An evaluation of lime Requirement methods for selected South African soils

Van der Berg, Vincent (2017-03)

Thesis (MSc)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: A laboratory study was conducted to evaluate seven widely used methods to predict soil lime requirement (LR) using 20 acidic South African top and sub-soils with a wide range of properties. The LR methods which were evaluated against a standard CaCO3 incubation LR procedure, included: the original Eksteen method with organic matter correction factor (OMCF) , commonly used in the Western Cape; two modifications of the Eksteen method, namely: (i) Eksteen-KCl method, involving the use of 1 M KCl exchangeable acidity instead of titratable acidity at pH 7, and (ii) Modified-Eksteen method, where a correction factor was applied to titratable acidity that was derived from soil data obtained in this study; the Cedara method, most commonly used in KwaZulu-Natal; the ARC-SGI method, developed primarily for Free State soils by the ARC- Small Grain Institute in Bethlehem; the Shoemaker-McLean-Pratt single buffer (SMP-SB) method most commonly used in the North East and North Central regions of the USA; the Adams and Evans single buffer (AE-SB) method most commonly used in the South East and Mid-Atlantic regions of the USA. The original Eksteen method, although highly correlated with incubation LR, was found to be a relatively inaccurate predictor of LR. The Eksteen-KCl and Cedara methods were found to be highly correlated with incubation LR, yet consistently underestimated LR. The modified-Eksteen method was found to be highly correlated with incubation LR, and was a good predictor of LR. The ARC-SGI method was a considerably poorer predictor of LR, and tended to grossly overestimate LR. The SMPSB method was found to be highly correlated with incubation LR, and was shown to be reasonably accurate to achieve a target pHKCl of 5.5. Recalibration of the SMP-SB soil-buffer pH with incubation LR resulted in considerable increases in accuracy. The AE-SB method was found to be highly correlated with incubation LR, yet tended to overestimate LR. Recalibration of the AE-SB soil-buffer pH with incubation LR resulted in a sufficient increase in accuracy. A correlation study was conducted to investigate the relationship between soil properties and both incubation LR and LR methods. It was revealed that soil properties other than soil pH, which are useful indicators of LR are: soil C > variable charge > CECpH 7 > clay + silt. Soil C was found to be a significant contributor to LR due to its association with exchangeable Al and due to its high pH dependent acidity. Titratable acidity was found to be the soil property that most strongly related to soil LR. Variable charge was also shown to exhibit significant relationships with soil parameters that most strongly influence LR. For these reasons, a multiple regression equation was developed that utilised only titratable acidity and variable charge. The multiple regression equation was able to predict 96.76% of the variation observed for incubation LR, and was 97.86% accurate in predicting the LR for each specific soil to obtain a target pHKCl of 5.5. Regarding the relationship between soil properties and LR methods, it was revealed that the local methods, except the ARC-SGI method, were most strongly influenced by exchangeable acidity and Al, and had significant relationships with soil C. The American direct buffer methods were strongly correlated with essentially all of the soil properties studied. This is indicative of the ability of these methods to directly determine the amount of acidity that may originate from various sources in the soil, in order to make a sufficiently accurate LR. It is therefore recommended that the application of direct buffer methods be further developed for use on South African soils in order to further improve the accuracy of LR determination in South Africa. The existing method that was found to most accurately predict LR on a wide range of soils was the modified-Eksteen method.

