The effect of once-off tillage on selected soil physical and chemical properties and resultant crop response on a shale derived soil under no-till in the Swartland sub-region of the Western Cape

Leygonie, Izane Riana (2016-03)

Thesis (MSc)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: Conservation agriculture is widely adopted by farmers who claim that any soil disturbance will be detrimental to both soil physical and chemical properties. However, as the effect of reduced tillage and maximum stubble retention becomes more prevalent, secondary effects, positive or negative, may develop at soil surface or in the upper layers of the soil profile. Therefore, an increased interest in strategic tillage have arose in order to address the emerging CA constraints. CA can be defined as the avoidance of mechanical soil disturbance combined with the maintenance of a permanent soil cover and the implementation of a crop rotation system. The study formed part of a long-term field trial established in 2007 investigating different crop and crop/pasture systems under no-tillage at the Langgewens Research Farm of the Western Cape Department of Agriculture, near Moorreesburg, Swartland, Western Cape, South Africa. The study was conducted during the 2014 and 2015 growing seasons to assess the effect of once-off tillage (till once in 10 years) in long-term medic/wheat/medic/wheat (McWMcW), wheat/lupin/wheat/canola (WLWC), lupin/wheat/canola/wheat (LWCW), wheat/medic/wheat/medic (WMcWMc) and canola/wheat/lupin/wheat (CWLW) cropping systems (the last letter in the sequence represents the crop that was in field at sampling time). Three once-off tillage treatments namely: continuous no-till (NT, soil left undisturbed until planting and then planted with an Ausplow), deep tine non-inversion tillage (DT) and mouldboard inversion tillage (MP) were conducted. Tillage treatments were conducted on 26 and 27 May, in 2014 and 2015, respectively. NT treatments were regarded as a control reference for this study. In February 2014 and May 2015, soil samples were taken at 0-50, 50-100, 100-200, 200-300 and 300-400 mm depth increments. Soil water content measurements were taken weekly (during growing season) and monthly (during fallow season) to a depth of 800 mm in order to determine the soil water balance and the resultant crop performance. The objectives of the study were to investigate the effect of once-off tillage of no-till soil on: i) the soil physical and chemical properties, ii) the soil water balance parameters, (iii) and the resultant crop performance (water use efficiency and rainfall use efficiency) as affected by physical, chemical and soil water balance related properties. Significant differences in particle size distribution between tillage treatments at different depths were found (P = 0.05). Tillage had no significant effect on coarse fragment percentage in both wheat systems investigated in the study (P = 0.05), although, in the canola after wheat system the DT treatment resulted in a significant higher coarse fragment percentage in the 200-300 mm soil depth (P ≤ 0.05). The general coarse fragment percentage trend observed was an increase with depth. This result is however not indicative of a mechanical sieving action which is usually expected after the repeatable conduction of conventional tillage practices. DT was the only treatment to result in significant aggregate stability decreases in both the WLWC and LWCW crop rotation systems. In the 300-400 mm soil depth (LWCW) a significantly higher aggregate stability was observed for the NT treatment compared to the DT treatment (P = 0.1514) while in the 0-100 mm (WLWC) soil depth a significantly higher aggregate stability was observed for the NT treatment compared to the DT treatment (P = 0.0078). The aggressive mechanical action of the deep tine implement was responsible for the aggregate stability decrease. Aggregate stability decreased with depth and therefore results correlated with SOC results due to increases in aggregate stability percentages in soil depths where increases in SOC were observed. According to results obtained once-off tillage had no significant effect on macro-aggregate density (P = 0.05) and it was concluded that the sample clods used for measurement were not representative of the prevailing soil conditions after tillage conduction. Hydraulic conductivity showed significant differences between treatments. Although not always significant, both the NT and DT treatments showed the highest hydraulic conductivity compared to MP for all cropping systems investigated (P = 0.05). The increase in hydraulic conductivity for the DT treatment can be explained by a more favourable soil structure created by the rip action, while the increase under NT was contributed to the preservation of soil macro-pores which is formed by earthworms and decayed plant roots as well as the present mulch layer. A MP tillage action leaves a soil surface bare which exposes the soil surface to compacting effects of rainfall and soil resettlement. MP tillage had a significant effect on pH (KCl and H₂O) while DT tillage had no significant effect. Significant differences were however only observed in the 0-50 mm and 50-100 mm soil depth increments (P = 0.05). The general trend was an increase in pH (KCl and H₂O) with