Agronomy Journal
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Optimizing Soil and Fertilizer Nitrogen Use by Intensively Managed Winter Wheat. II. Critical Levels and Optimum Rates of Nitrogen Fertilizer Abstract The assessment of optimum N fertilizer rates for winter wheat (Triticum aestivum L.) production is a major unsolved problem, particularly in humid climates. Eight field experiments were conducted in the Coastal Plain and Ridge and Valley regions of Virginia during the 1981–1982 through 1985–1986 winter wheat growing seasons to determine critical N levels and optimum rates of N fertilization. The treatments in these experiments consisted of various N fertilizer rates applied at Zadoks growth stage (GS) 25 and GS 30. Plant N concentration (NCON30) and crop N uptake (NUP30) at GS 30 were measured immediately prior to the N fertilizer application. Nitrogen critical levels were defined as the NCON30 or NUP30 values that produced 90% of maximum grain yields in each experimental site with no further N fertilizer application. Two nonlinear models were developed to determine critical levels at GS 30 using NCON30 and NUP30 as the independent variable. The calculated critical levels for the models using NCON30 and NUP30 were 39.5 g N/kg and 95 kg N/ha, respectively, with R2 values for the models being 0.87 (NCON30) and 0.79 (NUP30). A family of response curves of grain yield to N fertilizer applied at GS 30 was fitted at each ex‐ perimental site. The amount of N fertilizer required at GS 30 to obtain maximum grain yield (NMAX30) was calculated for each response curve. Regression analysis was used to develop modells for estimating the amount of N fertilizer required at GS 30 to produce maximum grain yields for any given value of NCON30 or NUP30. Best models were selected considering fit, significance of the regression coefficients, and predictive ability. The best models in both cases were simple linear regressions. The model using NUP30 as the independent variable had a larger correlation coefficient (r = –0.87) and a better predicting ability than the model using NCON30. Regression analysis was also used to develop models to estimate the amount of N fertilizer required at GS 30 to produce economically optimum yields at three fertilizer/wheat price. ratios (2.0, 4.0, and 6.0) using NUP30 as the independent variable. The three simple linear regression models were similar and presented large correlation coefficients (–0.88, –0.89, and –0.88, respectively). The results indicate that further field experimentation using the same methodology described in this research should be conducted under varying soils and climatic conditions to develop a model for predicting economically optimum N fertilizer rates for grower fields.
Agronomy Journal - Tập 81 Số 1 - Trang 120-125 - 1989
Nitrogen Management Strategies to Reduce Nitrate Leaching in Tile‐Drained Midwestern Soils Balancing the amount of N needed for optimum plant growth while minimizing the NO3 that is transported to ground and surface waters remains a major challenge for everyone attempting to understand and improve agricultural nutrient use efficiency. Our objectives for this review are to examine how changes in agricultural management practices during the past century have affected N in midwestern soils and to identify the types of research and management practices needed to reduce the potential for nonpoint NO3 leakage into water resources. Inherent soil characteristics and management practices contributing to nonpoint NO3 loss from midwestern soils, the impact of NO3 loading on surface water quality, improved N management strategies, and research needs are discussed. Artificial drainage systems can have a significant impact on water quality because they behave like shallow, direct conduits to surface waters. Nonpoint loss of NO3 from fields to water resources, however, is not caused by any single factor. Rather, it is caused by a combination of factors, including tillage, drainage, crop selection, soil organic matter levels, hydrology, and temperature and precipitation patterns. Strategies for reducing NO3 loss through drainage include improved timing of N application at appropriate rates, using soil tests and plant monitoring, diversifying crop rotations, using cover crops, reducing tillage, optimizing N application techniques, and using nitrification inhibitors. Nitrate can also be removed from water by establishing wetlands or biofilters. Research that is focused on understanding methods to minimize NO3 contamination of water resources should also be used to educate the public about the complexity of the problem and the need for multiple management strategies to solve the problem across agricultural landscapes.
