NUTRNT CYCL ENVIR QUAL
NUTRNT CYCL ENVIR QUAL SOIL 5813
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Date Created: 11/01/15
The Notill path during the last 20 years in Argentina Agustin Bianchini Aapresid bianchiniaapresidorgar 1 We started with notill to reduce soil erosion because this was a serious problem for our farmers Physical Degradation Consequences Intensive ecological Inte 2004 2Thenquot we realized that more water was available and that with the adoption of notill the water economy was changing That water had to be used by the farmer Mwm1p www 39 an I 39b 12 11 WATER LOSS DUE TO TRANSPIRATION AND EVAPOTRANSPIRATION NOTILL AND CONVENTIONAL TILLAGE Conventional Tillage Month 397 G 39 m transpiration evaporation May 3 25 June 2 July H 08 August 39 oe September gt 1o AAPRESID Sou rce Aa presid Crop Rotation Intensity D Beck 1996 Put the stored water in NT to work Less fallow and more water used by crops Climate soil latitude Appropriate intensity reduces risks Native vegetation is the best indicator of the appropriate intensity What to do to improve the rainfall water use efficiency Cover the soil with crop residues in an homogeneous and durable way Maintain a stable structure mainly in the first inches on the soil profile 3 New regions could be brought into production with NT so this allowed an expansion of our agricultural area Increased cropped area bean Planted Area 4 Legend Planted Area for Soy as Percent aiTotal mm 25Drsn nmmmHvLI m Bum Ira um Swim Waning mummy smmummr a say muonwmm g siumulmyanravnme r um um 4 Carbon dynamics were modified NT alone was not enough for increasing the C levels we needed to think on crop rotation intensification balanced fertilization etc Depth in Organic N in a noTill field and convenTional Tillage afTer 10 years mg N 1009391 dry soil 0 50 100 150 200 250 300 0 4 NT 8 12 16 The highesT proporTion of The OM 20 increase comes from The labile fracTions 24 28 Source Moraes S Crop Rotation Planned and ordered crop sequence with the objective 0 Maximize productivity 0 minimize risks 0 and preserve the involved resources Fertilization of the crop rotation o Balanced fertilization 0 Higher yield response in the rotation 0 Nutrient residual effects 0 Balance inmovilizationrelease 0 Soil biological activity Management to increase Soil Organic Carbon Paustian 1997 0 Reduce or eliminate tillage o Rotations with corn grain sorghum pastures 0 Include permanent gramineae and legumes 0 Increase time of soil covered with vegetation 0 Increase production and return residue to the soil No Till Crop Rotation Diversity Intensity Fertilization 5 A new paradigm started with Nitrogen because in NT N dynamics are modified and more quotbiologicalquot N is available for crops but difficult to quantify iological Nitrogen with legumes Inoculation and PS fertilization in soybean AAPRESIDNitraginRizobacterASP 200405 4 sites Santa Fe and Buenos Aires Provinces so 54 57 6 50 47 50 m 3 4o 5 3 3o 2 gt 20 E E 10 o 0 Control Inoculated PS Inoc PS Why hairy vetch Because notill conceptually evolves Crop rotation intensification and diversification Transform water in d matter zero fallow increase the size of the water storage tank Soil covered with residues and presence of live roots Nutrient cycling and deep water utilization Improve Carbon Nitrogen and Organic Matter balances Lorenzatti 2008 How much N can hairy vetch add to the system 80 to 90 lb Nac to the following corn crop Ebelhar et al 1984 Agron J 765155 67 to 112 lb Nac to the following corn or grain sorghum Blevins et al1990 Agron J 82769772 The accumulation and N contribution via hairy vetch as a cover crop was higher with the late burning 2 weeks Same trend in corn grain yield planted after the cover crop Sainju and Singh2001 Agron J 93878886 6 We need to think that we are farmers that are managing an offer of environmental resources nutrients water light C02 etc TRADITIONAL AGRICULTURE 41 THE TKLWL 4 ORGANIC MATTER dynamics and ll774 distribution 39 geometryind stabiliEY lt11 1 39 v I I39m1 I 39 co 39 39 v 2 gt 4 2 L g v 4 v quotquotquotquotquot7 39 39f j 39 7 co2 1 cozquot g 39 co2 g 1 th Wm 3 w Tb mbr eg neyr g V e39Ve y y r if el39 hdif my quot39g quotril 5 quot 39 Abiom g39sm v 39 y To u 2 End ange efi i y ti quot for vj39ha 602 of the sfoma l39allev f Io bvoduc I phptl39dsqutes39 quot 6 wj Tra ir onn c To convert efjcign y The psir ila res in a spur LR Gi l 20061 39 x hasve39l39able form forage or gram 1 u 5 quotThe humanity faces today a dilemma with no apparent solution between the ghost of the lack of food for an increasing demand in quantity and quality or a