AFRIKAANSE OPSOMMING: ʼn Laboratorium studie was uitgevoer om sewe metodes te evalueer, wat algemeen gebruik word om kalkbehoefte (KB) te bepaal, op 20 suur Suid-Afrikaanse bo- en ondergronde wat oor ʼn wye reeks eienskappe beskik. Die KB metodes wat geëvalueer was, teenoor ʼn standaard CaCO3 inkubasie KB prosedure, was: die Eksteen metode met organiese materiaal korreksie faktor (OMKF) wat algemeen in die Wes-Kaap gebruik word; ingesluit was twee veranderinge tot die oorspronklike metode, naamlik: (i) die Eksteen-KCl metode, wat die gebruik van 1 M KCl uitruilbare suur bewerkstellig, i.p.v. titreerbare suur by pH 7, en (ii) die gemodifiseerde Eksteen metode, waartoe ʼn korreksie tot die titreerbare suur data toegepas is soos verkry in die studie; die Cedara metode, wat algemeen in KwaZulu Natal gebruik word; die ARC-SGI metode, wat primêr vir Vrystaatse gronde ontwikkel is deur die LNR Klein Graan Instituut in Bethlehem; die Shoemaker-McLean-Pratt enkel buffer (SMP-SB) metode, wat algemeen in die Noord-Oostelike en sentrale Noordelike streke van die V.S.A gebruik word; die Adams en Evans enkel buffer (AE-SB) metode, wat algemeen in die Suid-Oostelike en mid-Atlantiese streke van die V.S.A. gebruik word. Dit was gevind dat die oorspronklike Eksteen metode ʼn relatiewe onakkurate voorspeller vir KBbepaling was, alhoewel ʼn sterk verwantskap gevind was tussen dié metode en die inkubasie KB metode. Die Eksteen-KCl en Cedara metodes het sterk gekorreleer met die inkubasie KB, maar het in meeste gevalle die KB om ʼn teiken pH te bereik onderskat. Die gemodifiseerde Eksteen metode het ʼn sterk verwantskap met die inkubasie KB getoon, en was oor die algemeen ʼn goeie voorspeller van KB om ʼn teiken pH te bereik. Die ARC-SGI metode was veral ʼn hoogs onakkurate voorspeller van KB, en het daartoe geneig om KB drasties te oorskat. Die SMP-SB metode het ʼn sterk verwantskap met die inkubasie KB getoon, en was gevind om bevredigend akkuraat te wees in die geval van KB bepaling om ʼn teiken pH te bereik. Kalibrasie van die SMP-SB grond-buffer pH met inkubasie KB het ʼn toename in akkuraatheid tot gevolg gehad. Die AE-SB metode het ʼn sterk verwantskap met die inkubasie KB getoon, maar het daartoe geneig om KB te oorskat. Kalibrasie van die AE-SB grond-buffer pH met inkubasie KB het egter ʼn bevredigende toename in akkuraatheid tot gevolg gehad. ʼn Korrelasie studie was uitgevoer om die verhouding tussen grondeienskappe en beide inkubasie en KB metodes te ondersoek. Dit was bevind dat grondeienskappe anders as grond pH wat KB bepaal die volgende insluit: grond C > veranderlike lading > KUKpH 7 > klei + slik. Dit was bevind dat grond C die grootste bydrae tot KB gehad het, a.g.v. C se assosiasie met uitruilbare Al en die groot bydrae daarvan tot pH veranderlike suurheid. Titreerbare suur was bevind om die grondeienskap te wees wat die sterkste verwantskap met KB het. Veranderlike lading het ook sterk verhoudings met grondeienskappe wat KB sterk beïnvloed getoon. Vir hierdie redes was ʼn veelvoudige regressie formule ontwikkel wat slegs titreerbare suurheid en veranderlike lading in ag neem. Die veelvoudige regressie model was in staat daartoe om 96.76% van die variasie te beskryf om ʼn KB te bepaal vir ʼn spesifieke grond om ʼn teiken pHKCl van 5.5 te bereik. Rakende die verhouding tussen grondeienskappe en KB metodes, was dit bevind dat plaaslike metodes – behalwe die ARC-SGI metode – die meeste deur uitruilbare suur, Al en C beïnvloed was. Die Amerikaanse direkte buffer metodes het sterk verwantskappe met feitlik alle geëvalueerde grondeienskappe getoon. Hierdie bevindinge toon dat die bestudeerde direkte KB metodes daartoe in staat is om alle suurheidsbronne in ag te neem tydens die voorspelling van KB. Dit word daarom aanbeveel dat die toepassing van sogenaamde direkte buffer metodes verder ontwikkel word vir gebruik op Suid-Afrikaanse gronde, om sodoende die akkuraatheid van KB bepalings te verhoog. Die bestaande metode wat bevind was om oor die algemeen die meeste akkuraat te wees op ʼn wye reeks gronde, was die gemodifiseerde Eksteen metode.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/101100
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