Improving phosphate fertilizer recommendations using soil phosphorus buffer capacity and evaluation of various P extraction tests on a variety of South-African soils
| dc.contributor.advisor | Hardie-Pieters, Ailsa G. | en_ZA |
| dc.contributor.advisor | Raath, Pieter | en_ZA |
| dc.contributor.author | White, Vivian George | en_ZA |
| dc.contributor.other | Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science. | en_ZA |
| dc.date.accessioned | 2019-02-15T07:25:08Z | |
| dc.date.accessioned | 2019-04-17T08:12:07Z | |
| dc.date.available | 2019-02-15T07:25:08Z | |
| dc.date.available | 2019-04-17T08:12:07Z | |
| dc.date.issued | 2019-04 | |
| dc.description | Thesis (MScAgric)--Stellenbosch University, 2019. | en_ZA |
| dc.description.abstract | ENGLISH ABSTRACT: Phosphorus (P) fertiliser recommendations are based on extractable P levels as measured by a variety of extraction tests and should incorporate factors such as depth, bulk density and the soils phosphorus buffering capacity. Internationally and in South-Africa a variety of extraction test are used with contrasting norms and crop norms are not available for all of these extraction tests. Therefore the first aim of this study was to examine the relative aggressiveness and relationships between the various P extraction tests using 49 soils with widely varying physicochemical properties. The relative efficiency as compared to Total P (acid digestion) of the extraction tests were as follows; 1:2 H2O < Olsen < Colwell < Bray I < Ambic 1 < Bray II < Mehlich III ≈ Citric acid. Bray II and Mehlich III were the only extractions test that correlated significantly (R2 = 0.78 & 0.76) with total P. Strong linear relationships were observed between Bray II, Olsen and Mehlich III extractions tests and based on the weighted standard error of measurement, direct conversions between these tests were most reliable. It is thus possible to convert with confidence from Olsen to Bray II and Bray I by using a factor of 5.20 and 3.88 respectively. For the conversion from Bray II to Mehlich III a factor of 1.10 can be used. Phosphorous buffering capacity (PBC) can be determined by a multiple-point sorption isotherm and through fitting the Freundlich equation or through a single-point isotherm method by making various assumptions. A subsample set of 10 soils varying in P sorbing capabilities was used, a large variance in parameter b was observed and ultimately two distinct groups with both a different value for b were determined. By equilibrating the samples with 1000 mg P kg-1 and by using the untransformed variation of the Freundlich equation and using the two distinct manually selected values for b (one for the low PBC soils and one for the high PBC soils) the best results were obtained. The single point estimate of PBC correlated significantly with PBC (Ozanne and Shaw, 1968) (Eq. 1 & 2). However, a large RMSE was observed and this predicted estimate of sorption is not reliable. These estimations utilise unrealistically high P levels and a 1:10 soil: solution, completely saturating the soil and allowing for most of the added P to react with the soil. This would be highly unlikely when incorporating P fertilizer into the soil thus a simple laboratory incubation method was investigated. A strong linear relationship existed between applied P and percentage extractable P by the three extraction tests used (Bray II, Mehlich III and Olsen) using the incubation method. It was found that at an application rate exceeding 150 mg P kg-1 the regression lines plateaued and that and the percentage extractable P at rates (100 – 150 mg kg-1) correlated highly significantly (R2 = 0.92 and 0.99 respectively) with the percentage extractable P derived from the slope of the regression line of applied P against extractable P. The influence of time also needed to be investigated and it was found that Bray II extractable P only significantly decreased after 1 month and that after 24 hours of equilibration the percentage extractable P remains fairly constant up to one month. Therefore it is possible to get an estimate of the amount of applied P that will be plant available with a single-application incubation method allowing for 24 – 72 h of equilibration. Lastly the Bray II and Olsen extractable P was correlated to plant response in high pH soils of the Northern cape planted to grapevine and citrus, no significant response was observed to the applied P fertilizer due to soil P already being at significant levels. Bray II was most sensitive in detecting applied P and excessive soil P levels, where Olsen was the least effective in detecting applied P. Therefore these soils do run the risk of reaching excessively high P levels which can in turn lead to P loss, micronutrient deficiencies and groundwater contamination. | en_ZA |
