University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Historical Materials from University of Nebraska-Lincoln Extension Extension 2005 EC05-705 Precision Agriculture: Site-Specific of Soil pH (FAQ) Viacheslav I. Adamchuk University of Nebraska-Lincoln, viacheslav.adamchuk@mcgill.ca Jerry Mulliken Independent Crop Consultant Follow this and additional works at: https://digitalcommons.unl.edu/extensionhist Part of the Agriculture Commons, and the Curriculum and Instruction Commons Adamchuk, Viacheslav I. and Mulliken, Jerry, "EC05-705 Precision Agriculture: Site-Specific of Soil pH (FAQ)" (2005). Historical Materials from University of Nebraska-Lincoln Extension. 713. https://digitalcommons.unl.edu/extensionhist/713 This Article is brought to you for free and open access by the Extension at DigitalCommons@University of Nebraska Lincoln. It has been accepted for inclusion in Historical Materials from University of Nebraska-Lincoln Extension by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. University of Nebraska–Lincoln Extension EC 05-705 Site-Specific Management of Soil pH (FAQ) Viacheslav฀I.฀Adamchuk,฀Precision฀Agriculture฀Engineer฀ Jerry฀Mulliken,฀Independent฀Crop฀Consultant RESOURCES For฀more฀information฀about฀precision฀ agriculture฀research,฀ education฀and฀demonstration฀programs฀ S ite-specific฀management฀of฀soil฀pH฀is฀a฀precision฀ agriculture฀ practice฀ that฀ can฀ provide฀ positive฀ economic฀ and฀ environmental฀ impacts฀ on฀ modern฀ crop฀production.฀This฀publication฀addresses฀several฀ frequently฀asked฀questions฀related฀to฀the฀meaning฀of฀ soil฀pH,฀lime฀requirement,฀and฀quality฀of฀data฀used฀to฀ prescribe฀site-specific฀management฀of฀soil฀pH. at฀the฀University฀of฀ What฀is฀soil฀pH? Nebraska–Lincoln,฀visit฀ The฀term฀“pH”฀is฀defined฀as฀the฀negative฀logarithm฀ of฀the฀hydrogen฀ion฀activity, ฀and฀values฀range฀from฀1฀ (very฀acidic)฀to฀14฀(very฀basic). ฀A฀neutral฀solution, ฀ such฀as฀pure฀water฀at฀23฀0C, ฀has฀a฀pH฀of฀7.0. ฀Soil฀pH฀ is฀a฀major฀characteristic฀of฀the฀crop-growing฀environment฀as฀it฀affects฀nutrient฀availability, ฀microbial฀ activity, ฀and฀the฀potential฀for฀toxicity฀problems. ฀Soil฀ acidification฀may฀be฀caused฀by฀acid-forming฀fertilizers, ฀removing฀bases฀with฀harvested฀crops, ฀leaching฀ nitrate฀ and฀ basic฀ elements, ฀ and฀ organic฀ material฀ decomposition฀(Management฀Strategies฀to฀Reduce฀the฀ Rate฀of฀Soil฀Acidification,฀NebGuide฀03-1503). In฀general, ฀optimal฀soil฀pH฀varies฀with฀the฀crop. ฀ When฀soil฀pH฀falls฀below฀the฀desired฀level, ฀soil฀acidification฀may฀cause฀toxic฀concentrations฀of฀aluminum฀ and฀manganese. ฀The฀activity฀of฀soil฀micro-organisms฀ that฀affect฀nitrogen, ฀sulfur, ฀and฀phosphorus฀availability฀may฀be฀altered฀as฀well. ฀Calcium฀may฀be฀deficient฀ when฀the฀percent฀base฀saturation, ฀and฀usually฀cation฀ exchange฀ capacity฀ (CEC), ฀ of฀ the฀ soil฀ is฀ extremely฀ low฀(as฀in฀sandy฀soils). ฀Acidic฀soils฀may฀be฀poorly฀ aggregated฀with฀poor฀tilth, ฀especially฀for฀low฀organic฀ matter฀soils. ฀The฀availability฀of฀phosphorus฀and฀other฀ nutrients฀also฀is฀frequently฀reduced. ฀On฀the฀other฀ hand, ฀a฀high฀soil฀pH฀may฀reduce฀the฀availability฀of฀ phosphorous฀and฀certain฀micronutrients, ฀and฀injury฀ or฀carryover฀with฀some฀classes฀of฀herbicides. ฀ the฀Web฀site฀at฀http:// precision฀agriculture. unl.edu/ Extension Institute฀of฀Agriculture฀and฀ Natural฀Resources University฀of฀Nebraska– Lincoln How฀is฀soil฀pH฀measured?฀ A฀ pH฀ measurement฀ is฀ normally฀ made฀ by฀ either฀ colorimetric฀ or฀ electrometric฀ methods.