technology and intensified production often involve a greater need for commercial fertilizer nutrients to avoid Nutrient inputs in crop production systems have come under innutrient depletion and ensure soil quality and crop procreased scrutiny in recent years because of the potential for environductivity. The need for increased inputs correctly raises mental impact from inputs such as N and P. The benefits of nutrient inputs are often minimized in discussions of potential risk. The purpose questions about associated risks. Potential risks are ofof this article is to examine existing data and approximate the effects ten widely publicized while the associated benefits of of nutrient inputs, specifically from commercial fertilizers, on crop an abundant, affordable, and healthful food supply can yield. Several long-term studies in the USA, England, and the tropics, be overlooked or understated. To judge any such pracalong with the results from an agricultural chemical use study and tice or system, the risks must be evaluated in comparison nutrient budget information, were evaluated. A total of 362 seasons with the benefits. While misuses of agricultural fertilizof crop production were included in the long-term study evaluations. ers have undoubtedly occurred and concerns about how Crops utilized in these studies included corn (Zea mays L.), wheat fertilizers affect the environment have sometimes been (Triticum aestivum L.), soybean [Glycine max (L.) Merr.], rice (Oroverstated, the purpose of this article is not to address yza sativa L.), and cowpea [Vigna unguiculata (L.) Walp.]. The these issues but to provide evidence of the impact comaverage percentage of yield attributable to fertilizer generally ranged from about 40 to 60% in the USA and England and tended to be mercial fertilizers have had on agricultural production. much higher in the tropics. Recently calculated budgets for N, P, and Several attempts have previously been made to esti-K indicate that commercial fertilizer makes up the majority of nutrient mate how much of the crop production in the USA is inputs necessary to sustain current crop yields in the USA. The results attributable to commercial nutrient inputs. These estiof this investigation indicate that the commonly cited generalizationmates usually range from about 30 to 50% for major that at least 30 to 50% of crop yield is attributable to commercial grain crops (Nelson, 1990). Determining these estimates fertilizer nutrient inputs is a reasonable, if not conservative estimate. W.M. Stewart, Potash and Phosphate Inst., 2423 Rogers Key, San CHEMICAL USE STUDY Antonio, TX 78258; D.W. Dibb, Potash and Phosphate Inst., 655 Engineering Dr., Suite 110, Norcross, GA 30092; A.E. Johnston, Ro-The impact of eliminating the use of several chemical thamsted Research, Harpenden AL5 2JQ, England; and T.J. Smyth, inputs, including inorganic N fertilizer, on corn, cotton

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Eleven precipitated phosphates were evaluated as sources of phosphorus (P)

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SummaryLong‐term field experiments that test a range of treatments and are intended to assess the sustainability of crop production, and thus food security, must be managed actively to identify any treatment that is failing to maintain or increase yields. Once identified, carefully considered changes can be made to the treatment or management, and if they are successful yields will change. If suitable changes cannot be made to an experiment to ensure its continued relevance to sustainable crop production, then it should be stopped. Long‐term experiments have many other uses. They provide a field resource and samples for research on plant and soil processes and properties, especially those properties where change occurs slowly and affects soil fertility. Archived samples of all inputs and outputs are an invaluable source of material for future research, and data from current and archived samples can be used to develop models to describe soil and plant processes. Such changes and uses in the Rothamsted experiments are described, and demonstrate that with the appropriate crop, soil and management, acceptable yields can be maintained for many years, with either organic manure or inorganic fertilizers.Highlights Long‐term experiments demonstrate sustainability and increases in crop yield when managed to optimize soil fertility.Shifting individual response curves into coincidence increases understanding of the factors involved.Changes in inorganic and organic pollutants in archived crop and soil samples are related to inputs over time.Models describing soil processes are developed from current and archived soil data.

Automatic mobile shelters were used to keep rain off a barley crop in a drought experiment. The treatments ranged from no water during the growing season to regular weekly irrigation. This paper reports the effect of drought on the harvest yield and its components, on water use and nutrient uptake.Drought caused large decreases in yield, and affected each component of the grain yield. The magnitude of each component varied by up to 25 % between treatments, and much of the variation could be accounted for by linear regression against the mean soil water deficit in one of three periods. For the number of grains per ear, the relevant period included tillering and ear formation; for the number of ears per unit ground area, the period included stem extension and tiller death; for grain mass, the period included grain filling.The harvest yields were linearly related to water use, with no indication of a critical period of drought sensitivity. The relation of grain yield to the maximum potential soil water deficit did show that a prolonged early drought had an exceptionally large effect on both yield and water use.Two unsheltered irrigation experiments, also on barley, were made in the same year on a nearby site. The effects of drought on yield in these experiments were in good agreement with the effects observed on the mobile shelter site.When fully irrigated, the small plots under the mobile shelters used water 11 % faster than larger areas of crop, because of advection. The maximum depth from which water was extracted "was unaffected toy the drought treatment. When 50 % of the available soil water had been used the uptake rate decreased, but the maximum depth of uptake continued to increase.Measurements of crop nutrients at harvest showed that nitrogen uptake was large, because of site history, and that phosphate uptake was decreased by drought to such an extent that phosphate shortage may have limited yield.

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The sequestration in soil of organic carbon (SOC) derived from atmospheric carbon dioxide (CO2) by replacing arable crops with leys, has been measured over 70 years on a sandy loam soil. The experiment was designed initially to test the effect of leys on the yields of arable crops. A 3‐year grazed grass with clover (grass + clover) ley in a 5‐year rotation with arable crops increased percentage organic carbon (%OC) in the top 25 cm of the soil from 0.98 to 1.23 in 28 years, but with little further increase during the next 40 years with all‐grass leys given fertilizer nitrogen (N). In this second period, OC inputs were balanced by losses, suggesting that about 1.3% OC might be near the equilibrium content for this rotation. Including 3‐year lucerne (Medicago sativa) leys had little effect on %OC over 28 years, but after changing to grass + clover leys, %OC increased to 1.24 during the next 40 years. Eight‐year leys (all grass with N or grass + clover) in 10‐year rotations with arable crops were started in the 1970s, and after three rotations %OC had increased to ca. 1.40 in 2000–2009. Over 70 years, %OC declined from 0.98 to 0.94 in an all‐arable rotation with mainly cereals and to 0.82 with more root crops. Applications of 38 t ha−1 farmyard manure (FYM) every fifth year increased %OC by 0.13% by the mid‐1960s when applications ceased. Soil treated with FYM still contained 0.10% more OC in 2000–2009. Changes in the amount of OC have been modelled with RothC‐26.3 and estimated inputs of C for selected rotations. Little of the OC input during the 70 years has been retained; most was retained in the grazed ley rotation, but 9 t ha−1 only of a total input of 189 t ha−1. In other rotations more than 98% of the total OC input was lost. Despite large losses of C, annual increases in OC of 4‰ are possible on this soil type with the inclusion of grass or grass + clover leys or the application of FYM, but only for a limited period. Such increases in SOC might help to limit increases in atmospheric CO2.Highlights Can leys sequester significant amounts of atmospheric CO 2 in SOM and contribute to the 4‰ initiative?Changes in the percentage and amount of OC were measured and modelled over 70 years and OC losses estimated.Three‐year grass or grass + clover leys increased %OC, but only to an equilibrium level that was then maintained.Despite large losses, sequestering CO 2‐C at 4‰ year−1 by growing grass or grass + clover leys is possible.