@article{sweeney_diaz_pedreira_havlin_2022, title={Long-term yield response of corn, wheat, and double-crop soybean to tillage and N placement}, ISSN={["1435-0645"]}, DOI={10.1002/agj2.20997}, abstractNote={AbstractOn claypan soils of the eastern Great Plains, yield of no‐till crops often lags yield of crops grown with tillage, especially cereals requiring sound N management. The objective of this 14‐yr study was to determine the yield response of corn (Zea mays L.), winter wheat (Triticum aestivum L.), and double‐crop soybean [Glycine max (L.) Merr.] in a 2‐yr rotation to tillage system and N fertilizer placement. Tillage, N placement, or their interaction affected corn yield in all seven‐corn crop‐years. Corn yields averaged about 25% lower with no‐till than with conventional or reduced tillage and were never greater with no‐till in any year. Nitrogen fertilization increased corn yield by increasing kernels per ear; however, corn yield increase to subsurface band (knife) N applications compared with surface (broadcast or dribble) applications was only about 10%. In the lower‐yielding no‐till system, corn yield increase to knifed N fertilization was greater than in tilled systems and may partially ameliorate the no‐till yield decline. Average wheat yield was unaffected by tillage. Adding N more than doubled wheat yield, but differences in wheat yield between preplant N placements methods were small. Average double‐crop soybean yield was unaffected by tillage with few, inconsistent differences by year. Compared with no‐till, tillage tended to increase soil P and K, but soil organic matter was just redistributed with no net increase with no‐till. Thus, tillage and N placement treatments may affect corn but have lesser effects on wheat and double‐crop soybean and soil properties in this rotation.}, journal={AGRONOMY JOURNAL}, author={Sweeney, Daniel W. and Diaz, Dorivar A. Ruiz and Pedreira, Bruno C. and Havlin, John L.}, year={2022}, month={Feb} }
 @article{havlin_austin_hardy_howard_heitman_2022, title={Nutrient Management Effects on Wine Grape Tissue Nutrient Content}, volume={11}, ISSN={["2223-7747"]}, url={https://doi.org/10.3390/plants11020158}, DOI={10.3390/plants11020158}, abstractNote={With limited research supporting local nutrient management decisions in North Carolina grape (Vitis vinifera) production, field studies (2015–17) were conducted to evaluate late season foliar nitrogen (N) application on leaf and petiole N concentration and yeast assimilable N (YAN) in the fruit. Foliar urea (1% v/v) was applied at different rates and application times beginning pre-and post-veraison. Compared to soil applied N, late season foliar N substantially enhanced petiole N and grape YAN. Smaller split N applications were generally more effective in increasing YAN than single larger N rates. These data demonstrate the value of assessing plant N content at full bloom with petiole N analysis or remote sensing to guide foliar N management decisions. Additional field studies (2008–11) were conducted to evaluate pre-bud soil applied phosphorus (P) and potassium (K) effects on petiole P and K nutrient status. Fertilizer P and K were initially broadcast applied (0–896 kg P2O5 ha−1; 0–672 kg K2O ha−1) prior to bud-break in 2008–09 and petiole P and K at full bloom soil test P and K were monitored for three to four years after application. Soil test and petiole P and K were significantly increased with increasing P and K rates, which subsequently declined to near unfertilized levels over the sampling time depending on site and P and K rate applied. These data demonstrate the value of annually monitoring petiole P and K levels to accurately assess plant P and K status to better inform nutrient management decisions.}, number={2}, journal={PLANTS-BASEL}, author={Havlin, John L. and Austin, Robert and Hardy, David and Howard, Adam and Heitman, Josh L.}, year={2022}, month={Jan} }
 @misc{havlin_schlegel_2021, title={Review of Phosphite as a Plant Nutrient and Fungicide}, volume={5}, ISSN={["2571-8789"]}, DOI={10.3390/soilsystems5030052}, abstractNote={Phosphite (Phi)-containing products are marketed for their antifungal and nutritional value. Substantial evidence of the anti-fungal properties of Phi on a wide variety of plants has been documented. Although Phi is readily absorbed by plant leaves and/or roots, the plant response to Phi used as a phosphorus (P) source is variable. Negative effects of Phi on plant growth are commonly observed under P deficiency compared to near adequate plant P levels. Positive responses to Phi may be attributed to some level of fungal disease control. While only a few studies have provided evidence of Phi oxidation through cellular enzymes genetically controlled in plant cells, increasing evidence exists for the potential to manipulate plant genes to enhance oxidation of Phi to phosphate (Pi) in plants. Advances in genetic engineering to sustain growth and yield with Phi + Pi potentially provides a dual fertilization and weed control system. Further advances in genetic manipulation of plants to utilize Phi are warranted. Since Phi oxidation occurs slowly in soils, additional information is needed to characterize Phi oxidation kinetics under variable soil and environmental conditions.}, number={3}, journal={SOIL SYSTEMS}, author={Havlin, John L. and Schlegel, Alan J.}, year={2021}, month={Sep} }