฀The฀ former฀ involves฀ suitable฀ dyes฀ or฀ acid-base฀ indicators, ฀ the฀ colors฀of฀which฀change฀with฀hydrogen฀ion฀activity.฀The฀ latter฀involves฀a฀glass฀electrode฀paired฀with฀a฀reference฀ electrode฀attached฀to฀a฀suitable฀meter฀for฀measuring฀ electromotive฀ force฀ (emf)฀ in฀ proportion฀ to฀ the฀ pH.฀ The฀colorimetric฀method฀is฀not฀reliable฀and฀provides฀ much฀lower฀accuracy.฀In฀the฀United฀States,฀soil฀pH฀is฀ commonly฀determined฀using฀an฀ion-selective฀electrode฀ in฀a฀solution฀obtained฀by฀mixing฀soil฀and฀water฀together฀ in฀a฀1:1฀ratio. The฀most฀common฀procedure฀for฀measuring฀soil฀pH฀ in฀a฀laboratory฀consists฀of฀five฀primary฀steps: 1.฀ Calibrate฀the฀pH฀meter฀over฀the฀appropriate฀range฀ using฀a฀minimum฀of฀two฀standard฀buffer฀solutions,฀ typically฀having฀pH฀7฀and฀4฀(and/or฀10฀for฀alkaline฀ soils).฀ 2.฀ Measure฀a฀sample฀of฀air-dried,฀crushed฀and฀sieved฀ soil฀into฀a฀cup฀(5,฀10฀or฀20฀g฀are฀recommended). 3.฀ Add฀distilled฀or฀double-deionized฀water,฀or฀another฀ extracting฀ solution฀ (e.g., ฀ 0.01M฀ CaCl2), ฀ to฀ the฀ sample฀to฀bring฀the฀solution฀to฀a฀weight-to-weight฀ ratio฀of฀1:1.฀ 4.฀ Stir฀vigorously฀for฀5-10฀seconds฀and฀let฀stand฀for฀ 10-30฀minutes. 5.฀ Place฀the฀electrode฀in฀the฀slurry,฀swirl฀carefully,฀and฀ read฀the฀pH. ฀ How฀can฀I฀raise฀low฀soil฀pH? Liming฀is฀a฀common฀practice฀used฀to฀neutralize฀soil฀ acidity.฀Lime฀requirement฀is฀defined฀as฀the฀amount฀of฀ agricultural฀limestone฀or฀other฀basic฀material฀needed฀to฀ increase฀soil฀pH฀from฀an฀unacceptably฀acidic฀condition฀ to฀a฀value฀that฀is฀considered฀optimum฀for฀the฀desired฀ use฀of฀the฀soil.฀Lime฀rates฀usually฀range฀between฀1฀and฀ 3-4฀ton฀per฀acre฀(greater฀rates฀should฀be฀split฀between฀ How฀variable฀is฀soil฀pH? With฀the฀advent฀of฀precision฀agriculture,฀soil฀variability฀within฀ an฀agricultural฀field฀has฀become฀the฀focus฀of฀many฀studies. ฀It฀ has฀ been฀ shown฀ that฀ the฀ natural฀ variation฀ in฀ field฀ landscape฀ (including฀ terrain, ฀ parent฀ material, ฀ surface฀ water฀ movement, ฀ etc.)฀and฀past฀and/or฀present฀management฀can฀cause฀significant฀ variation฀in฀soil฀pH, ฀lime฀requirement, ฀and฀other฀soil฀properties. ฀For฀example, ฀Figure฀1฀illustrates฀the฀distribution฀of฀soil฀pH฀ within฀three฀Nebraska฀fields. ฀In฀these฀fields฀the฀coefficients฀of฀ variation฀(one฀of฀the฀indicators฀of฀relative฀variability)฀were฀4%, ฀ 9%, ฀and฀8%, ฀respectively. ฀This฀means฀that฀the฀majority฀of฀a฀field฀ with฀an฀average฀pH฀of฀6.0฀may฀have฀soil฀pH฀varying฀between฀ 5.0฀and฀7.0. ฀Small฀areas฀with฀a฀soil฀pH฀outside฀this฀range฀are฀ not฀uncommon. ฀ 70% Percentage฀of฀Samples two฀or฀more฀applications).฀Soil฀pH฀indicates฀the฀need฀for฀lime฀but฀ buffer฀pH฀is฀needed฀to฀estimate฀the฀amount฀of฀exchangeable฀acidity฀to฀be฀neutralized฀and,฀therefore,฀the฀amount฀of฀lime฀required฀ to฀raise฀the฀soil฀pH฀to฀the฀desired฀level.฀Lime฀requirement฀is฀ affected฀ by฀ soil฀ properties,฀ including฀ parent฀ material,฀ clay฀ and฀ organic฀matter฀contents,฀the฀cation฀exchange฀capacity,฀forms฀of฀ acidity฀present,฀and฀initial฀and฀final฀pH฀of฀soil฀(Lime฀Use฀for฀Soil฀ Acidity฀Management,฀NebGuide฀G03-1504).฀ Currently,฀three฀methods฀are฀used฀to฀estimate฀the฀amount฀of฀ exchangeable฀acidity฀that฀must฀be฀neutralized฀to฀raise฀the฀pH฀to฀ the฀desired฀level.฀The฀first฀involves฀estimating฀the฀lime฀requirement฀from฀soil฀properties฀such฀as฀soil฀pH,฀texture,฀type฀of฀clay,฀ and฀organic฀matter฀content.฀The฀second฀method฀is฀direct฀titration฀ of฀soils฀with฀Ca(OH)2.฀The฀third฀and฀most฀common฀procedure฀ uses฀buffer฀methods฀to฀estimate฀the฀lime-test฀index.