destruction of the natural resources needed to produce themquot Certi ed Agriculture 39 Aapresid Theevnlutinn of NT The notill system Consequences 96 less soil erosion 66 less fuel use Maintenance or improvement of the organic matter Higher water use efficiency Increase in soil fertility Lower production costs Higher production stability and higher yield potential TANGIBLE BENEFITS FOR THE FARMER 7 Certified AapreSId mm Th evolution of m The notill system Benefits beyond the farmer Better soils Less competition for drinkable water strategic resource Higher water quality lower erosion and contamination risk Better atmosphere positive impact in the climate change Lower pressure on more fragile areas by an increase in yields Possibility of producing in more fragile areas without the known risks of Conventional Tillage CT BENEFITS TANGIBLE FOR THE SOCIETY EXTERNALITY 7 Certified AapreSId mm Th evolution of m Productive and environmental quality management system in CA OMSCA Objectives To provide tools for a professional agronomical management by the ordered registry of information and the analysis of the soil quality and efficiency indicators To show to the rest of the society how are the production processes and its impact on the environment allowingto capture the value of the positive externaliy that the CA makes in it 7 certified AapreSId mm Th evolution of m Productive and environmental quality management system in CA OMSCA Components Principles amp Criteria RTRS RSB ISGA RTSPO FSC FAO Management indicators in the soil resource use efficiency Good Agricultural Practices Protocol GAP s Certified Aapresid Agriculture 1h vulutiun of m lll Productive and environmental quality management system in CA QMSCA Potential uses 1 Associated to the agronomical management Decision makin in an 39 quot quot39 39 39 e Analysis of the evolution of the impact management in the wstem time tc 2 Associated to existing business or easily accessible Land rental as a requirement of the owner or as a differentiation tool Real estate Histon agronomically certi ed Cr and 4 Tax reductions 3 Associated to new businesses Business by 39 39 biofuels seeds Countn brand or provinces Better price access to preferential markets J 5quotquot foods 39 Certified AapreSId A Th evolution of m Certified Agriculture It is the production alternative that better combines the interests many times confronted of reaching a production Economically viable for farmers Environmentallysustainable Socially accepted Energetically efficient Certi ed Agriculture 39 Aapresid Theevnlutinn of NT Certified Agriculture A commitment that Aapresid as organization assumes to contribute to the increase of the wellbeing of the local and global society in the conflict solution Productivity vs Environment Certi ed Agriculture Aapresid Thewnlutinn of NT Thank you bianchiniaapresidorgar wwwaapresidorgarengish Certi ed AapreSId Agriwlture T waluuan nf NT NoTill Adoption Benefits all greater stabiliw and yield increase alt increase in cropped area a lower production costs Vatiety RR Soybean expansion of the agricultural boundaries WATER LOSS DUE TO TRANSPIRATION AND EVAPOTRANSPIRATION NOTILL AND CONVENTIONAL TILLAGE Conventional Tlllage Month 7 r i v I transpiration evaporation May 2 7 0 63 June 64 68 July August September AAP RE SID Depth cm Organic N in a noTill field and convenTional Tillage afTer 10 years mg N 1009391 dry soil 0 50 100 150 200 250 300 0 10 20 30 40 The highesT proporTion of The OM 50 increase comes from The labile fracTions 60 70 Source Moraes S Inoculation and PS fertilization in soybean AAPRESIDNitraginRizobacterASP 200405 4 sites Santa Fe and Buenos Aires Provinces 4500 4000 3583 3833 E 3500 3205 3373 E 3000 v 2500 E 2000 gt 1500 1000 5 500 0 l l Control Inoculated S Inoc PS P 0 39 quotA rigqltura edAapreSId csmm La evolumc m de la SD How much N can hairy vetch add to the system 90 to 100 kg Nha to the following corn crop Ebelhar et al 1984 Agron J 765155 75 to 125 kg Nha to the following corn or grain sorghum Blevins et al1990 Agron J 82769772 The accumulation and N contribution via hairy vetch as a cover crop was higher with the late burning 2 weeks Same trend in corn grain yield planted after the cover crop Sainju and Singh2001 Agron J 93878886 SOIL 5813 Questions For each of the processes in the NCyclequot be able to answer the following questions 1 Why does the process Ntransformation exist a What would happen without the process 2 How significant is the process to plant growth To crop production a Shortterm within the growing season effect b Longterm equilibrium condition for the system effect 3 