depth and therefore it was concluded that differences were not attributed to a tillage effect but rather to the inherent mother material properties. It is well known that mother materials have higher pH values compared to weathered soil materials. A decreasing trend with soil depth was observed for electrical conductivity. DT proved to be the least favourable in terms of the leaching of salts due to EC increases while NT proved to be the most favourable. The SOC content was not influenced by the single tillage operation as no significant differences were observed between all tillage treatments at all measured depths (P = 0.05). The highest SOC content was observed in the 0-100 mm soil depth where after SOC decreased with each measured depth. The Active C content was not significantly influenced by a once-off tillage operation as was expected in 2014 (P = 0.0005). The prevailing low soil temperatures did not allow active microorganism activity. In 2015 (1 year after tillage) a significant increase in active C content (P = 0.0258) was observed for DT for both the medic MP tillage had a significant effect on pH (KCl and H₂O) while DT tillage had no significant effect. Significant differences were however only observed in the 0-50 mm and 50-100 mm soil depth increments (P = 0.05). The general trend was an increase in pH (KCl and H₂O) with depth and therefore it was concluded that differences were not attributed to a tillage effect but rather to the inherent mother material properties. It is well known that mother materials have higher pH values compared to weathered soil materials. A decreasing trend with soil depth was observed for electrical conductivity. DT proved to be the least favourable in terms of the leaching of salts due to EC increases while NT proved to be the most favourable. The SOC content was not influenced by the single tillage operation as no significant differences were observed between all tillage treatments at all measured depths (P = 0.05). The highest SOC content was observed in the 0-100 mm soil depth where after SOC decreased with each measured depth. The Active C content was not significantly influenced by a once-off tillage operation as was expected in 2014 (P = 0.0005). The prevailing low soil temperatures did not allow active microorganism activity. In 2015 (1 year after tillage) a significant increase in active C content (P = 0.0258) was observed for DT for both the medic after wheat and the wheat after canola systems which was explained by an increase in microbe activity due to favourable soil conditions. Tillage had a negligible small effect on SWC during the 2014 growing season. Significant differences were only observed between NT and DT after big rainfall events and at the end of the growing season (P = 0.05). SWC readings in 2014 ranged between 35-270 mm for all tillage treatments and crop rotation systems measured. ET values varied according to the crops developing stage as well as the available SWC and external environmental factors (rainfall and temperature). Tillage had no significant effect on ET as no significant difference in ΣET was observed at the end of the 2014 growing season (P = 0.05). Tillage had no effect on the ability of the soil to store water during the 2014/2015 fallow season as no significant difference in SWC were observed between treatments. More valuable results would have been obtained during the fallow season if technical difficulties could have been solved. The total amount of rainfall recorded during the 2015 growing season was 31% lower compared to rainfall recorded during 2014 and therefore SWC readings during 2015 ranged between 10-140 mm. Generally, the highest SWC was observed for the MP treatment for all crop rotation systems and on several dates significant higher SWC readings were observed for MP compared to NT and DT (P ≤ 0.05). MP resulted in a significantly higher ΣET at the end of the 2015 growing season (P = 0.05). Due to limited information on the topic a valuable explanation for the higher SWC and ΣET under MP was not found. In the 2014 growing season no significant differences in wheat yield were found between all tillage treatments and crop rotation systems investigated while for the 2015 growing season NT resulted in a significantly higher wheat yield (P =0.0274) compared to both DT and MP in the CWLW system which was attributed to the crop residues present. Although the lowest biomass was obtained for the MP treatment no significant difference between treatments were observed for medics (P = 0.9010). The lower biomass under medics was contributed to the transportation of the self-regenerating medic seeds during the inversion plough action which resulted in seedling emergence delays and poor crop stand. When comparing the 2014 and 2015 grain yield results a 71% decrease in canola yield was observed while a 57% decrease in wheat grain yield was observed in the 2015 growing season. Even though the crops underwent a longer growing season during 2015 a total of 31% less rainfall was recorded during the 2015 growing season compared to the 2014 growing season and therefore lower grain yields were expected. Once-off tillage had no significant effect on WUE and RUE in both the 2014 and 2015 growing seasons for all tillage treatments and crop rotation systems tested.

