionally. This situation apparently meant that very little natural selection for resistance to the myriad of insect pests that attack cotton today took place from the time that cotton was introduced as a commercial crop in the United States through the 1800s. But the situation changed dramatically in 1892 when the Mexican boll weevil (Anthonomus grandis Boh.) migrated into South Texas. Thus, began a continuing battle between the American cotton producer and insects that would deprive the producer of the fruits of his labor. By 1965 over 100 species of insects and mites were known to attack cotton in the United States. To understand the present status of host plant resistance in cotton to insects, one must follow the evolution of cotton production in the United States since its introduction in 1607 at Jamestown, Virginia, and its continual, and expanded, production in this country. It is found that plant breeding research teams in the United States have made giant strides toward controlling cotton's insect pests through host plant resistance (HPR). Another benchmark is on the horizon. Genetically engineered plants await the identification of genes in almost any organism for transplant into cotton's genome. Genes from bacteria, Bacillus thuringiensis, that code for the production of a deadly toxin that dissolves the gut membrane of the tobacco budworm, cotton bollworm, and the pink bollworm have already been transferred into cotton.

Electrochemical Techniques for Characterizing Soil Chemical Properties

Abstract: Publisher Summary Electrochemical techniques are a category of methods used for the study of the composition and properties of a medium based on electrochemical reactions occurring at the electrode-solution interface. This chapter reviews the principles and applications of most important electrochemical techniques in soil science. The basic principle of electrochemical techniques is to convert one chemical parameter of a medium into an electrical parameter by an appropriate device, usually an electrode or an electrode pair, and then to measure this electrical parameter using an instrument. As in the case of electrical measurements, in electrochemical techniques four parameters can be utilized. The electrode potential may be a function of the composition of the solution. Techniques based on the measurement of this potential are called potentiometry. When an external voltage is applied to an electrode, an electrical current may flow through the circuit. Techniques utilizing this current-voltage relationship are called voltammetry. One can also measure the electrical resistance (conductance) between two electrodes in the medium, and such a technique is called conductometry. In all these techniques, the electrode is the site at which electrochemical reactions take place and is the key element in electrochemical analyses. Therefore, in the discussion the electrode potential at the electrode- solution interface when no current flows is first examined, and then the effect of current on electrode potential, electrode polarization, is discussed.