How significant is the process for conserving N in the soilplant system 4 How is the importance of each process altered by climate especially moisture how would your ranking of importance be different for an arid versus a humid climate General One might assume that the biological processes are favored by heat and require moisture Most of the chemical reactions will increase with increased heat and while most will require moisture water some NH3 volatilization will be favored by loss of water For all processes reactions the rate generally increases in direct proportion to the concentration of reactants and inversely to the concentration of the products Mineralization a Requirements for process to proceed i OM that is easy to decay i Aerobic andor anaerobic environment iii Removal of endproducts b Most abundant group of microorganisms in soil ll Immobilization a Requirements for process to proceed i Supply of mineral N ii Organism requiring deficient in N lll Ammonia volatilization a Excess NH4 present accumulated i Basic pH environment ii IV Ammonium adsorption CEC V Ammonium entrapment fixation in clays Vl Nitrification Vll Denitrification Vlll Leaching lX Plant crop uptake IMPROVING THE EFFICIENCY OF CORN PRODUCTION BY FERTILIZER N MANAGEMENT WITH THE ILLINOIS SOIL N TEST S A Khan T R Ellsworth R L Mulvaney and T J Smith S A Khan is a Research Specialist T R Ellsworth is an Associate Professor and R L Mulvaney is a Professor Department of Natural Resources and Environmental Sciences University of Illinois Urbana IL T J Smith is an Agronomist Cropsmith Monticello 1L INTRODUCTION Traditional yieldbased recommendations for N fertilization of corn are based on the assumption that fertilizers contribute twice as much plantavailable N as the soil regardless of soil type or fertilizer practices Yet numerous 15Ntracer investigations have revealed that uptake is actually much more extensive for soil than fertilizer N when corn is grown in the Midwest with typical or even excessive fertilization eg Bigeriego et a1 1979 Olson 1980 Kitur et a1 1984 Blackmer and Sanchez 1988 Torbert et a1 1993 Jokela and Randall 1997 Omay et a1 1998 Stevens et a1 2005 These findings are veri ed by the fact that unfertilized check plot yields in Nresponse studies often exceed the yield increase obtained with fertilization Lory and Scharf 2003 and in many such studies sites have been detected where corn is completely nonresponsive to fertilizer N eg Bundy and Malone 1988 Blackmer et a1 1989 Fox et a1 1989 Schmitt and Randall 1994 There are inherent differences among soils in their capacity to supply plantavailable N which can also be markedly affected by management and cropping practices The implication is that fertilizer practices should account for differences in soil N availability and ideally should be implemented on a sitespeci c basis This approach has only become feasible with the development of the Illinois soil N test ISNT which was designed to estimate potentially mineralizable N as a means of detecting sites where corn is nonresponsive to N fertilization Khan et a1 2001 There is an obvious potential for sitespeci c N management because recent work by Ruffo 2004 has demonstrated that the ISNT is highly predictive of spatial variability in yield and can be adequately mapped with a relatively sparse sampling grid Ruffo et a1 2005 Recent work by Mulvaney et a1 2006 has demonstrated not only that nonresponsive sites can be reliably detected by the ISNT but has provided ample evidence of a relationship between test values and crop N requirement although consideration must be given to other factors that can have a marked effect on mineralization of soil N or the efficiency of crop N utilization Plant population was identified as being especially important because crop N demand is increased along with the input of C from crop residues which promotes the tieup of plantavailable N through immobilization during microbial decomposition Higher plant populations necessarily affect soil test calibrations Bray 1948 Melsted and Peck 1973 and have long been recognized as an important means of increasing yield on highly productive soils Dungan et al 1958 OBJECTIVES The work reported was designed to explore the possibility that the ISNT can be utilized to increase the efficiency of corn production by exploiting the interaction of plant population and crop N demand The speci c objectives were 1 To evaluate the effect of plant population on fertilizer N requirement when corn is grown on soils that test high by the ISNT 2 To compare the impact on yield and profitability of N fertilizer management