 @article{schlegel_havlin_2021, title={Irrigated grain sorghum response to 55 years of nitrogen, phosphorus, and potassium fertilization}, volume={113}, ISSN={["1435-0645"]}, DOI={10.1002/agj2.20453}, abstractNote={AbstractAlthough long‐term field experiments provide a valuable resource to assess crop yield response to climate and fertilizer, few studies have included grain sorghum (Sorghum bicolor L.). The objectives of this study were to quantify the effects of 55 yr of annual nitrogen (N), phosphorus (P), and potassium (K) application on irrigated continuous sorghum grain yield, grain nutrient uptake, and economic optimum N rates. Six N rates (0, 45, 90, 134, 179, and 224 kg N ha−1) and three combinations of P and K (0 P with 0 K, 20 kg P ha−1 with 0 K, and 20 kg P ha−1 with 37 kg K ha−1) were applied annually from 1961 to 2015 to a Ulysses silt loam near Tribune, KS. Average maximum grain yield with N was 53% greater than with no N applied; however, application of 20 kg P ha−1 with N resulted in a 70% increase in average maximum grain yield. Potassium fertilization had no effect on grain yield. The N rate required for maximum profit at 20 kg P ha−1 averaged 137 kg N ha−1. At the economic optimum N rate, apparent fertilizer N recovery in grain was 25% with no P and increased to 42% with P. Apparent fertilizer P recovery at the economic optimum N rate was 51% with 20 kg P ha−1. Fifty‐five yr of irrigated sorghum response to N and P fertilization demonstrated a strong positive interaction between N and P on grain yield, apparent N and P recovery, and profitability.}, number={1}, journal={AGRONOMY JOURNAL}, author={Schlegel, Alan J. and Havlin, John L.}, year={2021}, month={Jan}, pages={464–477} }
 @article{havlin_heiniger_2020, title={Soil Fertility Management for Better Crop Production}, volume={10}, ISSN={["2073-4395"]}, DOI={10.3390/agronomy10091349}, abstractNote={Increasing crop productivity per unit of land area to meet future food and fiber demand increases both soil nutrient removal and the importance of replenishing soil fertility through efficient nutrient management practices. Significant progress in enhancing nutrient-use efficiency in production agriculture requires improved estimates of plant-available nutrients in the root zone, enhanced crop response to applied nutrients, and reduced offsite nutrient transport. This special issue, Soil Fertility Management for Better Crop Production, presents 15 manuscripts that advance our knowledge of interrelated soil, plant, and management factors important to increasing the nutrient availability and crop recovery of applied nutrients.}, number={9}, journal={AGRONOMY-BASEL}, author={Havlin, John and Heiniger, Ron}, year={2020}, month={Sep} }
 @article{dold_heitman_giese_howard_havlin_sauer_2019, title={Upscaling Evapotranspiration with Parsimonious Models in a North Carolina Vineyard}, volume={9}, ISSN={2073-4395}, url={http://dx.doi.org/10.3390/agronomy9030152}, DOI={10.3390/agronomy9030152}, abstractNote={Water stress can positively or negatively impact grape yield and yield quality, and there is a need for wine growers to accurately regulate water use. In a four-year study (2010–2013), energy balance fluxes were measured with an eddy-covariance (EC) system in a North Carolina vineyard (Vitis vinifera cv. Chardonnay), and evapotranspiration (ET) and the Crop Water Stress Index (CWSI) calculated. A multiple linear regression model was developed to upscale ET using air temperature (Ta), vapor pressure deficit (VPD), and Landsat-derived Land Surface Temperature (LST) and Enhanced Vegetation Index (EVI). Daily ET reached values of up to 7.7 mm day−1, and the annual ET was 752 ± 59 mm, as measured with the EC system. The grapevine CWSI was between 0.53–0.85, which indicated moderate water stress levels. Median vineyard EVI was between 0.22 and 0.72, and the EVI range (max–min) within the vineyard was 0.18. The empirical models explained 75%–84% of the variation in ET, and all parameters had a positive linear relationship to ET. The Root Mean Square Error (RMSE) was 0.52–0.62 mm. This study presents easily applicable approaches to analyzing water dynamics and ET. This may help wine growers to cost-effectively quantify water use in vineyards.}, number={3}, journal={Agronomy}, publisher={MDPI AG}, author={Dold, Christian and Heitman, Joshua and Giese, Gill and Howard, Adam and Havlin, John and Sauer, Tom}, year={2019}, month={Mar}, pages={152} }
 @article{schlegel_havlin_2017, title={Corn Yield and Grain Nutrient Uptake from 50 Years of Nitrogen and Phosphorus Fertilization}, volume={109}, ISSN={["1435-0645"]}, DOI={10.2134/agronj2016.05.0294}, abstractNote={Core Ideas
Initiated in 1961, a 50‐yr field study quantified continuous irrigated corn response to annual N and P rates.
