David W. Meek

Crop rotation effects on soil quality at three northern corn/soybean belt locations

This paper examines how three different rotations effect on soil quality and profitability.

Relationship of photosynthetically active radiation to shortwave radiation in the San Joaquin Valley

Abstract Photosynthetically active radiation (PAR) (0.4–0.7 μm) and shortwave radiation (SI) (0.285–2.8 μm) measurements were made for a year at two weather stations near Fresno, California. Solar radiation quantity and daylength did not appear to affect the relation of PAR to SI in this environment. The effects of diurnal variations in the relationship of the diffuse to the direct radiation components are also negligible on a daily basis. For each location, linear equations were developed to express the relationship of PAR as a function of SI. Additional measurements (which are not part of the development data) taken at each site were used to validate each of the annual models.

Nitrogen fertilizer effects on soil carbon balances in Midwestern U.S. agricultural systems

A single ecosystem dominates the Midwestern United States, occupying 26 million hectares in five states alone: the corn-soybean agroecosystem [Zea mays L.-Glycine max (L.) Merr.]. Nitrogen (N) fertilization could influence the soil carbon (C) balance in this system because the corn phase is fertilized in 97-100% of farms, at an average rate of 135 kg N x ha(-1) x yr(-1). We evaluated the impacts on two major processes that determine the soil C balance, the rates of organic-carbon (OC) inputs and decay, at four levels of N fertilization, 0, 90, 180, and 270 kg/ha, in two long-term experimental sites in Mollisols in Iowa, USA. We compared the corn-soybean system with other experimental cropping systems fertilized with N in the corn phases only: continuous corn for grain; corn-corn-oats (Avena sativa L.)-alfalfa (Medicago sativa L.; corn-oats-alfalfa-alfalfa; and continuous soybean. In all systems, we estimated long-term OC inputs and decay rates over all phases of the rotations, based on long-term yield data, harvest indices (HI), and root:shoot data. For corn, we measured these two ratios in the four N treatments in a single year in each site; for other crops we used published ratios. Total OC inputs were calculated as aboveground plus belowground net primary production (NPP) minus harvested yield. For corn, measured total OC inputs increased with N fertilization (P < 0.05, both sites). Belowground NPP, comprising only 6-22% of total corn NPP, was not significantly influenced by N fertilization. When all phases of the crop rotations were evaluated over the long term, OC decay rates increased concomitantly with OC input rates in several systems. Increases in decay rates with N fertilization apparently offset gains in carbon inputs to the soil in such a way that soil C sequestration was virtually nil in 78% of the systems studied, despite up to 48 years of N additions. The quantity of belowground OC inputs was the best predictor of long-term soil C storage. This indicates that, in these systems, in comparison with increased N-fertilizer additions, selection of crops with high belowground NPP is a more effective management practice for increasing soil C sequestration.

Relationship Between Six Years of Corn Yields and Terrain Attributes

Crop yield, soil properties, and erosion are strongly related to terrain attributes. The objectives of our study were to examine the relationship between six years of corn (Zea mays L.) yield data and relative elevation, slope, and curvature, and to develop a linear regression model to describe the spatial patterns of corn yield for a 16 ha field in central Iowa, USA. Corn grain yield was measured in six crop years, and relative elevation was measured using a kinematic global positioning system. Slope and curvature were then determined using digital terrain analysis. Our data showed that in the four years with less than normal growing season precipitation, corn yield was negatively correlated with relative elevation, slope, and curvature. In the two years with greater than normal precipitation, yield was positively correlated with relative elevation and slope. A multiple linear regression model based on relative elevation, slope, and curvature was developed that predicted 78% of the spatial variability of the average yield of the transect plots for the four dry years. This model also adequately identified the spatial patterns within the entire field for yield monitor data from 1997, which was one of the dry years. The relationship between terrain attributes and corn yield spatial patterns may provide opportunities for implementing site-specific management.

Selenium accumulation in selected vegetables

Absract Greenhouse experiments were conducted to determine selenium (Se) uptake by sulfur‐accumulating vegetables. Cabbage (Brassica oleracea var. capitata), broccoli (Brassica oleracea var. botrytis), Swiss chard (Beta vulgaris var. cicla) and collards (Brassica Oleracea var. acephda) were grown in a soil mix to which 4.5 mg of selenate or selenite had been added per kg of soil. Plants were grown to maturity, separated into plant organs, and the tissues analyzed for Se and sulfate (SO4). Vegetables grown in selenate laden soil significantly (P<0.05) accumulated higher concentrations of Se than plants grown in selenite laden soil. The highest concentrations of Se and SO4 were found in the broccoli floret and vegetable leaf tissues. A second greenhouse experiment examined the uptake of Se and SO4 in broccoli (Brassica oleracea var. botrytis) grown hydroponically with increasing Se concentrations. Treatments consisted of three Se concentrations (2, 6, and 15 mg of selenate, added as Na2SeO4/L to a synthetic...