with or without plant population adjustment MATERIALS AND METHODS Nitrogenresponse experiments were conducted during 2005 on University of Illinois cropland managed by the Department of Agricultural Engineering site 1 or the Department of Animal Sciences site 2 The soil series in both cases was a Drummer silty clay loam but there was a difference in management with site 1 under a com soybean rotation with no manure history while site 2 was cropped to continuous corn with annual manuring A compact randomized triangular plot design Fig 1 was adopted so as to allow equidistant plant spacing for optimal light interception and root growth while minimizing spatial variability in soil N supply With this design 96 plots were established within approximately 07 A accommodating three populations 20000 24000 or 40000 plantsA and eight N rates 0 30 60 90 120 150 180 or 210 lbA with four replications Soil sampling for the ISNT was done in midApril by collecting a 4core composite 0 12 and 1224 inches from the central part of each plot where yield data would be collected The samples were immediately transported to the laboratory and were thoroughly dried in a forcedair oven maintained at 40 C The dried samples were crushed with a mechanical grinder to pass a 2mm screen and were then mixed thoroughly before duplicate analysis by the ISNT as described in a technical note 15N Analysis Service 2004 Prior to planting in early May site 1 was ripped to a depth of approximately 18 inches so as to alleviate subsoil compaction Within a week fertilizer N was applied as urea NH4N03 solution 2800 using a pressurized sprayer system mounted on a manually propelled tricycle chassis equipped with a pressure regulator and speedometer Immediately following application the fertilizer was incorporated by rototilling It 1quot l All iii in lllllllllllllllllllllllllllllllllllllllllllllll a Filmle i ii llllllllllm mill A Research Area X BorderArea Fig 1 Plot layout and illustration of equidistant planting scheme for the low planting rate 20000 plants Al Plots were planted Pioneer 33124 with jab planters after being covered by atarpaulin that had been perforated with holes pierced at an equidistant spacing of 17 ft 20000 plantsA 15 ft 24000 plantsA or 12 ft 40000 plantsA One replicate was subsequently sacrificed at site 1 due to reduced plant stands caused by pheasant feeding Weed control was accomplished with a postemergent herbicide treatment with hand hoeing as necessary Insecticide applications were made to control root pruning and silk clipping Owing to dry weather during the growing season water was applied as needed by sprinkler irrigation beginning on 7 June for site 1 and on 1 July for site 2 At physiological maturity grain yield was determined by handharvesting the central 61 ft2 20000 plantsA 65 ft2 24000 plants A or 60 ft2 40000 plantsA and was adjusted to a constant moisture content 155 Corn response to N fertilization was based on regression analyses performed by tting a linear linear plateau quadratic or quadratic plateau model to N rate and yield data Among these response models the best fit was obtained by quadratic regression which was subsequently employed for quantifying economically optimum N rate EONR and economically optimum yield EOY at Ncom price ratios of0 1 015 02 and 025 RESULTS AND DISCUSSION Objective 1 Modern corn production relies on highyielding varieties high populations and soils managed for high productivity The benefit is evident from yield trends in longterm trials such as the Morrow Plots Aref and Wander 1998 and has also been documented more generally in the scientific literature eg Cardwell 1982 Walters et al 2004 High plant populations create more pressure on soil nutrient supply and thereby increase fertilizer requirements This has often been found to hold in Nresponse studies with corn whether conducted in Illinois or elsewhere eg Lang et al 1956 Duncan 1958 Pesek et al 1959 Colyer and Kroth 1968 Mulvaney 1971 Rhoads et al 1988 Thomison et al 1992 Alam et al 2003 Mulvaney et al 2006 The present study provides further evidence that even with highly productive soils higher plant populations increase the yield response by corn to N fertilization This is clearly apparent from Fig 2 and 3 both of which show a marked increase with population in EONR estimated assuming current prices for N 035lb and corn 175bu Table 1 shows that the same relationship was observed across a range of Ncom price ratios Further examination of Fig 2 and 3 demonstrates that the economic risk from underfertilization with N is far greater when plant populations are high as re