All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. N itrogen is an important and costly input for nonleguminous grain crops, and producers are applying N fertilizer in large amounts to ensure high yields over a range of environmental conditions (Kyveryga et al., 2007). However, excessive N fertilization may lead to runoff , leaching, and nitrate pollution. A delayed N application and the use of remote sensing tools might allow a producer to apply a more economically benefi cial N rate to their fi elds. Scharf and Lory (2002) gave several reasons to delay N applications, including avoiding extra work during the busy planting season and lowering the in-season N loss during wet years. Th ey also suggested that diagnostic tools for plant N might increase fertilizer use effi ciency, and these tools include the SPAD meter, refl ectance measurements, and color analysis. Th e SPAD meter is used to make an optimum fertilizer N-rate decision by measuring N stress relative to an optimum N-rate strip within a fi eld (Hawkins et al., 2007). Th e SPAD meter is well documented as an accurate measure of the N status of corn at diff erent developmental stages (Piekielek and Fox, 1992; Blackmer et al., 1994; Schepers, 1994). Piekielek et al. (1995) showed that SPAD values expressed relative to SPAD values from a high-N strip (relative or normalized SPAD) could be compared over a wide range of sampling times when using a common critical value. Normalized SPAD values lessen the eff ect of diff erences in hybrid, soil type, growth stage, or environmental conditions (Piekielek et al., 1995). Scharf et al. (2006) found that the relationship between SPAD values and economically optimum N rate was much stronger when using normalized values as opposed to absolute values. Th e SPAD meter is a useful tool, but it has some potential limitations. Th e SPAD meter costs about $1500 USD, has a small sampling area (6 mm 2), is subject to operator bias, and Zhang et al. (2008) showed that SPAD meters have diffi culty in estimating chlorophyll levels when they are near or above optimum. Th eir observations indicate that increases in chlorophyll are not necessarily associated with increases in yield. Spectral refl ectance of crop leaves can be a valuable tool to estimate plant N status (Li et al., 2005). Spectral refl ectance is the refl ectance of certain plant components that are controlled by their visual properties and radiant energy exchange in a canopy (Huete, 1988). Th e refl ectance of certain wavelengths is related to diff erent amounts of chlorophyll a and b, which can be used to estimate the N status of certain crops (Huete, 1988). Th is method shows great potential because it off ers a method to deliver variable-rate N applications from a vehicle-mounted sensor (Kitchen et al., 2010). Tools for measuring refl ectance, howe...
The ability of soil tests to identify nutrient‐deficient soils and recommend fertilizer rates that optimize agronomic yield is essential for profitable soybean [Glycine max (L.) Merr.] production. Our objectives were to correlate relative soybean yield to Mehlich‐3 and 1 mol L−1 HNO3–extractable soil K and trifoliolate‐leaf K concentration at the R1 to R2 development stage and calibrate the K rates for Mehlich‐3‐extractable soil K. Experiments were established on silt loams at 34 site‐years planted with a Maturity Group IV or V cultivar and fertilized at five K rates (0–148 kg K ha−1). Mehlich‐3‐extractable soil K ranged from 46 to 167 mg K kg−1 and produced relative soybean yields of 59 to 100% when no K was applied. Eleven sites had Mehlich‐3‐extractable K < 91 mg K kg−1 and all responded positively to K fertilization. Soybean grown in soil having 91 to 130 mg K kg−1 responded positively at nine of 15 sites. Mehlich‐3 soil K explained 76 to 79% of the variability in relative yields and had critical concentrations of 108 to 114 mg K kg−1, depending on the model. The linear‐plateau model predicted the critical HNO3–extractable soil K to be 480 mg K kg−1 Trifoliolate‐leaf K concentration increased significantly, positively, and linearly as Mehlich‐3‐ and HNO3–extractable soil K increased, but Mehlich‐3 soil K explained only 49 to 53% of the variation in trifoliolate‐leaf K. Mehlich‐3‐extractable K is an excellent predictor of soil K availability for soybean grown on silt loams in eastern Arkansas.
Spatial variability and distributions of soil chemical properties and the relationships between soil chemical and biological properties are not well characterized in agroecosystems that have been land leveled to facilitate more uniform delivery of irrigation water. The objectives of this study were to characterize the short‐term impacts of land leveling on the magnitudes, spatial variability, and spatial distributions of soil chemical properties and to evaluate the impact of land leveling on the relationships between soil chemical properties and microbial biomass in a Stuttgart silt loam (fine, smectitic, thermic Albaqultic Hapludalf) used for irrigated soybean [Glycine max (L.) Merr.] and rice (Oryza sativa L.) production in the Mississippi Delta region of eastern Arkansas. A grid‐sampling approach was used to characterize pre‐ and postleveling soil chemical properties and microbial biomass. Results of this study demonstrate that land leveling, a severe form of anthropogenic soil disturbance, causes significant alteration of the magnitudes, spatial variability, and spatial distributions of many soil chemical properties. Soil electrical conductivity (EC) and the contents of P, K, Mg, Na, S, Mn, and Cu in the top 10 cm significantly increased, while soil pH and organic matter (OM) and Fe contents significantly decreased, as a result of land leveling. Land leveling also significantly altered many linear relationships among soil chemical properties and microbial biomass. The benefit of improved water distribution must be weighed against the relatively severe and immediate alteration of soil properties and natural processes brought on by land leveling. Further research is required to ascertain long‐term effects of altered soil biogeochemical properties on crop growth as a result of land leveling.
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