| dc.description.abstract | AFRIKAANSE OPSOMMING: Fosfaat (P) bemestings aanbeveelings word gebasseer op ekstraheerbare P vlakke soos gemeet deur een van ‘n verskeindenheid fosfaat ekstraksie toetse en moet faktore soos grond diepte, bulk digthied en die grond se fosfaat buffer kapasiteit insluit. Internationaal en in Suid –Afrika word ‘n verskeindenheid P ekstraksie toetse gebruik met varieerende gewas norms,norms is ook nie beskikbaar vir al die ekstraksie toetse nie. Dus, is die eerste doel van hierdie studie was om die relatiewe agresiwiteit van die onderskeie ekstraksies en die verhouding tussen die ekstraksies te ondersoek op 49 verskillende gronde met varieerde grond fisiese en chemise eienskappe. Die relatiewe doeltreffendheid soos vergelyk met totale P van die ekstraksie toetse was soos volg; 1:2 H2O < Olsen < Colwell < Bray I < Ambic 1 < Bray II < Mehlich III ≈ Citroen suur. Bray II en Mehlich III was die enigste ekstraksies wat statisties bedeuidend met totale P gekorroleer het (R2= 0.78 & 0.76). Sterk liniêre verwantskappe was gevind tussen Bray II. Olsen en Mehlich III ekstraksies en gebasseer op die standard foout van meting sal direkte omskakelings tussen hierdie P ekstraksie toetse die mees betroubaarste wees. Daar kan dus met sekerheid van Olsen na Bray I en Bray II omgeskakel word met ‘n factor van 5.20 en 3.88 respektiewelik. Vir die omskakeling van Bray II na Mehlich III kan ‘n factor van 1.10 gebruik word. Fosfaat buffer kapasiteit (FBK) kan bepaal word meervoudige-punt isotermes en deur gebruik te maak van die Freundlich vergelyking of deur ‘n enkel-punt isterme deur om verskeie aannames te maak. ‘n Stel van 10 verskillende grond-monsters was gebruik met varieërende P bindings bindings vermoëns. ‘n Groot variasie in konstante b was was op gemerk en uiteindellik was daar twee unieke groepe geïdentifiseer met statisties verskillende b waardes. Die beste resultate was verkry deur die grond-monsters met 1000 mg P kg-1 te ekwilibreer en gebruik te maak van die standard Freundlich vergelyking met die twee geselekteerde waardes vir konstante b (een vir die hoë FBK gronde en een vir die lae FBK gronde). Hierdie enkel-punt FBK waarde het statisties beduidend gekorroleer met FBK (Ozanne & Shaw, 1968). Ongeag hierdie sterk korrolasie was ‘n groot RMSE waarde bepaal en hierdie geskatte enkel-punt indikasie van FBK is nie betroubaar nie. Hierdie sorpsie-isotemre skattings van FBK gebruik egter onrealistiese hoë vlakke van P en ‘n 1:10 grond : oplossing, dit lei tot totale water versadiging van die grond end at meeste van die toegediende P met die grond reageer. Dit is onwaarskynlik vir veld toestande en gevolglik was ‘n eenvoudige inkubasie metode ondersoek. ‘n Sterk liniêre verhouding was waargeneem tussen toegediende P en die persentasie ekstraheerbare P vir die drie ekstraksies wat gebruik was ( Bray II, Mehlich III en Olsen ). Dit was vas gestel dat by toedienings hoeveelhede groter as 150 mg P kg-1 het die regressive lyne begin af plat. Die persentasie ekstraheerbare P by toedienigs (100 – 150 mg kg-1) het sterk gekoroleer (R2 = 0.92 and 0.99 onderskeidellik) met die persentasie ekstraheerbare P soos verky van die liniëre regressive lyn van toegediende P teenoor ekstraheerbare P. Die Inloed van tyd was ook ondersoek en daar was bevind dat Bray II ekstraheerbare beduidend afgeneem het na 1 maand van inkubasie end at na 24 uur van ekwilibrasie die persentasie ekstraheerbare P relatief constant bly tot en met 1 maand. Dit is dus moontlik om ‘n indikasie van die hoeveelheid toegediende P wat plant beskikbaar sal wees te kry van ‘n enkele P toediening met ‘n inkubasie periode van 24 – 72 uur. Daar was ook besluit om Bray II en Olsen ekstraheerbare P te korroleer met gewas-reaksie van tafel druiwe en sitrus op hoë pH gronde van die Noordkaap. Geen beduidende effek was waar geneem nie, dit kan toegeskryf word aan die reeds voldoende P vlakke van hierdie gronde. Daar was wel vas gestel dat Bray II meer sensitief was om toegediende P en oormatige P vlakke te meet. Daar is dus ‘n risiko dat P vlakke in hierdie gronde kan op bou tot onreëlmatige vlakke wat tot P verliese, mikro-nutiriënt tekorte en grond-water kontaminasie kan lei. | af_ZA |
| dc.format.extent | 121 pages : illustrations | en_ZA |
| dc.identifier.uri | http://hdl.handle.net/10019.1/105768 | |
| dc.language.iso | en_ZA | en_ZA |
| dc.publisher | Stellenbosch : Stellenbosch University | en_ZA |
| dc.rights.holder | Stellenbosch University | en_ZA |
| dc.subject | Phosphorus fertilizer recommendations | en_ZA |
| dc.subject | Soil fertility | en_ZA |
| dc.subject | Phosphorus extraction tests | en_ZA |
| dc.subject | Soil phosphorus fixation | en_ZA |
| dc.subject | UCTD | en_ZA |
| dc.title | Improving phosphate fertilizer recommendations using soil phosphorus buffer capacity and evaluation of various P extraction tests on a variety of South-African soils | en_ZA |
| dc.type | Thesis | en_ZA |