ected in a much steeper response curve In contrast response was quite limited with low populations as would be expected for soils that test high by the ISNT The importance of soil N mineralization is evident from a comparison of Fig 2 and 3 which reveals that fertilizer N response was greater under continuous corn site 2 than in a comsoybean rotation site 1 This difference is consistent with the higher surface and subsoil ISNT values obtained for site 1 Table 2 and can also be attributed to earlier irrigation These same factors are implicated in lower checkplot yields for site 2 which were reduced substantially by increasing plant population from 24000 to 40000 plantsA Fig 3 With greater soil N availability at site 1 N fertilization was not economically profitable for the lowest plant population with the highest Ncorn price ratio N N O 1 13 A 250 40000 plA 230 210 24000 pIA Yield buA O O x N in 0 El l a E a N 0 O O O 395 gt o 50 100 150 200 250 N Applied lbA Fig 2 Interaction of plant population and N rate for site 1 with mean ISNT values of 292299 ppm for 012 inches Optimum N rates EONR indicated above arrows were calculated assuming a Ncom price ratio of 02 225 m A 147 40000 pIA O 2 175 24000 plA 3 150 a a 20000 plA E 125 2 gt 100 1 75 50 I 0 50 100 150 200 250 N Applied lbA Fig 3 Interaction ofplant population and N rate for Site 2 with mean ISNT values of 249261 ppm for 012 inches Optimum N rates EONR indicated above arrows were calculated assuming a Ncorn price ratio of02 Table 1 Economically optimum N rates EONR estimated for different plant populations Ncorn price ratioT Site Population 010 015 020 025 plantsA 777 EONR lbA 777 1 20000 100 61 22 0 24000 114 102 80 78 40000 128 121 113 106 2 20000 123 110 97 84 24000 162 146 130 114 40000 160 153 147 141 TValues reported as a mean of three site 1 or four site 2 replicates estimated by quadratic regression Table 2 Summary of ISNT data obtained for sites 1 and 2 Sampling depth inchesT 012 1224 Site Population Range Mean Range Mean plantsA iiii ISNT value ppm iiii 1 20000 236360 299 86239 190 24000 229339 293 138240 197 40000 229342 292 148258 198 2 20000 220284 249 99177 149 24000 235295 261 99245 157 40000 240281 260 118240 162 TValues summarized from duplicate analyses of soil samples collected from 24 site 1 or 32 site 2 plots Objective 2 On the highly productive soils studied higher EOY values were always obtained by increasing plant population as is documented by Table 3 across all Ncorn price ratios Of particular interest is the greater uniformity among these values at the highest plant population regardless of fertilizer cost which is attributable to the shape of the response Table 3 Economically optimum yields EOY estimated for different plant populations Ncorn price ratioT Site Population 010 015 020 025 plantsA i777 EOY buA iiii 1 20000 189 185 179 169 24000 216 214 212 210 40000 261 260 258 257 2 20000 160 159 156 153 24000 183 181 178 175 40000 206 205 204 203 TValues reported as a mean of three site 1 or four site 2 replicates estimated by quadratic regression curves Fig 2 and 3 The highest plant populations were also the most pro table increasing the average return by 56A as compared to the lowest planting rate assuming 175bu and adjusting for seed cost The latter finding has important implications for maximizing the pro tability of crop production since plant uptake of fertilizer N will be promoted when population pressure exceeds soil Nsupplying capacity CONCLUSONS Nitrogen fertilizers cannot be utilized efficiently in corn production without accounting for the soil s capacity to supply plantavailable N which has become feasible with the development of the ISNT With hightesting soils fertilizer N response yield and profitability can be increased by boosting plant populations This strategy was highly effective in the present study for irrigated plots with equidistant plant spacing and will be evaluated through further field trials in 2006 REFERENCES Alam M M M M Basher A Karim M A Rahman and M R Islam 2003 Effect of rate of nitrogen fertilizer and population density on the yield and yield attributes of maize Zea mays Pakistan Journal of Biological Sciences 6 17701773 Aref S and M M Wander 1998 Longterm trends of corn yield and soil organic matter in different crop sequences and soil fertility treatments on the Morrow Plots Advances in Agronomy 62 153 197 Bigereigo M R D Hauck and R A Olson 1979 Uptake translocation and utilization of 15N depleted fertilizer in irrigated corn Soil Science Society ofAmerica Journal 43528533 Blackmer A M D Pottker M E Cerrato and J Webb 1989 Correlations