AFRIKAANSE OPSOMMING: Bewaringsboerdery word wêreldwyd deur boere aangeneem wie vasgestel het dat enige grondversteuring noodlottig is vir beide grondfisiese en grondchemiese eienskappe. Alhoewel, soos wat die effek van verminderde bewerking en maksimum stoppelbehoud meer opvallend raak, kan sekondêre effekte, positief of negatief, ontwikkel by die grondoppervlak of in die boonste grondlae van die grondprofiel. Daarom het ‘n toenemende belangstelling in strategiese bewerking na vore gekom om sodoende die bewaringsbewerking beperkings te adresseer. Die studie het deel gevorm van ‘n langtermyn proef, gevestig in 2007, wat verskillende gewas/weiding sisteme onder geen bewerking ondersoek by Langgewens Navorsingsplaas van die Departement Landbou Wes-Kaap naby Moorreesburg, Swartland, Suid-Afrika. Die studie is uitgevoer gedurende die 2014 en 2015 groeiseisoen om sodoende die effek van eenmalige bewerking (bewerk eenmalig in 10 jaar) van geen bewerkde grond in langtermyn medic/koring/medic/koring (McKMcK), koring/lupiene/koring/kanola (KLKC) and lupiene/koring/kanola/koring (LKCK), koring/medic/koring/medic (KMcKMc) en kanola/koring/lupiene/koring (CKLK) gewassisteme (die laaste letter in die volgorde verteenwoordig die gewas teenwoordig in die veld by monsterneming). Drie eenmalige bewerkingsbehandelings is uitgevoer naamlik: aanhoudende geen-bewerking (NT, grond onversteur gelaat tot plant en dan word geplant met ‘n Ausplow), diep tand geen omkering bewerking (DT) skaarploeg omkeer bewerking (MP). Bewerkings is uitgevoer op 26 en 27 Mei 2014. Geen bewerkings behandelings is geag as kontrole vir die studie. In Februarie 2014 en 2015 is grondmonsters geneem by 0-50, 50-100, 100-200, 200-300 en 300-400 mm diepte inkremente. Grondwaterinhoud metings is weekliks geneem (gedurende groeiseisoen) en maandeliks (gedurende braakseisoen) tot ‘n diepte van 800 mm om sodoende die grondwaterbalans te bepaal en die resulterende gewasprestasie. Die objektiewe van die studie was om die effek van eenmalige bewerking van geen bewerkde grond te ondersoek op: i) die grond fisiese en chemise eienskappe soos geaffekteer deur eenmalige bewerking, ii) die grondwaterbalans soos geaffekteer deur eenmalige bewerking, iii) en die resulterende gewasprestasie (waterverbruiksdoeltreffendheid en reënvalverbruiksdoeltreffendheid) soos geaffekteer deur fisiese, chemiese en grondwaterbalans verwante eienskappe na die uitvoer van eenmalige bewerking. Betekenisvolle verskille in partikelgrootte verspreiding tussen behandelings is gevind (P = 0.05). Betekenisvolle verskille is mees kenmerklik gevind vir die growwe- en fyn sandfraksie en tussen NT en DT behandelings. Bewerking het geen betekenisvolle effek gehad op growwe fragment persentasie in beide koring stelsels (P = 0.05), alhoewel, DT het geresulteer in betekenisvolle hoër growwe fragment persentasies vergelykend met MP en NT behandelings in die 200-300 mm gronddiepte (P ≤ 0.05). Die algemene tendens was ‘n verhoging in growwe fragment persentasie met diepte. Die tendens was alhoewel nie verteenwoordigend van ‘n meganiese siftings aksie wat gewoonlik resulteer na aanhoudende uitvoer van konvensionele bewerkingspraktyke nie. DT het ‘n betekenisvolle effek op aggregaatstabiliteit gehad. DT het in ‘n betekenisvolle laer aggregaatstabiliteit geresulteer vergelykend met NT in die 300-400 mm gronddiepte in ‘n LKCK stelsel (P = 0.1514), asook, DT het ‘n betekenisvolle laer aggregaatstabiliteit gehad vergelykend met NT in die 0-100 mm diepte vir WLWC (P = 0.0078). Aggregaatstabiliteit het verlaag met diepte en daarom korrelleer resultate met grond organiese koolstof (GOK) resultate as gevolg van verhogings in aggregaatstabiliteit persentasies in gronddieptes waar verhogings in GOK opgemerk is. Eenmalige bewerking het geen betekenisvolle effek op makro-aggregaat digtheid gehad nie (P = 0.05). Daar was tot die gevolgtrekking gekom dat kluitmonsters nie ‘n aanduiding was van die heersende grondkondisies soos na bewerking te verwagte is nie. Hidrouliese geleiding het betekenisvolle verskille tuseen