Positive N–P interactions on grain yield and grain N/P concentrations were documented.
Economic optimum N rate varied greatly over years, but averaged ∼175 kg N ha−1 (1992‐2010).
Significantly greater apparent fertilizer N recovery in grain (∼45%) occurred with P fertilization compared to no P (∼20%).
Long‐term field studies can be used to improve nutrient effects on productivity.
Long‐term agricultural field experiments provide valuable information regarding the effects of nutrient inputs on crop productivity. The objectives of this study were to quantify the effects of 50 yr of annual N and P application on irrigated continuous corn (Zea mays L.) grain yield, grain nutrient uptake, and economic optimum N rates. Six N (0, 45, 90, 134, 179, and 224 kg N ha−1) and three P rates (0, 20, and 40 kg P ha−1) in a factorial arrangement were applied annually from 1992 to 2010 to a Ulysses silt loam near Tribune, KS. From 1961 to 1991, only two P rates (0 and 20 kg P ha−1) were applied with the six N rates. During the last 19 yr, grain yield increased 20% with P alone and 103% with N alone; however, N and P applied together increased grain yields up to 225% compared to the unfertilized control. The N rate required for maximum profit at 20 and 40 kg P ha−1 averaged 172 and 180 kg N ha−1, respectively. At the economic optimum N rate of 172 kg N ha−1, apparent fertilizer nitrogen recovery in grain (AFNRg) was 44% and apparent fertilizer phosphorus recovery (AFPRg) was 63 and 44% with 20 and 40 kg P ha−1, respectively. Fifty years of irrigated corn response to N and P fertilization demonstrated a strong positive interaction between N and P on grain yield, apparent N and P recovery, and profitability.}, number={1}, journal={AGRONOMY JOURNAL}, author={Schlegel, Alan J. and Havlin, John L.}, year={2017}, pages={335–342} }
 @article{haines_gehl_havlin_ranney_2015, title={Nitrogen and Phosphorus Fertilizer Effects on Establishment of Giant Miscanthus}, volume={8}, ISSN={["1939-1242"]}, DOI={10.1007/s12155-014-9499-4}, number={1}, journal={BIOENERGY RESEARCH}, author={Haines, S. A. and Gehl, R. J. and Havlin, J. L. and Ranney, T. G.}, year={2015}, month={Mar}, pages={17–27} }
 @article{holland_heitman_howard_sauer_giese_ben-gal_agam_kool_havlin_2013, title={Micro-Bowen ratio system for measuring evapotranspiration in a vineyard interrow}, volume={177}, ISSN={["1873-2240"]}, DOI={10.1016/j.agrformet.2013.04.009}, abstractNote={Sparse canopy systems such as vineyards are comprised of multiple components (e.g., vines, interrow soil and/or groundcover) that each contribute to system water and energy balance. Understanding component water and energy fluxes is critical for informing management decisions aimed at improving productivity and water use efficiency. Few methods are available to accurately and continuously measure component fluxes. We tested a novel micro-Bowen ratio (MBR) energy balance system for determining interrow evapotranspiration (ET) flux within a vineyard. Our objectives were to develop MBR methodology to measure ET flux from the vineyard interrow and to compare MBR ET measurements for bare soil and fescue interrow conditions to independent ET estimates. MBR methodology utilized measurement of air temperature and water vapor concentration at 1 and 6 cm heights within 2.7 m wide interrows. Measured ET rates were well correlated between MBR systems and micro-lysimeters for both fescue (R2 = 0.99) and bare surface (R2 = 0.89) interrow conditions, though MBR ET rates were larger than those determined from micro-lysimeters in both cases (20 and 60%, respectively). MBR daily ET estimates, determined by compositing measurements from fescue interrows and bare soil under vines, were also well correlated to (R2 = 0.70) and of similar magnitude as vineyard eddy covariance ET measurements during periods when the vines were dormant. Overall, MBR systems appeared to provide a reasonable approach to determine ET for the interrow component within the vineyard. Similar methodology may be useful to better understand components' contributions to water and energy fluxes in other complex or sparse canopy systems.}, journal={AGRICULTURAL AND FOREST METEOROLOGY}, author={Holland, S. and Heitman, J. L. and Howard, A. and Sauer, T. J. and Giese, W. and Ben-Gal, A. and Agam, N. and Kool, D. and Havlin, J.}, year={2013}, month={Aug}, pages={93–100} }
 @book{havlin_2013, title={Soil fertility and fertilizers: An introduction to nutrient management}, publisher={Upper Saddle River, N.J.: Pearson}, author={Havlin, J.}, year={2013} }