between soil nitrate concentrations in late spring and corn elds in Iowa Journal of Production Agriculture 2 103 109 Blackmer A M and C A Sanchez 1988 Response of corn to nitrogen15labeled anhydrous ammonia with and without nitrapyrin in Iowa Agronomy Journal 8095102 Bray R H 1948 Requirements for successful soil tests Soil Science 668389 Bundy L G and E S Malone 1988 Effect of residual profile nitrate on corn response to applied nitrogen Soil Science Society ofAmerica Journal 52 13771383 Cardwell V B 1982 Fifty years of Minnesota corn production Sources ofyield increase Agronomy Journal 74984990 Colyer D and E M Kroth 1968 Corn yield response and economic optima for nitrogen treatments and plant population over a sevenyear period Agronomy Journal 60524529 Duncan E R 1958 The relationship between corn population and yield Agronomy Journal 508284 Dungan G H A L Lang and J W Pendleton 1958 Corn plant population in relation to soil productivity Advances in Agronomy 10435473 Fox R H G W Roth K V Iversen and W P Piekielek 1989 Soil and tissue nitrate tests compared for predicting soil nitrogen availability to corn Agronomy Journal 81971974 Jokela W E and G W Randall 1997 Fate of fertilizer nitrogen as affected by time and rate of application to corn Soil Science Society ofAmerica Journal 61 1695 1703 Khan S A R L Mulvaney and R G Hoeft 2001 A simple soil test for detecting sites that are nonresponsive to nitrogen fertilization Soil Science Society ofAmerica Journal 65 175 1 1760 Kitur B K M S Smith R L Blevins and W W Frye 1984 Fate oflsNdepleted ammonium nitrate applied to notillage and conventional tillage corn Agronomy Journal 76240242 Lang A L J W Pendleton and G H Dungan 1956 In uence of population and nitrogen levels on yield and protein and oil contents of nine corn hybrids Agronomy Journal 48284289 Lory J A and P C Scharf 2003 Yield goal versus delta yield for predicting fertilizer nitrogen need in corn Agronomy Journal 95994999 Melsted S W and T R Peck 1973 The principles of soil testing In Soil Testing and PlantAnalysis L M Walsh and J D Beaton ed pp 1321 Mulvaney D L 1971 Corn plant population and yield response to nitrogen In Illinois Fertilizer Clinics 1971 pp 12 Mulvaney R L S A Khan and T R Ellsworth 2006 Need for a soilbased approach in managing nitrogen fertilizers for pro table corn production Soil Science Society ofAmerica Journal 70 172182 15N Analysis Service 2004 The Illinois soil nitrogen test for amino sugarN Estimation of potentially mineralizable soil N and 15N Technical Note 02 01 Revision f Available online at httpillinoissoilntestnresuiucedupapersTN0201fpdf Olson R V 1980 Fate oftagged nitrogen fertilizer applied to irrigated corn Soil Science Society ofAmerica Journal 44514517 Omay A B C W Rice L D MadduX and W B Gordon 1998 Corn yield and nitrogen uptake in monoculture and in rotation with soybean Soil Science Society ofAmerica Journal 62 15961603 Pesek J T E O Heady J P Doll and R P Nicholson 1959 Production surfaces and economic optima for corn yields with respect to stand and nitrogen levels Iowa Agricultural and Home Economics Experiment Station Research Bulletin 4 72 Rhoads F M F G Martin and R L Stanley Jr 1988 Plant population as a guide to N fertilization of irrigated corn Journal of Fertilizer Issues 56771 Schmitt M A and G W Randall 1994 Developing a soil nitrogen test for improved recommendations for corn Journal of Production Agriculture 7328334 Stevens W B R G Hoeft and R L Mulvaney 2005 Fate ofnitrogen15 in a long term nitrogen rate study II Nitrogen uptake efficiency Agronomy Journal 97 10461053 Ruffo M L 2004 Spatial variability of corn response to nitrogen fertilizer Implications for variable rate fertilization PhD thesis University of Illinois Ruffo M L G A Bollero R G Hoeft and D G Bullock 2005 Spatial variability of the Illinois soil nitrogen test Implications for soil sampling Agronomy Journal 97 14851492 Thomison P R J W Johnson and D J Eckert 1992 Nitrogen fertility interactions with plant population and hybrid plant type in corn In Fluid Fertilizer Foundation 1992 Fluid Forum Proceedings pp 226231 Torbert H A R G Hoeft R M Vanden Heuvel R L Mulvaney and S E Hollinger 1993 Shortterm excess water impact on corn yield and nitrogen recovery Journal ofProduction Agriculture 6 337344 Walters D T A Dobermann K G Cassman R Drijber J Lindquist J Specht and H Yang 2004 Changes in nitrogen use efficiency and soil quality after five years of managing for high yield corn and soybean In North Central Extension Industry SoilFertility Conference Vol 20 pp 4148
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