฀Numerous฀ buffer฀methods฀have฀been฀developed฀over฀the฀years.฀The฀SMP฀and฀ Woodruff฀single-buffer฀methods฀for฀rapid฀measurement฀of฀lime฀ requirement฀have฀been฀adopted฀by฀many฀soil-testing฀laboratories,฀ including฀those฀in฀Nebraska.฀ The฀ most฀ common฀ alternatives฀ to฀ buffers฀ are฀ some฀ sort฀ of฀an฀estimate฀of฀the฀lime฀requirement฀based฀on฀soil฀pH฀and฀ a฀ measured, ฀ or฀ recorded, ฀ factor฀ that฀ is฀ associated฀ with฀ soil฀ buffering฀capacity.฀Examples฀include฀soil฀organic฀matter฀content, ฀ estimated฀CEC, ฀and฀soil฀series. ฀Many฀experiment฀stations฀and฀ soil-testing฀laboratories฀have฀determined฀the฀recommendations฀ for฀computing฀lime฀requirements฀of฀the฀major฀soil฀series฀and฀ types฀in฀the฀areas฀they฀serve. ฀Once฀this฀has฀been฀done, ฀knowledge฀of฀the฀pH฀and฀the฀soil฀type฀will฀make฀an฀immediate฀liming฀ recommendation฀possible. ฀ Most฀current฀recommendations฀provide฀application฀rates฀for฀ a฀given฀effective฀calcium฀carbonate฀equivalent฀(ECCE),฀relative฀ neutralizing฀value,฀effective฀neutralizing฀material,฀or฀similar฀characteristic฀of฀liming฀material,฀which฀vary฀with฀its฀quality฀(purity฀and฀ fineness).฀Therefore,฀it฀is฀necessary฀to฀adjust฀application฀rates฀ for฀the฀quality฀of฀material฀actually฀being฀applied.฀In฀addition,฀lime฀ recommendations฀are฀based฀on฀the฀assumption฀that฀lime฀will฀be฀ incorporated฀to฀a฀depth฀of฀6฀to฀9฀inches฀(4฀inches฀in฀the฀case฀of฀ no-till)฀following฀the฀application.฀Thus,฀the฀application฀rate฀should฀ be฀adjusted฀for฀the฀actual฀depth฀of฀lime฀incorporation. 60% Field฀1 50% Field฀2 Field฀3 40% 30% 20% 10% 0% <฀5.1฀ 5.1-5.5฀ 5.6-6.0฀ 6.1-6.5฀ Soil฀pH 6.6-7.0฀ >฀7.0 Figure 1. Distribution of soil pH within three agricultural fields in Nebraska (based on 182-186 soil samples collected in each field using a 1-acre grid pattern). In฀general, ฀soil฀pH฀is฀believed฀to฀have฀coefficients฀of฀variation฀ranging฀between฀2%฀and฀16%, ฀which฀is฀low฀compared฀to฀ soil฀ nutrients฀ or฀ certain฀ physical฀ properties฀ (e.g., ฀ saturated฀ hydraulic฀conductivity). ฀In฀addition, ฀soil฀pH฀does฀not฀change฀ abruptly, ฀and฀soil฀samples฀taken฀close฀together฀tend฀to฀have฀ smaller฀differences฀between฀pH฀measurements฀than฀samples฀ collected฀farther฀apart. ฀Therefore, ฀soil฀pH฀has฀“spatial฀structure.”฀Although฀ the฀ degree฀ of฀ this฀ spatial฀ structure฀ changes฀ from฀field฀to฀field, ฀similarities฀in฀soil฀pH฀measurements฀can฀be฀ observed฀at฀maximum฀distances฀of฀60฀-฀900฀ft. ฀ What฀is฀site-specific฀management฀of฀soil฀pH? One฀of฀the฀goals฀of฀precision฀agriculture฀is฀to฀manage฀agricultural฀inputs฀according฀to฀changing฀local฀field฀conditions฀in฀order฀ to฀increase฀profitability฀and฀reduce฀environmental฀waste฀of฀agricultural฀inputs.฀According฀to฀many฀adopters,฀variable฀rate฀liming฀ is฀one฀of฀the฀profitable฀and฀popular฀practices฀in฀site-specific฀crop฀ management.฀In฀addition฀to฀acidic฀field฀areas,฀having฀knowledge฀ of฀areas฀with฀alkaline฀soil฀conditions฀(high฀pH)฀can฀be฀useful฀to฀ avoid฀lime฀application฀in฀these฀areas฀and฀also฀aid฀in฀the฀selection฀ of฀crop฀varieties฀tolerant฀to฀problems฀associated฀with฀high฀pH฀ (e.g.,฀iron฀chlorosis). Currently, ฀ variable฀ rate฀ lime฀ prescription฀ maps฀ are฀ generated฀based฀on฀soil฀samples฀collected฀manually฀and฀analyzed฀in฀ laboratory฀conditions.฀These฀samples฀are฀usually฀obtained฀with฀a฀ 2.5-acre฀sampling฀frequency฀(Soil฀Sampling฀for฀Precision฀Agriculture,฀ EC00-154).฀ Is฀2.5-acre฀grid฀sampling฀an฀adequate฀approach?