Agronomy Journal - Tập 94 Số 1 - Trang 153-171 - 2002
The Growth and Rubber Content of Guayule as Affected by Variations in Soil Moisture Stresses<sup>1</sup>
Agronomy Journal - Tập 38 Số 2 - Trang 118-134 - 1946
Rice—A Step Toward Use of Allelopathy Rice (Oryza sativa L.) allelopathy has been on the research agenda for a decade. Now it is important to step back and look at its progress to enable priority setting for future research. This paper aims to do so primarily using the following five‐step protocol for allelopathy research: (i) carrying out laboratory, greenhouse, and field studies to illustrate the effect of released allelochemicals; (ii) isolating, identifying, and characterizing allelochemicals; (iii) establishing a correlation between growth inhibition and allelochemicals; (iv) performing genetic mapping of quantitative trait loci (QTLs) correlated with allelopathy; and (v) breeding for allelopathic cultivars tested for competitive ability in greenhouse and field experiments. Recent research on rice allelopathy has resulted in the following research milestones:
There is large variation in allelopathy among rice cultivars. Allelopathy plays a role under field conditions. Allelopathic rice can suppress both mono‐ and dicot weed species. Progress has been made in identifying rice allelochemicals. Quantitative trait loci correlated with allelopathy have been identified.
This paper discusses the progress made in recent years and suggests some direction for future research.
Agronomy Journal - Tập 93 Số 1 - Trang 3-8 - 2001
Theory and Practical Application of Heat Pulse to Measure Sap Flow Heat pulse methods can be used for accurate measurements of sap flow in plant stems provided a reliable calibration procedure is used to relate the measured heat pulse velocity to the actual sap flow. This paper reviews the theory underpinning both the compensation and T‐max heat pulse methods that use a linear heater and temperature probes inserted radially into the plant stem. These probes not only disrupt the sap stream, but they also alter the thermal homogeneity of the sapwood in the vicinity of the probes. The degree of disturbance depends on the size and geometry of the probes and the corresponding wound width of the nonconducting sapwood. A two‐dimensional model of heat and water flow was used here to derive appropriate correction factors to account for the influence of both probe thermal properties and flow blockage. Wound width has a large influence on the heat pulse measurements while sensor material appears to have little or no influence. A table of correction factors is presented for both the compensation and T‐max methods. These new correction factors are confirmed by comparing heat pulse measurements in the trunk of a willow (Salix alba L.) and a poplar (Populus deltoides W. Bartram ex Marsh), against actual rates of transpiration determined from measured weight loss of the trees growing in large lysimeters. On a daily basis, both heat pulse measurements were found to be within 5 to 10% of the actual transpiration. The compensation method accurately measured flows close to 2 cm/h. The T‐max method had difficulty resolving any flows slower than about 10 cm/h.
Agronomy Journal - Tập 95 Số 6 - Trang 1371-1379 - 2003
Exogenous Hormones Alleviated Salinity and Temperature Stresses on Germination and Early Seedling Growth of Sweet Sorghum Germination and vigorous growth are critical for seedling establishment, especially under salinity and high temperature. A laboratory experiment was done to determine the effects of salinity (0, 100, 200, and 300 mM NaCl) and temperature (25, 35, and 39°C) on germination and early seedling growth of two genotypes (Yajin 13 and Yajin 71) of sweet sorghum [Sorghum bicolor (L.) Moench]. Seeds were soaked in solutions with one of three hormones [288.7 μM gibberellic acid (GA3 ), 232.3μM kinetin, or their combination] and germinated under controlled conditions. Water imbibition, germination, radicle and shoot length, and the electrical conductivity of leachate were measured. Seed water imbibition increased with increasing temperature and decreased with increasing salinity level. Yajin 13 had 17.9% higher germination percentage with 16.3% higher germination velocity index than Yajin 71. Salinity and temperature stresses clearly affected germination and decreased subsequent shoot and root growth. The application of GA3 , kinetin, or their combination alleviated the negative effects of salinity and temperature stresses on all the measurements. Among the tested hormones, GA3 was better in alleviating salinity and temperature stress in both genotypes in germination and other parameters except water uptake. It was concluded that seeds treated with suitable exogenous hormones may serve as a possible way to improve water imbibition and to alleviate the negative effects produced by salinity and high temperature on germination and early seedling growth of sweet sorghum.
Agronomy Journal - Tập 106 Số 6 - Trang 2305-2315 - 2014
A Simple Turbidimetric Method of Determining Total Sulfur in Plant Materials<sup>1</sup> Abstract A rapid method of determining total sulfur in plant materials is described. The plant material is digested in a volumetric flask with nitric and perchloric acid, and the sulfate content of an aliquot of the digest is determined turbidimetrically as BaSO4 by a barium chloride‐gelatin procedure. The method is simple and precise, and its results agree closely with those obtained by the Johnson‐Nishita method.