behandelings getoon. Alhoewel nie altyd betekenisvol verskillend nie, het beide NT en DT behandelings die hoogste hidrouliese konduktiwiteit getoon vergelykend met MP vir alle stelsels betrokke (P = 0.05). Die verhoging in hidrouliese konduktiwiteit kan toegeskryf word aan ‘n meer gunstige grondstruktuur geskep deur die tandaksie, terwyl die verhoging onder NT toegeskryf word aan die preservering van die grond se makroporieë gevorm der erdwurms en verotte plantwortels asook die teenwoordigende restelaag. Die MP bewerking laat die grond naak na bewerking wat die grond blootstel aan partikel hervestiging en konsolidasie onder die invloed van reënval. MP het ‘n betekenisvolle effek op pH (KCl and H₂O) gehad terwyl DT geen betekenisvolle effek getoon het nie. Betekenisvolle verskille is slegs waargeneem in die 0-50 mm en 50-100 mm diepte. Die algemene tendens was ‘n verhoging in pH (KCl en H₂O) met diepte en daarvoor is daar tot die gevolgtrekking gekom dat verskille nie toegeskryf is aan bewerking nie maar eerder die teenwoordigende moedermateriaal. Dit is welbekend dat moedermateriale hoër pH’s besit as verweerde grondmateriale. ‘n Verlagende tendens met gronddiepte is opgemerk vir elektriese konduktiwiteit (EC). DT het bewys om ongunstig te wees ten opsigte van loging van soute as gevolg van EC verhogings terwyl NT die meeste gunstig was vir die doel. Die GOK inhoud was nie beinvloed deur die enkele bewerkingsoperasie nie omdat geen betekenisvolle verskille met diepte opgemerk is nie (P = 0.05). Die hoogste GOK inhoud is gesien in die 0-100 mm gronddiepte waar GOK met elke gemete diepte verlaag het vir alle behandelings. Die aktiewe koolstof inhoud was nie betekenisvol beinvloed deur ‘n eenmalige bewerkings operasie nie soos wat te verwagte was in 2014 as gevolg van heersende koue grondtemperature wat mikro organisme aktiwiteit vertraag (P= 0.0005). In 2015 (1 jaar na bewerking) is ‘n betekenisvolle verhoging in aktiewe koolstof opgemerk (P = 0.0258) vir DT in beide medic na koring en koring na canola stelsels as gevolg van ‘n verhoging in mikrobe aktiwiteit as gevolg van gunstige grondkondisies. Bewerking het ‘n weglaatbare klein effek gehad op grondwaterinhoud (GWI) die 2014 groeiseisoen. Betekenisvolle verskille is gesien tussen NT en DT na groot reënval gebeurtenisse en aan die einde van die groeiseisoen (P = 0.05). GWI lesings in 2014 het gevarieër tussen 35-270 mm vir alle bewerkingsbehandelings en gewasrotasiestelsels. Bewerking het geen betekenisvolle effek op ET gehad anngesien geen betekenisvolle verskil in ΣET opgemerk is aan die einde van die 2014 groeiseisoen (P = 0.05). Geen betekenisvolle verskil tussen bewerkingsbehandelings is opgemerk gedurende die 2014/2015 braakseisoen. Die totale hoeveelheid reënval opgeneem gedurende die 2015 groeiseisoen was 31% laer as die hoeveelheid opgeneem gedurende 2015. Die hoogste GWI is opgemerk vir die MP behandelings vir alle gewasrotasiesisteme en op verskeie datums is betekenisvol hoër GWI lesings opgeneem vir MP (P = 0.05). MP het geresulteer in ‘n betekenisvol hoër ΣET aan die einde van die 2015 groeiseisoen (P = 0.05). As gevolg van ‘n tekort aan navorsing oor die onderwerp is geen verklaarbare rede gevind vir die hoë waterinhoud en ΣET onder MP. In die 2014 groeiseisoen is geen betekenisvolle verskil gevind tussen bewerkingsbehandelings en gewasrotasiestelsels ondersoek terwyl daar in die 2015 groeiseisoen ‘n betekenisvolle hoër opbrengs opgeneem is vir NT in die CKLK stelsel (P = 0.9010). Alhoewel die laagste biomassa vir MP opgeneem is is geen betekenisvolle verskil gevind in biomassa resultate nie (P = 0.0274). Wanneer die 2014 en 2015 resultate vergelyk word is ‘n 71% verlaging in opbrengs resultate vir kanola opgemerk terwyl ‘n 57% verlaging in koring graanopbrengs opgemerk is in 2015. Eenmalige bewerking het geen betekenisvolle effek gehad op waterverbruiksdoeltreffendheid en reënvalverbruiksdoeltreffendheid in beide 2014 en 2015 vir alle bewerkingsbehandelings en gewasrotasiestelsels ondersoek.

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