฀ Figure฀2฀illustrates฀a฀common฀problem฀with฀creating฀a฀prescription฀map฀for฀applying฀variable฀rate฀lime฀using฀2.5-acre฀grid฀ sampling.฀ In฀ this฀ case,฀ 330-฀ by฀ 330-ft฀ (2.5-acre)฀ grid฀ cells฀ are฀ superimposed฀on฀a฀bare-soil฀infrared฀image.฀The฀field฀has฀terraces฀ which฀appear฀as฀dark฀lines฀so฀it฀is฀evident฀that฀there฀is฀a฀significant฀ slope฀in฀this฀field.฀The฀white฀areas฀are฀eroded฀Nora฀soil,฀with฀ alkaline฀(high฀pH)฀subsoil฀near฀the฀surface.฀The฀darker฀areas฀are฀ less฀eroded,฀and฀more฀acid฀in฀the฀upper฀horizon. ● ● ● ● Possible฀sample฀ locations฀within฀฀฀ a฀grid฀cell ● Could฀directed฀sampling฀be฀helpful? ● ● structure฀exists,฀certain฀map฀interpolation฀methods฀can฀be฀used฀ to฀better฀predict฀lime฀application฀rates฀in฀unsampled฀locations.฀ However,฀even฀with฀the฀best฀(from฀a฀scientific฀viewpoint)฀interpolation฀method,฀errors฀will฀remain.฀Any฀type฀of฀interpolation฀is฀ ineffective฀when฀substantial฀soil฀variability฀can฀be฀found฀between฀ nearest฀soil฀samples.฀ ● Center฀of฀a฀ grid฀cell Figure 2. A field image with 2.5-acre grid sampling pattern. Directed฀(also฀called฀guided)฀sampling฀according฀to฀relatively฀ uniform฀required฀lime฀application฀zones฀is฀a฀promising฀approach฀ for฀many฀fields.฀The฀zones฀are฀determined฀by฀considering฀the฀ variations฀in฀the฀field฀that฀may฀affect฀lime฀requirement,฀including฀ soil฀types,฀topographic฀position,฀past฀management,฀aerial฀images฀ of฀bare฀soil฀and฀growing฀crops,฀spatial฀variation฀in฀historical฀yields,฀ soil฀electrical฀conductivity฀maps฀and/or฀other฀data฀layers.฀ For฀example,฀Figure฀3a฀represents฀an฀aerial฀photo฀of฀a฀soybean฀ field฀in฀late฀July.฀The฀field฀is฀irrigated฀with฀a฀center฀pivot฀system.฀ Stand฀loss฀and฀plant฀death฀have฀occurred฀in฀the฀southwest฀corner,฀ which฀is฀not฀irrigated.฀The฀pH฀in฀the฀bare฀areas฀was฀below฀4.5฀due฀ to฀a฀history฀of฀seed-corn฀production฀with฀relatively฀high฀nitrogen฀ application฀rates.฀The฀irrigated฀parts฀of฀the฀field฀have฀pH฀above฀5.5฀ due฀to฀better฀uptake฀of฀nitrogen฀in฀previous฀crop฀years฀and฀high฀ amounts฀of฀calcium฀in฀the฀irrigation฀water.฀Compaction฀effects฀ are฀also฀relevant฀because฀a฀disk-tillage฀pan฀was฀present฀on฀the฀ west฀half฀but฀not฀on฀the฀east฀half,฀which฀was฀under฀ridge฀tillage.฀ The฀most฀severely฀degraded฀areas฀are฀relatively฀level.฀This฀can฀ be฀seen฀in฀Figure฀3b,฀which฀shows฀the฀same฀photo฀viewed฀from฀ the฀west,฀and฀overlaid฀on฀a฀digital฀elevation฀model฀(3-D฀view฀of฀ field฀terrain).฀On฀the฀steep฀slopes,฀alkaline฀subsoil฀is฀exposed฀ and฀ roots฀ grew฀ through฀ the฀ tillage฀ pan.฀ In฀ the฀ relatively฀ level฀ area฀near฀a฀field฀entrance฀in฀the฀northwest฀corner฀(indicated฀by฀ the฀red฀arrow),฀crop฀growth฀was฀affected฀by฀both฀compaction฀ and฀low฀pH.฀In฀summary,฀the฀spatial฀variability฀of฀pH฀in฀this฀field฀ is฀due฀to฀differences฀in฀past฀nitrogen฀use,฀calcium฀applied฀with฀ irrigation฀water,฀and฀differences฀in฀soil฀type฀as฀influenced฀by฀slope.฀ The฀effect฀of฀pH฀on฀crop฀growth฀also฀was฀influenced฀by฀tillage฀ history.฀Therefore,฀enough฀information฀is฀available฀to฀create฀a฀ useful฀directed฀sampling฀plan. N If฀a฀few฀cores฀are฀taken฀near฀the฀center฀of฀a฀grid฀cell฀(red฀dot),฀ the฀sample฀pH฀is฀likely฀to฀be฀greater฀than฀7฀since฀it฀is฀within฀the฀ white฀area.฀As฀a฀result,฀the฀entire฀grid฀cell฀will฀receive฀no฀lime.฀If฀ several฀cores฀are฀taken฀randomly฀throughout฀the฀grid฀cell,฀such฀as฀ at฀the฀yellow฀dots,฀and฀then฀combined,฀the฀result฀will฀be฀nearer฀ the฀average฀pH฀for฀the฀grid฀cell.฀However,฀the฀variability฀in฀this฀ grid฀cell฀is฀likely฀to฀be฀as฀high฀as฀it฀is฀across฀the฀field,฀so฀little฀is฀ accomplished.