Agronomy Journal - Tập 62 Số 6 - Trang 805-806 - 1970
Evapotranspiration Rates of Field Crops Determined by the Bowen Ratio Method<sup>1</sup> Abstract Simultaneous evapotranspiration rates were determined biweekly for the crop combinations of alfalfa and barley, alfalfa and cotton, alfalfa and sorghum, wheat and oats, and cotton from meteorological data by the Bowen ratio method. The crops were grown under irrigated conditions in the hot, arid region of south‐central Arizona. Calculated evapotranspiration rates ranged from 1.0 to 1.8 times net radiation, indicating that large amounts of energy were extracted from the air mass. Alfalfa prior to cutting tended to use more water than the other crops. Water use by cotton after canopy development approached that of alfalfa. Barley, wheat, and grain sorghum appeared to require the least water. The Bowen ratio method, as a survey technique or continuous sampling method, can be used to obtain short‐period evapotranspiration rates under field conditions. It appears to be one of the few methods which will yield valid results when crops are subjected to frequent and heavy irrigation or where water tables are present.
Agronomy Journal - Tập 58 Số 3 - Trang 339-342 - 1966
Bowen‐Ratio Comparisons with Lysimeter Evapotranspiration Abstract Water use in agriculture by different cropping systems is of interest in determining crop water use efficiency of different tillage practices that will lead to reduced crop production risk. Lysimeters are considered the standard for evapotranspiration (ET) measurements; however, these units are often not replicated and are few in number at any given location. Our objective was to determine if a simple Bowenratio system with nonexchanging psychrometers could provide accurate measurements of ET from lentil (Lens culinaris Medikus) in a semiarid climate. The study was conducted in 1993 and 1994 on two adjacent 180‐ by 180‐m fields with weighing lysimeters (1.68 by 1.68 by 1.83 m) located in the center of each field, on a Williams loam (fine‐loamy, mixed Typic Argiboroll) soil near Sidney, MT. A Bowen‐ratio system comprised of two nonexchanging psychrometers and anemometers at 0.25 and 1.25 m above the plant canopy surface was placed in the lentil field along with a net radiometer and soil heat flux plate. Precipitation during the growing season from planting to swathing was 367 mm in 1993 and 227 mm in 1994. In 1993, soil water content of the lysimeter was greater than the field after large precipitation events around Day of Year (DOY) 210, even though the lysimeter was drained. After this time, the lysimeter ET exceeded that measured by the Bowen‐ratio system. Agreement was closer in 1994, when precipitation was near normal and there was no excess soil water in the lysimeter. Cumulative ET totals from the lysimeter were reflective of the seasonal precipitation patterns. Differences between the lysimeter and Bowen‐ratio occurred when there was excess precipitation and inadequate drainage from the lysimeter. Halfhourly ET fluxes from lysimeter and Bowen‐ratio values agreed to within 10% throughout the season. Bowen‐ratio systems with nonexchanging psychrometers can provide satisfactory estimates of daily and seasonal ET and can be used to estimate ET in semiarid climates.
Agronomy Journal - Tập 89 Số 5 - Trang 730-736 - 1997
Model Evaluation by Comparison of Model‐Based Predictions and Measured Values The appropriateness of a statistical analysis for evaluating a model depends on the model's purpose. A common purpose for models in agricultural research and environmental management is accurate prediction. In this context, correlation and linear regression are frequently used to test or compare models, including tests of intercept a = 0 and slope b = 1, but unfortunately such results are related only obliquely to the specific matter of predictive success. The mean squared deviation (MSD) between model predictions X and measured values Y has been proposed as a directly relevant measure of predictive success, with MSD partitioned into three components to gain further insight into model performance. This paper proposes a different and better partitioning of MSD: squared bias (SB), nonunity slope (NU), and lack of correlation (LC). These MSD components are distinct and additive, they have straightforward geometric and analysis of variance (ANOVA) interpretations, and they relate transparently to regression parameters. Our MSD components are illustrated using several models for wheat (Triticum aestivum L.) yield. The MSD statistic and its components nicely complement correlation and linear regression in evaluating the predictive accuracy of models.
Agronomy Journal - Tập 95 Số 6 - Trang 1442-1446 - 2003
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