 @article{havlin_hardy_gehl_spayd_2012, title={Survey of Nutrient Status in Vitis vinifera Grapes in North Carolina}, volume={43}, ISSN={["0010-3624"]}, DOI={10.1080/00103624.2011.638600}, abstractNote={Grape (Vitis vinifera L.) production is a rapidly growing industry in North Carolina; however, no local-research-based information is available to support nutrient-management decisions. Field studies were initiated to survey soil and plant nutrient status over a wide range in geography and management conditions in the dominant V. vinifera regions in North Carolina. While the survey data are still being analyzed, significant information was initially obtained to identify aluminum toxicity and phosphorus and potassium deficiency as potential limiting factors to vine health and productivity. Field studies were initiated to quantify soil test and plant nutrient responses to lime, phosphorus, and potassium rates. Responses to phosphorus and potassium application were observed at most sites predicted by soil tests. However, lime responses were not observed in either petiole or blade samples collected at full bloom or veraison. These studies will be continued until sufficient response data are collected to establish soil test and plant nutrient diagnostic criteria for efficient nutrient management of V. vinifera in North Carolina.}, number={1-2}, journal={COMMUNICATIONS IN SOIL SCIENCE AND PLANT ANALYSIS}, author={Havlin, J. L. and Hardy, D. H. and Gehl, R. J. and Spayd, S. E.}, year={2012}, pages={299–314} }
 @article{megonigal_stauffer_starrs_pekarik_drohan_havlin_2010, title={"Dig It!": How an Exhibit Breathed Life into Soils Education}, volume={74}, ISSN={["1435-0661"]}, DOI={10.2136/sssaj2009.0409}, abstractNote={We propose that the primary goal of public soils education should not be to teach, but to inspire The goal to inspire guided the design of “Dig It! The Secrets of Soil,” a large exhibit in the Smithsonian's National Museum of Natural History, Washington, DC, for 18 mo beginning in June 2008. The “Dig It!” exhibit was designed from an ecosystem perspective, in which agriculture—the traditional context for soils education—is considered to be just one of many ecosystem types. For visitors inspired primarily by art and culture, there were objects chosen to surprise and expand the imagination about soils. The exhibit was designed to communicate to wide range of ages, centered on 12–14 yr olds. As such, it was rich in audiovisual media that included a cartoon, a movie, two looping videos, a kiosk for exploring the state soils, a quiz game, and a role‐playing game. It also included scale models and actual soil monoliths. The exhibit addressed the full spectrum of issues that concern contemporary soil scientists—climate change, aquatic eutrophication, soil degradation, sustainable farming, and others—by organizing the content according to scale (global, regional. and local). Interviews with visitors indicated that the exhibit had the potential to effectively alter the preconceived notions of more than 2 million visitors about soils. We fully expect the exhibit to continue inspiring the public about soils through the richness of the exhibit website (www.forces.si.edu/soils; verified 6 Mar. 2010), which includes videos, games, and interactive content.}, number={3}, journal={SOIL SCIENCE SOCIETY OF AMERICA JOURNAL}, author={Megonigal, J. Patrick and Stauffer, Barbara and Starrs, Siobhan and Pekarik, Andrew and Drohan, Patrick and Havlin, John}, year={2010}, pages={706–716} }
 @article{drohan_havlin_megonigal_cheng_2010, title={The "Dig It!" Smithsonian Soils Exhibition: Lessons Learned and Goals for the Future}, volume={74}, ISSN={["1435-0661"]}, DOI={10.2136/sssaj2009.0017}, abstractNote={The opening of “Dig It!: The Secrets of Soil,” a 5000‐ft2 exhibit on soil at the Smithsonian National Museum of Natural History in Washington, DC, presented an opportunity to reflect on the development process. The project generated important and new ways of thinking about soils education, and taught the SSSA much about itself and how to manage a project of this size and scope. While early struggles in organization and financing of the exhibit presented challenges never before faced by SSSA at this scale, persistence, flexibility, and some unconventional thinking won out in the end, and SSSA achieved its most significant educational success to date. Most importantly, the process taught many soil scientists that achieving greater visibility for soil science meant partnering with professionals outside of soil science, and trusting that they could convey the message delivered by the exhibit with the same enthusiasm they might have in the classroom. The lessons learned from the project, in the scope of its history, can help others further SSSA, soils education, and soil science as a respected, scientific field. We suggest six goals for the future of soil science that, if embraced with whole‐heart dedication and support, can help to embed the importance of soil in world thinking and policy. Our suggested goals are unconventional, like the experiences we faced in creating the exhibit, and thus perhaps will seem beyond the capability of most soil scientists. Hence our reminder: unconventional and bold thinking are what helped create the exhibit and will be what leads SSSA forward as a society best serving the world.}, number={3}, journal={SOIL SCIENCE SOCIETY OF AMERICA JOURNAL}, author={Drohan, Patrick J. and Havlin, John L. and Megonigal, J. Patrick and Cheng, H. H.}, year={2010}, pages={697–705} }