Evaporation from screened class a pans in a semi-arid climate

Abstract Evaporation measurements from ‘screened’ and standard Class A pans at two locations near Fresno, California were compared together and with ‘potential evapotranspiration’ estimates, using two combination equations with climate data from the sites. The screen covering on a Class A pan reduced the Class A pan evaporation by 10%. This difference was found to be due more to radiation than aerodynamic differences. The combination equation calculations were found to be highly correlated with pan evaporation. Pan coefficients of 0.81 and 0.91 were found for the open and ‘screened’ Class A pans, respectively, in the semi-arid climate.

THE INFLUENCE OF SELENIUM, SALINITY, AND BORON ON SELENIUM UPTAKE IN WILD MUSTARD

Wild brown mustard (Brassica juncea) was shown in greenhouse water cultures to determine the effects of selenium (Se), salinity (salt), and boron (B) in the root media on total Se concentrations in plant tissues. The experimental design was a three-way incomplete factorial with treatments consisting of four Se concentrations (0, 2, 6, and 15 mg Se L−1 as Na2SeO4), four B concentrations (0.1, 2, 6, and 15 mg B L−1 as boric acid) and four salt treatments (0.5, 3, 10, and 15 dS m−1 as NaCl and CaCl in approximately 5:1 ratio by weight). After 40 d of growing in the respective water culture treatment, plants were harvested, separated into shoots and roots and analyzed for total tissue Se and B, and shoot sulfate (SO4) concentrations.The treatments significantly influenced yield and uptake of Se, B, and SO4 by wild mustard. Shoot and root dry weight yields were reduced by 30% and 21%, respectively. Selenium and SO4 tissue concentrations were positively related to solution Se, while the Se model was independent of solution B and salinity. Similarly, B concentrations were positively related to solution B, while the B model was independent of solution Se and salinity. Therefore mustard is reasonably salt tolerant and accumulates Se and B when grown in waters laden with Se and B.

Estimation of maximum possible daily global solar radiation

The estimation of maximum possible daily global solar radiation is important in many applied sciences. This study develops and evaluates climatic extreme based modifications of two single-atmospheric-layer broadband solar shortwave irradiance models for the purpose of estimating a dynamic upper boundary for global solar radiation at any given location. Climatic component models were developed for five rural locations in the central United States: Ames, IA, Bismarck, ND, Columbia, MO, Dodge City, KS, and Wooster, OH. Each site had long-term (30 or 31 years) records of daily global solar radiation data available. Aerosol optical depth, precipitable water, and surface albedo were the input variables. Data for the first two inputs were obtained from the SAMSON database (Solar and Meteorological Observation Network). Albedo interpolating curves were estimated from the predecessor of SAMSON. For each site, precipitable water and aerosol optical depth daily data were used to develop annual trends in the climatic lows and normals for each variable. The normals were based on median daily values. Nonlinear generalized least squares regression analyses were used to develop the interpolating curves. To evaluate the global radiation models, maximum daily global solar radiation values were selected for each day in the year from the entire period of record for each site. In either broadband model, the use of the climatic normals in each input variable either interpolated or underestimated the selected radiometer data. The use of the climatic lows, however, yielded a reasonable upper boundary for the selected data. The result may be partially due to the fact that the climatic minima curves for each turbidity variable were generally significantly different throughout the year from the climatic normals (P < 0.05). The global solar radiation models were most sensitive to aerosol optical depth. Although it is more sensitive to input variation and it is somewhat less conservative, the simpler of the two broadband models is adequate for most applications. While results are site-specific, the methodology is general and provides a climatic-based definition for maximum possible daily global solar radiation. Published by Elsevier Science B.V.

A note on recognizing autocorrelation and using autoregression

The presence of autocorrelation in the analysis of a variable sampled sequentially at regular time intervals appears to be unknown to many agricultural meteorologists despite abundant documentation found in the traditional meteorological and statistical literature. It follows that the statistical consequences as well as methodological alternatives are also unknown. Through an example using paired radiometer observations, this note discusses recognition of autocorrelation as well as the importance of testing ordinary least squares regression parameters in the presence of autocorrelated residuals. An autoregression example is presented as one alternative way to analyze the given dataset.

Relations between phosphorus in drip irrigation water and selenium uptake by wild mustard

Abstract A field study determined that phosphorus concentrations influence the accumulation of selenium in the plant tissue of wild brown mustard (Brassica juncea czem L.). The main treatment was a one‐way H3PO4 application at five different concentrations (less than 1, 10, 25, 50, and 100 mg P/L). A constant concentration of 5 mg Se/L was added as Na2SeO4 with each application. The H3PO4 and Se additions were injected daily by a surface drip irrigation system. Plant dry weight yields did not vary significantly from one phosphorus treatment level to another, but plant tissue concentrations of Se and P increased, soil Se decreased, and soil P increased as H3PO4 application concentrations increased. Thus, adding phosphate to the soil in irrigation water contributed to Se‐accumulation in mustard and led to lower levels in the soil.