฀Using฀this฀method,฀it฀is฀likely฀that฀the฀grid฀cell฀will฀ receive฀too฀little฀lime฀in฀the฀non-eroded฀portion,฀and฀too฀much฀ lime฀on฀the฀eroded฀spot.฀Since฀the฀grid฀lines฀do฀not฀coincide฀ with฀the฀patterns฀of฀variability,฀the฀variable฀rate฀application฀is฀not฀ necessarily฀more฀appropriate฀than฀a฀uniform฀application.฀In฀this฀ example,฀the฀analysis฀cost฀would฀be฀eight฀times฀as฀high฀as฀when฀a฀ regular฀20-acre฀composite฀sampling฀strategy฀is฀used.฀Overall,฀grid฀ sampling฀with฀2.5-acre฀grids฀increases฀analysis฀cost฀and฀often฀fails฀ to฀adequately฀measure฀spatial฀pH฀variability,฀resulting฀in฀reduced฀ profitability฀of฀variable฀rate฀liming.฀ The฀quality฀of฀prescription฀maps฀generated฀using฀a฀grid฀sampling฀can฀be฀improved฀by฀decreasing฀the฀grid฀size฀to฀1฀acre;฀however,฀the฀cost฀of฀the฀laboratory฀analysis฀ for฀ pH฀ and฀ buffer฀ pH฀ will฀ increase. ฀ 5.8 N Although฀the฀procedure฀will฀not฀need฀ to฀be฀repeated฀for฀five฀or฀more฀years฀ 5.4 and฀ the฀ cost฀ can฀ be฀ prorated฀ over฀ that฀ time, ฀ the฀ profitability฀ of฀ variable฀ 5.8 5.4 5.4 6.6 rate฀liming฀using฀this฀sampling฀strategy฀ 5.4 6.6 remains฀questionable. ฀Even฀in฀a฀1-acre฀ 5.6 5.6 grid฀ cell, ฀ a฀ 50-ft฀ lime฀ spreader฀ can฀ 4.5 make฀four฀passes฀with฀several฀different฀ applied฀rates฀in฀each฀pass฀(more฀than฀ 4.5 16฀50฀by฀50-ft฀squares฀can฀be฀located฀ ฀฀฀Soil฀pH b a within฀1-acre฀grid฀cell). ฀Therefore, ฀the฀ mapping฀ method฀ still฀ does฀ not฀ match฀ Figure 3. Soybean field with crop stress due to pH variability caused by past management the฀application฀technique. represented as a) aerial photograph, and b) the same image combined with a 3D view of If฀ the฀ earlier฀ mentioned฀ spatial฀ the terrain. Usually฀the฀effects฀of฀soil฀pH฀on฀crop฀growth฀are฀more฀subtle฀ than฀those฀seen฀in฀the฀example฀above.฀When฀soil฀pH฀deviates฀ from฀the฀optimum฀range,฀root฀growth,฀legume฀nodulation,฀and฀ phosphorous฀uptake฀may฀be฀reduced.฀Also,฀soil฀applied฀herbicides฀ may฀be฀less฀effective฀in฀some฀cases.฀All฀of฀these฀effects฀can฀be฀ caused฀by฀other฀factors,฀such฀as฀compaction,฀lack฀of฀rhizobium฀ inoculants,฀ or฀ insect฀ damage.฀ Crop฀ scouting฀ observations฀ are฀ usually฀not฀adequate฀to฀detect฀these฀effects฀on฀plants฀and฀the฀ cumulative฀ impact฀ is฀ best฀ measured฀ by฀ crop฀ yield.฀Therefore,฀ knowing฀soil฀pH฀is฀essential฀for฀preventing฀potential฀yield฀loss฀ in฀the฀future.฀ How฀can฀the฀accuracy฀of฀soil฀pH฀maps฀be฀improved? Since฀the฀beginning฀of฀precision฀agriculture฀approach,฀several฀ researchers฀and฀manufacturers฀pursued฀the฀development฀of฀onthe-go฀soil฀sensors฀to฀accurately฀map฀pH฀(and฀other฀soil฀properties)฀at฀a฀relatively฀low฀cost฀(On-the-Go฀Vehicle-Based฀Soil฀Sensors,฀ EC02-178).฀Based฀on฀research฀conducted฀at฀Purdue฀University฀ and฀the฀University฀of฀Nebraska-Lincoln,฀Veris฀Technologies,฀Inc.,฀ based฀in฀Salina,฀Kan.,฀launched฀production฀of฀the฀world’s฀first฀ automated฀on-the-go฀soil฀pH฀mapping฀system฀in฀the฀summer฀of฀ 2003.฀This฀product฀is฀called฀the฀Mobile฀Sensor฀Platform฀(MSP).฀ It฀consists฀of฀a฀widely฀used฀electrical฀conductivity฀(EC)฀mapping฀ unit฀and฀a฀Soil฀pH฀ManagerTM฀(Figure฀4). Figure 4. Veris® Mobile Sensor Platform (MSP). During฀field฀operation,฀the฀Soil฀pH฀ManagerTM฀automatically฀ collects฀ and฀ measures฀ a฀ soil฀ sample฀ without฀ stopping. ฀While฀ mapping฀a฀field,฀row฀cleaners฀remove฀crop฀residue.฀A฀hydraulic฀ cylinder฀on฀a฀parallel฀linkage฀retracts฀to฀lower฀the฀cutting฀shoe฀ assembly฀into฀the฀soil,฀and฀the฀cutting฀shoe฀creates฀a฀soil฀core฀ which฀flows฀into฀the฀sampling฀trough.