 @article{havlin_balster_chapman_ferris_thompson_smith_2010, title={Trends in Soil Science Education and Employment}, volume={74}, ISSN={["0361-5995"]}, DOI={10.2136/sssaj2010.0143}, abstractNote={During the last several decades, members of the SSSA have discussed several trends related to soil science education, including: (i) declining academic programs and course offerings at land grant universities, (ii) decreased enrollments, and (iii) improved employment opportunities for soil science graduates (SSSA, 2006; Ferris et al., 2010). The SSSA Advocacy/Education Task Force met in 2007 and concluded that quantitative survey information was needed to document trends in soil science academic programs, student enrollment, faculty, and job opportunities for graduates. Suggested survey topics included:

· Has the recognition of soil science as a distinct discipline increased or decreased?
· How has the job market changed during the past decade, and how will job opportunities for soil scientists change in the future?
· How have undergraduate and graduate soils curricula changed during the last decade?
· Has enrollment in soil science degree programs and courses changed during the past decade?
· Has there been a change in the degree programs of students enrolling in soils courses in the past decade?
· Have soil science programs been combined with other programs?

Therefore, the objective of the survey was to quantify trends in student enrollment, faculty positions, pertinent educational issues in soil and related sciences, and career or job opportunities and trends. Expected outcomes included a better understanding of current educational practices and trends, and identification of specific opportunities for SSSA to enhance the practice and profession of soil science.}, number={5}, journal={SOIL SCIENCE SOCIETY OF AMERICA JOURNAL}, author={Havlin, J. and Balster, N. and Chapman, S. and Ferris, D. and Thompson, T. and Smith, T.}, year={2010}, pages={1429–1432} }
 @inproceedings{havlin_heiniger_2009, title={A variable-rate decision support tool}, volume={10}, DOI={10.1007/s11119-009-9121-5}, number={4}, booktitle={Precision Agriculture}, author={Havlin, J. L. and Heiniger, Ronnie}, year={2009}, pages={356–369} }
 @article{crozier_havlin_hoyt_rideout_mcdaniel_2009, title={Three Experimental Systems to Evaluate Phosphorus Supply from Enhanced Granulated Manure Ash}, volume={101}, ISSN={["0002-1962"]}, DOI={10.2134/agronj2008.0187x}, abstractNote={Three experimental systems were used to evaluate a new P fertilizer since residual P levels at typical farm sites may make response detection unlikely. The systems were (i) greenhouse with low P soil, (ii) long‐term research sites with preexisting soil P gradients, and (iii) agricultural fields with prior P fertilization based on agronomic recommendations. The new fertilizer (animal waste by‐product, AWP: 5% N, 28% P2O5, 4% K2O, and 1% S) is an enhanced granulated manure ash. Corn (Zea mays L.), wheat (Triticum aestivum L.), and soybean [Glycine max (L.) Merr.] growth, P uptake, and residual soil Mehlich‐3 P were measured with agronomic rates of AWP or triple superphosphate (TSP). Greenhouse corn and wheat P uptake, and soil Mehlich‐3 P increased similarly with either fertilizer at rates equivalent to 0, 10, 20, 40, and 80 kg P ha−1. In long‐term research sites, grain yield increased with P fertilization in 8 of 12 tests, and was greater with TSP than with AWP in 3 of 12 tests. Plant P uptake increased in all 12 tests, and was greater with TSP in 1 of 12 tests. In previously fertilized agricultural fields, soil Mehlich‐3 P, but not yield, increased due to P fertilization. Fertilizer source differences were infrequent and relatively minor, but possibly due to lower water soluble P content of the AWP (70% versus 78% for TSP). Evaluation of such products requires an appropriate experimental system with low P soils that may be difficult to find on typical North Carolina farms.}, number={4}, journal={AGRONOMY JOURNAL}, author={Crozier, C. R. and Havlin, J. L. and Hoyt, G. D. and Rideout, J. W. and McDaniel, R.}, year={2009}, pages={880–888} }
 @article{harrelson_hoyt_havlin_monks_2008, title={Effect of planting date and nitrogen fertilization rates on no-till pumpkins}, volume={43}, number={3}, journal={HortScience}, author={Harrelson, E. R. and Hoyt, G. D. and Havlin, J. L. and Monks, D. W.}, year={2008}, pages={857–861} }
