Research in plant nutrition is a continuing program. The development of new crop varieties and the introduction of new management practices to increase crop yields impart changes in nutrient requirements of plants. The increasing application of genomics is providing more understanding of the genetic basis for the efficiency with which different plants utilize nutrients. For example, a study of induction of Arabidopsis genes by nitrate confirmed that genes encoding nitrate reductase, the nitrate transporter NRT1 (but not the nitrate transporter NRT2), and glutamate synthase were all highly induced, and this work also demonstrated induction of a further 15 genes that had not previously been shown to be induced (87). Nitrate influences root architecture through induction of genes that control lateral root growth (88).
Research is conducted, and will continue to be conducted, to ensure that soil tests correlate with use of nutrients by plants and that fertilizer recommendations are calibrated for crops (89). These correlations must be developed for individual crops and different land areas. Some research is directed toward development of systems for evaluation of soil and crop conditions through methods other than traditional soil and plant analysis. Much of the past and current research addresses chemical, physical, and biological properties of soils (90,91). Some researchers have studied the interaction of these quantitative aspects to determine soil quality and to develop a soil quality index that correlates with crop productivity and environmental and health goals (92). Soil quality has been defined to include productivity, sustainability, environmental quality, and effects on human nutrition (93). To quantify soil quality, specific soil indicators are measured and integrated to form a soil quality index.
Research in plant nutrition addresses methods of economically and environmentally sound methods of fertilization. Worldwide, large increases have occurred in the use of fertilizers because of their effects on yields and availability. Traditionally, fertilizer use has followed Sprengel's law of the minimum, made famous by Liebig (94), and the application of the law of diminishing returns by Mitscherlich (95). Applying these two laws has given us fertilizers with the nutrients blended in the correct proportions for the world's major crops and rates of fertilizer use that lead to maximum yields commensurate with the cost of the fertilizer.
Research also gives attention to the accumulation of elements that are beneficial in plant, animal, and human nutrition. Accumulation of selenium is addressed in research and in this handbook (105,106). Sections on aluminum, cobalt, and silicon discuss research on these elements. Traditional soil testing provides information on patterns in soil fertility and management, and plant vigor provides an indication of plant response to soil properties and management often based on soil testing. Shortcomings of current soil testing methodology are the inability to predict yields, large soil test spatial and temporal variability, inability to reflect dynamics of field parameters that affect nutrient availability, lack of accurate tests for nutrient mineralization, and lack of accurate nutrient response functions (107).
Precision agriculture considers spatial variability across a field to optimize application of fertilizer and other inputs on a site-specific basis (76,90,108-110). Precision agriculture employs technologies of global positioning and geographic information systems and remote sensing. These technologies permit decisions to be made in the management of crop-yield-limiting biotic and abiotic factors and their interactions on a site-specific basis rather than on a whole-field basis (111-114). Remote sensing is a term applied to research that assesses soil fertility and plant responses through means other than on-the-ground sampling and analysis (115). Research has applied video image analysis in monitoring plant growth to assess soil fertility and management (116). Spectral reflection and digital processing of aerial photographs have been researched to assess soil fertility (117). In precision agriculture, it is possible for the fertilizer spreader on the back of a tractor to operate at different speeds in different parts of a field in response to data obtained on the growth of the crop underneath and stored in a geographic information system. These data may have been obtained by remote sensing, or even by continuous measurement of yields by the harvesting equipment operating in the same field at the previous harvest. The precise location of the fertilizer spreader at any moment of time is monitored by global positioning.
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