฀The฀previous฀core฀sample฀ is฀discharged฀at฀the฀rear฀of฀the฀trough฀as฀it฀is฀replaced฀by฀the฀new฀ sample฀core฀entering฀in฀the฀front.฀The฀hydraulic฀cylinder฀extends฀ to฀raise฀the฀sampling฀trough฀containing฀the฀soil฀core฀out฀of฀the฀soil฀ while฀bringing฀the฀new฀sample฀in฀contact฀with฀two฀ion-selective฀ pH฀electrodes฀(combination,฀gel-filled,฀epoxy-body,฀dome-glass฀ membrane).฀During฀sampling,฀the฀electrodes฀are฀washed฀with฀ two฀flat฀fan฀nozzles.฀Covering฀disks฀fill฀the฀soil฀trench฀and฀cover฀ the฀track.฀Measurement฀depth฀is฀adjustable฀from฀1.5฀to฀6฀inches,฀ typically฀ with฀ a฀ 3-inch฀ average฀ effective฀ measurement฀ depth.฀ Soil฀cores฀are฀brought฀into฀direct฀contact฀with฀the฀electrodes฀ and฀held฀in฀place฀for฀7-25฀seconds฀(depending฀on฀the฀electrode฀ response). ฀ Every฀ measurement฀ represents฀ an฀ average฀ of฀ the฀ outputs฀produced฀by฀the฀two฀electrodes.฀Two฀independent฀measurements฀allow฀cross-validation฀of฀electrode฀performances฀and฀ filtration฀of฀erroneous฀readings.฀The฀recorded฀electrode฀output฀ is฀converted฀to฀pH฀values฀according฀to฀the฀selected฀electrode฀ calibration฀ parameters.฀ Every฀ measurement฀ is฀ geo-referenced฀ using฀a฀Global฀Positioning฀System฀(GPS)฀receiver. Figure฀5฀illustrates฀the฀results฀of฀comparisons฀between฀conventional฀laboratory฀analysis฀conducted฀on฀manually฀extracted฀soil฀ samples฀and฀corresponding฀on-the-go฀measurements฀performed฀ within฀a฀25-ft฀radius.฀This฀comparison฀involved฀14฀fields฀in฀Kansas,฀ Nebraska,฀Iowa,฀Illinois,฀and฀Wisconsin.฀Although฀the฀degree฀of฀ correlation฀between฀the฀two฀methods฀is฀high,฀on-the-go฀measurements฀can฀have฀a฀standard฀error฀as฀high฀as฀0.2฀to฀0.3฀pH,฀ which฀is฀slightly฀higher฀than฀usual฀in฀a฀selected฀commercial฀soil฀ lab.฀On฀the฀other฀hand,฀on-the-go฀mapping฀allows฀for฀a฀significant฀ increase฀in฀sampling฀density.฀Table฀I฀illustrates฀the฀effect฀of฀travel฀ speed฀and฀distance฀between฀passes฀on฀sampling฀density฀with฀the฀ assumption฀that฀sampling฀occurs฀every฀10฀seconds. Apparent electrical conductivity mapping unit comprised of six coulters that provide two depths of investigation (0-1 ft and 0-3 ft). A soil pH mapping unit that includes a soil sampling mechanism with two ionselective electrodes and cleaning water supply system. Soil฀pH฀(on-the-go฀mapping) 9.0 8.0 r2฀=฀0.80 7.0 6.0 5.0 4.0 3.0 3.0฀ 4.0฀ 5.0฀ 6.0฀ 7.0฀ Soil฀pH฀(laboratory฀analysis) 8.0฀ Figure 5. Comparison between on-the-go and laboratory measurements of soil pH. 9.0 Travel฀speed฀ (mph)฀ ฀4฀ ฀ ฀6฀ ฀ ฀8฀ ฀ 10฀ ฀ * ฀฀฀฀฀Distance฀between฀passes฀(ft) 20฀ 40฀ 60฀ 80฀ 100 37.1฀ 18.6฀ 12.4฀ 9.3฀ 7.4 24.8฀ 12.4฀ 8.3฀ 6.2฀ 5.0 18.6฀ 9.3*฀ 6.2฀ 4.6฀ 3.7 14.9฀ 7.4฀ 5.0฀ 3.7฀ 3.0 ฀-฀sampling฀density฀currently฀recommended฀by฀researchers฀and฀the฀manufacturer This฀increase฀in฀sampling฀density฀frequently฀results฀in฀more฀ accurate฀soil฀pH฀maps.฀For฀example,฀Figure฀6฀illustrates฀a฀60acre฀Kansas฀field.฀The฀neutral฀soil฀band฀near฀the฀northwest฀field฀ boundary฀(caused฀by฀an฀adjacent฀gravel฀road)฀and฀a฀fuzzy฀pattern฀ of฀acidic฀soil฀in฀the฀middle฀of฀the฀field฀were฀hidden฀when฀the฀2.5acre฀grid฀sampling฀approach฀was฀applied.฀Laboratory฀analysis฀of฀ 10฀validation฀samples฀confirmed฀that฀the฀map฀based฀on฀on-the-go฀ sensing฀was฀more฀accurate฀than฀the฀interpolated฀map฀based฀on฀ 2.5-acre฀grid฀sampling.