 @article{robertson_allen_boody_boose_creamer_drinkwater_gosz_lynch_havlin_jackson_et al._2008, title={Long-term agricultural research: A research, education, and extension imperative}, volume={58}, ISSN={["0006-3568"]}, DOI={10.1641/B580711}, abstractNote={ABSTRACT For agriculture to meet goals that include profitability, environmental integrity, and the production of ecosystem services beyond food, fuel, and fiber requires a comprehensive, systems-level research approach that is long-term and geographically scalable. This approach is largely lacking from the US agricultural research portfolio. It is time to add it. A long-term agricultural research program would substantially improve the delivery of agricultural products and other ecosystem services to a society that calls for agriculture to be safe, environmentally sound, and socially responsible.}, number={7}, journal={BIOSCIENCE}, author={Robertson, G. Philip and Allen, Vivien G. and Boody, George and Boose, Emery R. and Creamer, Nancy G. and Drinkwater, Laurie E. and Gosz, James R. and Lynch, Lori and Havlin, John L. and Jackson, Louise E. and et al.}, year={2008}, pages={640–645} }
 @article{harrelson_hoyt_havlin_monks_2007, title={Effect of winter cover crop residue on no-till pumpkin yield}, volume={42}, number={7}, journal={HortScience}, author={Harrelson, E. R. and Hoyt, G. A. and Havlin, J. L. and Monks, D. W.}, year={2007}, pages={1568–1574} }
 @article{bond_maguire_havlin_2006, title={Change in soluble phosphorus in soils following fertilization is dependent on initial Mehlich-3 phosphorus}, volume={35}, ISSN={["1537-2537"]}, DOI={10.2134/jeq2005.0404}, abstractNote={ABSTRACTThere is a lack of information on how fertilization and initial Mehlich‐3 phosphorus (M3P) interact to affect water soluble P (WSP) in soils. Our objectives were to (i) quantify the relationship between WSP and M3P for four textural diverse benchmark soils of North Carolina (NC) and (ii) quantify the change in WSP concentrations following P additions to soils over a wide range of initial M3P. Soils known to represent a wide range in M3P were collected from an Autryville loamy sand (loamy, siliceous, subactive, thermic Arenic Paleudults), Wasda muck (fine‐loamy, mixed, semiactive, acid, thermic Histic Humaquepts), Georgeville silt loam (fine, kaolinitic, thermic Typic Kanhapludults), and Pacolet sandy clay loam (fine, kaolinitic, thermic Typic Kanhapludults) and analyzed for M3P, Fe, Al, and WSP. An incubation study was also conducted where four samples representing a range in M3P from each series were fertilized at rates of 150 and 300 kg P ha−1, and WSP was measured at 1, 7, and 21 d after fertilization. The Wasda muck exhibited a change point at 115 mg P kg−1 across a broad range of M3P concentrations (60–238 mg kg−1) while Autryville, Georgeville, and Pacolet series (with ranges in M3P of 32–328, 119–524, 0–1034 mg P kg−1, respectively) maintained linear relationships between WSP and M3P. For the fertilized soils, significant increases in WSP occurred regardless of P rate. Yet, WSP concentrations were greater in soils with greater initial M3P. Thus, these data suggest that shifting animal waste applications to fields of relatively lower M3P concentrations would have an immediate impact on reducing risk for P losses, if all other factors are equal.}, number={5}, journal={JOURNAL OF ENVIRONMENTAL QUALITY}, author={Bond, C. Ryan and Maguire, R. O. and Havlin, J. L.}, year={2006}, pages={1818–1824} }
 @article{schlegel_grant_havlin_2005, title={Challenging approaches to nitrogen fertilizer recommendations in continuous cropping systems in the Great Plains}, volume={97}, ISSN={["1435-0645"]}, DOI={10.2134/agronj2005.0391}, abstractNote={Cropping systems in the Great Plains have evolved over the past two decades from reliance on summer fallowing to continuous cropping under reduced or no‐tillage. Most N recommendation models were developed in fallow systems under conventional tillage and were based on average yield goal, with adjustments for soil profile N content. The objective of this review is to examine the impact of continuous cropping on N requirements. With high‐residue continuous cropping systems, N requirements may increase because of increased annualized production, reduced contribution of N mineralization, and increased immobilization and volatilization potential of surface‐applied fertilizer N. Mitigating these effects on N availability and supplemental N requirements are the reduction in yield per crop, reduced nitrate (NO3) leaching potential, increased N use efficiency (NUE), and increased rates of N mineralization due to higher soil organic matter (OM) content. Unfortunately, increased year‐to‐year yield variability with continuous cropping increases the difficulty in accurately estimating yield goals. Also, reducing the frequency and duration of fallow may reduce the usefulness of the preplant soil N tests in estimating N availability. Recent research has evaluated the use of optical sensors during the growing season to assess N stress and to estimate crop N requirements. If proved feasible for many crops, this would provide a drastic change for determining N recommendations. In the absence of a reasonable yield goal and known residual soil N content, a fertilizer N rate near 70 kg N ha−1 or less was generally sufficient to optimize small‐grain or oilseed yields in several continuous cropping studies.}, number={2}, journal={AGRONOMY JOURNAL}, author={Schlegel, AJ and Grant, CA and Havlin, JL}, year={2005}, pages={391–398} }
