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A wet digestion method for plant tissue analysis for use with Inductively Coupled Plasma -Atomic Emission Spectrometry has been developed. This procedure uses only nitric acid and eliminates the use of perchloric acid and subsequently eliminates the danger of HClO 4 . explosions, and the problem of KClO 4 precipitation. Analysis of National Bureau of Standards plant materials -orchard, tomato, and spinach leaves -digested with nitric-perchloric acid or nitric acid compared very well with the NBS certified values. The analytical results for eight other plant species were comparable for the two methods of digestion. 970 HAVLIN AND SOLTANPOUR A summary of the recommended procedure for use with ICP is given below: 1. Weigh 0.5 to 1.0 g of plant tissue into a 50 ml Taylor tube. 2. Add 10 ml conc, nitric acid. Let stand overnight. 3. Heat samples at 125° C for 4 hours. Let cool. 4. Dilute to 12.5 ml with conc, nitric acid. Dilute to 50 ml with distilled water. Mix and let amorphous silica settle. 5. Aspirate directly into plasma for ICP analysis of P, K, Cu,

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Use of fallow to store soil water is a common practice in semiarid regions. In the central Great Plains, the most common dryland cropping system is winter wheat (Triticum aestivum L.)‐fallow. Stubble mulching involving tillage is the predominant weed‐control practice during the 14‐mo fallow period. As a result of tillage, soil organic matter content has declined 40 to 70% since the early 1900s. This decline has called for development of cropping practices that control soil erosion and increase soil organic matter. Green fallow is the practice of growing a legume during the time period not devoted to crop production. Water is a major limiting factor for crop production in the central Plains, and water use by the legume could reduce grain yields. Field studies were conducted near Tribune, KS, from 1990 to 1994 to evaluate green fallow in the central Great Plains. The objectives were to (i) evaluate the production potential of several dryland forage legumes, (ii) quantify the water use of dryland legumes as a function of growth period, and (iii) measure the effects of legume growth on grain yield of subsequent crops. Of 11 legume species evaluated, hairy vetch (Vicia villosa Roth) and yellow sweetclover (Melilotus officinalis Lam.) were the most promising in terms of biomass production and weed control. Hairy vetch was planted in a green fallow system and allowed to grow for selected periods of time. In all cases, green fallow depleted soil water and reduced grain yield of subsequent crops. Allowing hairy vetch growth throughout the fallow period reduced soil water by up to 178 mm and reduced grain yield by 42 to 83%. For every millimeter of soil water depletion by vetch, grain yields decreased by 15 kg ha−1. Although green fallow is too detrimental to subsequent crop yields to be recommended in the central Great Plains, dryland legumes may have potential as forage crops.

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T o meet the growing world demand for food, fuel, and fiber, and at the same time sustain the environment's ability to provide economic, social, and environmental services to society, agricultural innovations are essential. Such innovations must derive from a comprehensive understanding of the long-term functioning of agricultural systems and their resiliency. Soil, water, and energy limitations pose longstanding and persistent problems for agricultural productivity, profitability, and social acceptability; for global agricultural competitiveness; and for environmental quality and security. Long-lasting solutions to these problems require a comprehensive, systems-level understanding of the linkages among basic biophysical processes and human activity, an understanding that can serve as a solid foundation for informed management and policy decisions. This understanding can be achieved best-or perhaps only-through long-term research that integrates multiple processes, both biophysical and socioeconomic, across multiple spatial and temporal scales. Practical solutions depend on long-term research because robust solutions to many of the problems facing agriculture require evaluation in the context of climatic, social, ecological, and other factors that change on decadal (or longer) time scales. Long-term research also allows the impacts of management to be distinguished from impacts caused by long-term environmental trends such as land use and regional climate change.Frontiers in Agricultural Research (NRC 2003), the most recent comprehensive review of the US agricultural research portfolio, identified five major challenges for US agricultural G. Philip Robertson (e-mail: robertson@kbs.msu.edu

Improving N management for corn (Zea mays L.) production with precision agriculture technologies requires that spatial N recommendations adequately represent in‐field variability in N availability. Our objective was to evaluate corn response to increasing N rates in several in‐field locations that represented the range of soil organic matter (OM) content in the field. In a 2‐yr study, three center pivot–irrigated fields were selected in south‐central Kansas and south‐central Nebraska. Four or five locations were selected within each field. At each location, five or six N treatments (0–336 kg N ha−1) were surface‐applied early in the growing season. The minimum N rate to achieve maximum yield varied by as much as 130 kg N ha−1 among in‐field locations at three site‐years. The least amount of N to achieve maximum yield did not coincide with locations representing greater soil OM. Yield response at two site‐years was the same among in‐field locations; however, mean yield among in‐field locations varied by as much as 4.2 Mg ha−1, representing potential for improvement in N use efficiency. Leaf tissue N was below the critical threshold for 60 to 100% of observations at three different in‐field locations but below the critical threshold for <35% of the observations at all other in‐field locations. The reason for the discrepancy in N availability among in‐field locations was not conclusively identified but was not only related to soil OM content. Variable N recommendations based only on soil OM is too simplistic to reflect variability in N availability within a field.