฀ validation฀samples Soil฀pH <฀5.0 5.0-5.5 5.5-6.0 6.0-6.5 6.5-7.0 >฀7.0 On-the-go฀Sensing Conventional฀2.5฀acre฀Grid฀Sampling Figure 6. Comparison between soil pH maps obtained through on-the-go mapping and conventional 2.5-acre grid sampling. Can฀on-the-go฀soil฀sensing฀be฀used฀directly฀to฀ prescribe฀lime฀application฀rates? Soil฀pH฀maps฀based฀on฀on-the-go฀measurements฀indicate฀the฀ variability฀of฀soil฀acidity/alkalinity฀but฀need฀to฀be฀translated฀to฀lime฀ application฀maps฀prior฀to฀variable฀rate฀liming.฀This฀is฀somewhat฀ challenging฀as฀soil฀buffering฀capacity฀typically฀varies฀across฀the฀ field,฀and฀the฀amount฀of฀lime฀needed฀to฀change฀soil฀pH฀by฀one฀ unit฀ is฀ not฀ constant.฀Therefore,฀ the฀Veris®฀ MSP฀ combines฀ soil฀ pH฀and฀electrical฀conductivity฀mapping฀capabilities฀as฀electrical฀ conductivity฀maps฀often฀reflect฀changes฀in฀soil฀texture฀(percentage฀ of฀clay,฀silt,฀and฀sand),฀the฀major฀factor฀affecting฀soil฀buffering฀ capability.฀Therefore,฀lime฀prescription฀maps฀can฀be฀calculated฀ from฀the฀simultaneously฀obtained฀electrical฀conductivity฀and฀soil฀ pH฀measurements. For฀example,฀the฀calibration฀of฀lime฀requirement฀measurements฀can฀be฀done฀by฀using฀laboratory฀analysis฀of฀eight฀to฀10฀soil฀ samples฀from฀parts฀of฀the฀field฀with฀either฀relatively฀low฀or฀high฀ soil฀pH฀and฀co-aligning฀these฀results฀with฀corresponding฀on-thego฀measurements฀of฀soil฀pH฀and฀EC.฀A฀multivariate฀regression฀ approach฀can฀be฀applied฀to฀develop฀a฀field-specific฀equation฀for฀ predicting฀the฀lime฀requirement฀based฀on฀a฀linear฀combination฀of฀ soil฀pH฀and฀EC฀data฀collected฀on-the-go.฀Although฀this฀approach฀ appears฀complex,฀a฀straight-forward฀technique฀is฀being฀developed฀ to฀integrate฀soil฀sensor฀measurements฀with฀results฀from฀laboratory฀analysis฀of฀a฀few฀samples.฀This฀will฀make฀variable฀rate฀liming฀ prescriptions฀easier฀to฀create฀in฀the฀future.฀Additional฀sources฀ of฀spatial฀soil฀data฀might฀also฀be฀used฀to฀improve฀the฀quality฀of฀ lime฀ application฀ maps.฀ Currently฀ under฀ development,฀ sensors฀ for฀mapping฀soil฀optical฀reflectance฀(predictor฀of฀organic฀matter฀ content)฀and฀conventional฀bare฀soil฀imagery฀also฀could฀serve฀as฀ additional฀data฀sources.฀ Does฀variable฀rate฀liming฀pay? As฀with฀other฀site-specific฀crop฀management฀strategies,฀the฀ profitability฀ of฀ variable-rate฀ liming฀ depends฀ on: ฀ 1)฀ quality฀ of฀ information,฀ 2)฀ additional฀ application฀ cost฀ and฀ data฀ collection฀ and฀processing฀costs,฀and฀3)฀the฀variability฀in฀lime฀requirement฀ for฀the฀particular฀field.฀ For฀instance,฀variable-rate฀liming฀will฀not฀be฀profitable฀if฀lime฀ requirement฀is฀uniform฀or฀soil฀acidity฀is฀not฀limiting฀the฀yield.฀Also,฀ liming฀may฀require฀several฀years฀to฀impact฀the฀yield฀and฀should฀be฀ considered฀a฀long-term฀investment.฀Finally,฀poor฀quality฀of฀information฀used฀to฀prescribe฀variable-rate฀liming฀may฀result฀in฀inappropriate฀changes฀of฀lime฀application฀rates฀and฀therefore฀increase฀(rather฀ than฀reduce)฀soil฀pH฀variability฀at฀the฀farmer’s฀expense.฀ In฀a฀recent฀University฀of฀Nebraska–Lincoln฀study฀of฀the฀value฀ of฀soil฀pH฀maps,฀it฀has฀been฀shown฀that฀the฀expected฀net฀return฀ (crop฀sale฀revenue)฀over฀cost฀of฀lime฀(NRCL)฀during฀a฀four-year฀ corn-soybean฀growing฀cycle฀is฀affected฀by฀the฀errors฀associated฀ with฀different฀mapping฀approaches.฀Based฀on฀the฀model฀developed, ฀ higher฀ errors฀ mean฀ lower฀ potential฀ benefit฀ (Figure฀ 7).฀฀ 930 Expected฀NRCL,฀$/acre Table฀1.