 @book{havlin_2005, title={Soil fertility and fertilizers: An introduction to nutrient management (7th ed.)}, ISBN={0130278246}, publisher={Upper Saddle River, NJ: Pearson/Prentice Hall}, author={Havlin, J. L.}, year={2005} }
 @article{schlegel_dhuyvetter_havlin_2003, title={Placement of UAN for dryland winter wheat in the central high plains}, volume={95}, ISSN={["0002-1962"]}, DOI={10.2134/agronj2003.1532}, abstractNote={Research was initiated in 1993 to determine the N fertilizer requirement for dryland winter wheat (Triticum aestivum L.) grown under reduced tillage systems in western Kansas. Six sites in west‐central Kansas were selected each year for 4 yr in cooperation with area farmers. The typical cropping system was wheat‐fallow with reduced tillage practices. All sites were on silt loam soil that ranged in residual soil nitrate‐N content from 2 to 9 mg kg−1 (0‐ to 60‐cm sample). Crop residue cover at wheat planting averaged 28%. Fluid N (28% N as urea‐ammonium nitrate solution, UAN) was injected in the fall and spring and surface broadcast during the winter and spring at five rates (22, 45, 67, 90, and 112 kg ha−1) along with a zero N control. Typical production practices consisted of planting winter wheat in mid‐September with a hoe‐type drill. Grain protein increased linearly with increased N rates with greater than 130 g kg−1 when 112 kg N ha−1 was injected. Apparent fertilizer N recovery decreased with increased N rates, but was consistently higher with fall or spring injected rather than winter or spring broadcast UAN. The straw/yield ratio was greater than 2 across all N rates, which is greater than the commonly used value of 1.7. The soil N test was an indicator of yield response to N fertilization. Grain yields increased in 10 of 13 site‐years with N fertilizer. Average grain yields were 8% greater from spring injected than broadcast UAN. The time of N application had little effect on grain yield. Economic analysis indicated that injecting UAN in the fall or spring was more profitable than topdressing UAN in the winter or spring because of improved yields and/or lower total N costs. These data suggest that N rate recommendations should vary with application method.}, number={6}, journal={AGRONOMY JOURNAL}, author={Schlegel, AJ and Dhuyvetter, KC and Havlin, JL}, year={2003}, pages={1532–1541} }
 @article{schmidt_dejoia_ferguson_taylor_young_havlin_2002, title={Corn yield response to nitrogen at multiple in-field locations}, volume={94}, DOI={10.2134/agronj2002.0798}, abstractNote={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.}, number={4}, journal={Agronomy Journal}, author={Schmidt, J. P. and Dejoia, A. J. and Ferguson, R. B. and Taylor, R. K. and Young, R. K. and Havlin, J. L.}, year={2002}, pages={798–806} }
 @article{heiniger_havlin_crouse_kvien_t._2002, title={Seeing is believing: The role of field days and tours in precision agriculture education}, volume={3}, ISBN={1385-2256}, number={4}, journal={Precision Agriculture}, author={Heiniger, R. W. and Havlin, J. L. and Crouse, D. A. and Kvien, C. and T., Knowles.}, year={2002}, pages={309} }
 @article{taylor_kluitenberg_schrock_zhang_schmidt_havlin_2001, title={Using yield monitor data to determine spatial crop production potential}, volume={44}, DOI={10.13031/2013.7007}, abstractNote={Consistent spatial–temporal yield patterns should help determine spatial production potential. Our objective was 
to evaluate methods for using yield monitor data to develop spatial yield goal maps. Three to seven years of yield monitor 
data were analyzed for five sprinkler–irrigated cornfields in central and western Kansas. Yield data were block–averaged 
to 55 m square cells, normalized based on the mean yield, and then used to develop spatial yield goals for subsequent years 
using six different methods. One method used a uniform yield goal, two methods combined normalized yield monitor data with 
a uniform yield goal (transitional), and three methods used only normalized yield monitor data from previous years. Methods 
were evaluated based on their ability to predict the spatial yield pattern of the subsequent year better than the uniform method. 