฀Sampling฀density฀(samples฀per฀acre)฀for฀on-the-go฀soil฀ pH฀mapping 920 High฀accuracy฀map฀ (on-the-go฀mapping,฀1-acre฀or฀ proper฀directed฀sampling) Low฀accuracy฀map฀ (2.5-acre฀grid฀or฀whole฀ field฀composite฀sampling) 910 900 890 0.1฀ Expected฀value฀of฀high฀versus฀ low฀accuracy฀soil฀maps฀ (typically฀$5฀-฀$15฀per฀acre฀for฀ four฀years) 0.2฀ 0.3฀ 0.4฀ 0.5฀ 0.6฀ Variance฀of฀soil฀pH฀estimation฀error 0.7฀ 0.8 Figure 7. The effect of soil pH mapping quality on liming profitability (NRCL represents four years of corn and soybean crop revenue minus the cost of lime). For฀the฀selected฀field฀conditions฀with฀slightly฀acidic฀(5.8฀average฀ pH)฀soil฀and฀9%฀variability,฀“low฀accuracy฀map”฀means฀either฀a฀map฀ obtained฀using฀2.5-acre฀grid฀sampling฀or฀simply฀assuming฀that฀soil฀ pH฀is฀constant฀across฀the฀field฀(composite฀field฀sampling).฀A฀“high฀ accuracy฀map”฀can฀be฀obtained฀through฀1-acre฀grid฀sampling,฀on- the-go฀mapping,฀or฀properly฀conducted฀directed฀sampling.฀The฀ difference฀between฀expected฀NRCL฀corresponding฀to฀high฀and฀ low฀accuracy฀maps฀represents฀the฀expected฀economic฀benefit฀ that฀typically฀ranges฀between฀$5฀and฀$15฀per฀acre.฀Of฀course,฀this฀ benefit฀should฀cover฀the฀difference฀in฀costs฀associated฀with฀both฀ methods,฀which฀ranges฀between฀$0฀and฀$20/acre.฀For฀example,฀ the฀cost฀of฀on-the-go฀mapping฀can฀be฀similar฀to฀the฀2.5-acre฀grid฀ sampling,฀and฀the฀1-acre฀grid฀sampling฀costs฀$20/acre฀more฀than฀ the฀whole-field฀composite฀sampling. Summary When฀implementing฀different฀precision฀agriculture฀practices,฀ site-specific฀management฀of฀soil฀pH฀has฀been฀shown฀to฀be฀one฀of฀ the฀most฀promising฀strategies฀in฀fields฀with฀substantial฀variability฀ in฀soil฀pH.฀Justification฀of฀variable-rate฀liming฀is฀complicated฀by฀ the฀following:฀liming฀is฀a฀long-term฀investment;฀lime฀requirements฀ across฀fields฀are฀not฀always฀highly฀variable;฀and฀the฀conventionally฀ implemented฀2.5-acre฀grid฀soil฀sampling฀does฀not฀provide฀the฀ sampling฀density฀needed฀to฀accurately฀determine฀the฀variability฀ of฀soil฀pH฀in฀many฀fields.฀The฀recently฀commercialized฀technology฀ of฀on-the-go฀soil฀mapping฀provides฀a฀better฀basis฀of฀information฀ about฀spatial฀variability฀of฀soil฀pH฀and฀other฀properties฀related฀ to฀ buffering฀ characteristics฀ (i.e.,฀ electrical฀ conductivity).฀With฀ proper฀consideration฀of฀all฀the฀information฀available,฀an฀optimized฀ strategy฀for฀site-specific฀pH฀management฀can฀be฀developed฀and฀ positive฀economic฀and฀environmental฀impacts฀can฀be฀achieved. Additional฀recommended฀reading Adamchuk฀V.I฀ and฀ P.J.฀ Jasa.฀ 2002.฀ On-the-go฀Vehicle-Based฀ Soil฀ Sensors.฀University฀of฀Nebraska–Lincoln฀Extension฀EC฀02-178. Ferguson฀ R.B. ฀ and฀ G.W. ฀ Hergert. ฀ 2000. ฀ Soil฀ Sampling฀ for฀ Precision฀Agriculture. ฀ University฀ of฀ Nebraska–Lincoln฀ Extension฀ EC00-154. Mamo,฀M.฀C.฀Wortmann฀and฀C.฀Shapiro.฀2003.฀Lime฀Use฀For฀ Soil฀Acidity฀Management.฀University฀of฀Nebraska–Lincoln฀Extension฀ NebGuide฀G03-1504. Veris฀Technologies,฀Inc.฀2003.฀Mobile฀Sensor฀Platform฀(MSP).฀ Product฀Bulletin.฀Available฀at฀http://www.veristech.com. Wortmann฀C.,฀M.฀Mamo฀and฀C.฀Shapiro.฀2003.฀Management฀ Strategies฀to฀Reduce฀the฀Rate฀of฀Soil฀Acidification.฀University฀of฀฀ Nebraska–Lincoln฀Extension฀NebGuide฀G03-1503. ฀ The฀information฀in฀this฀publication฀is฀supplied฀with฀the฀understanding฀that฀ no฀endorsement฀of฀specific฀products฀named,฀nor฀discrimination฀of฀products฀ not฀named,฀is฀implied฀by฀University฀of฀Nebraska–Lincoln฀Extension. University of Nebraska–Lincoln Extension educational programs abide with the non-discrimination policies of the University of Nebraska–Lincoln and the United States Department of Agriculture. Extension is a Division of the Institute of Agriculture and Natural Resources at the University of Nebraska–Lincoln cooperating with the Counties and the U.S. Department of Agriculture.