Yield monitor data were also segregated based on the temporal CV of each field during the time of the study, and the six 
methods were evaluated only on the data that were deemed temporally stable. 
The result of incorporating yield monitor data into yield goals was inconsistent across sites and years. For one site, the 
two transitional and three yield monitor methods were significantly better predictors of normalized yield. On another field, 
the uniform method was a better predictor of normalized yield than the yield monitor methods in three of six years, while the 
yield monitor methods were better than the uniform method in another year. On a third field, the yield monitor method 
predicted normalized yield better than the uniform method in one of four years with no difference in the other three years. 
In general, when the correlation coefficient between two years of yield monitor data exceeded 0.70, the methods that 
incorporated yield monitor data into the yield goal were better predictors of normalized yield than the uniform method. 
Evaluating these methods using only data from cells where the temporal CV was less than the average temporal CV for the 
field did not improve the results sufficiently to warrant widespread use of this practice.}, number={6}, journal={Transactions of the ASAE}, author={Taylor, R. K. and Kluitenberg, G. J. and Schrock, A. D. and Zhang, N. and Schmidt, J. P. and Havlin, J. L.}, year={2001}, pages={1409–1414} }
 @article{crouse_havlin_mcbride_white_heiniger_weisz_roberson_2000, title={Precision farming education at NC State University}, journal={Proceedings of the 5th International conference on precision agriculture, Bloomington, Minnesota, USA, 16-19 July, 2000}, publisher={Madison, WI : Precision Agriculture Center, University of Minnesota, ASA-CSSA-SSSA}, author={Crouse, D. A. and Havlin, J. L. and McBride, R. G. and White, J. G. and Heiniger, R. and Weisz, R. and Roberson, G.}, year={2000}, pages={1} }
 @article{austin_crouse_havlin_hodges_2000, title={The Spatial Information Research Laboratory at North Carolina State University}, journal={Proceedings of the 5th International conference on precision agriculture, Bloomington, Minnesota, USA, 16-19 July, 2000}, publisher={Madison, WI : Precision Agriculture Center, University of Minnesota, ASA-CSSA-SSSA}, author={Austin, R. E. and Crouse, D. A. and Havlin, J. L. and Hodges, S. C.}, year={2000}, pages={1} }
 @book{havlin_beaton_tisdale_nelson_1999, title={Soil fertility and fertilizers: An introduction to nutrient management}, ISBN={0136268064}, publisher={Medford, NJ: Prentice Hall}, author={Havlin, J. L. and Beaton, J. D. and Tisdale, S.L. and Nelson, W. L.}, year={1999} }
 @article{havlin_schlegel_1997, title={Dryland conservation technologies: Enhancing agricultural profitability and sustainability}, volume={36}, number={3}, journal={Annals of Arid Zone}, author={Havlin, J. L. and Schlegel, A. J.}, year={1997}, pages={291–303} }
 @article{schlegel_havlin_1997, title={Green fallow for the central Great Plains}, volume={89}, ISSN={["0002-1962"]}, DOI={10.2134/agronj1997.00021962008900050009x}, abstractNote={AbstractUse 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.}, number={5}, journal={AGRONOMY JOURNAL}, author={Schlegel, AJ and Havlin, JL}, year={1997}, pages={762–767} }
 @article{halvorson_havlin_schlegel_1997, title={Nutrient management for sustainable dryland farming systems}, volume={36}, number={3}, journal={Annals of Arid Zone}, author={Halvorson, A. D. and Havlin, J. L. and Schlegel, A. J.}, year={1997}, pages={233–254} }