Influence of tillage and cropping sequence on the soil physical properties, water balance and water use efficiency of wheat (Triticum aestivum L.) and canola (Brassica napus) under rainfed-conditions in the Rȗens sub-region of the Western Cape

Vorster, Abraham Hercules (2015-12)

Thesis (MSc)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: Conservation agriculture has become a very important management strategy around the globe with research showing that decreases in soil disturbance tend to improve soil physical properties over time and therefore positively influencing, amongst others, soil water storage capacity and water usage by crops. In a water scarce country like South Africa, the efficient use of water by crops under rain-fed conditions is of absolute importance. Identifying management practices that increase water use efficiency (WUE) is necessary to develop management strategies ensuring maximum production from seasonal rainfall. A soil water balance is a helpful tool to evaluate the effectiveness of different treatments in terms of WUE. The study was a component trial (2013 and 2014) within a long-term research programme investigating the effect of soil tillage and crop rotation on soil quality at the Tygerhoek Research Farm near Riviersonderend. Three crop rotations: continuous wheat (WWWW), wheat / medic-clover / wheat / medic-clover (WMcWMc) and wheat / canola / wheat / lupin (WCWL) as well as all sequences of the latter two mentioned rotations (WMcWMc & WCWL) were allocated to main plots. The first and last letter in the rotation code refers to the first and last crop planted in the sequence, respectively. Two tillage methods, conventional (CT) and no-till (NT) were allocated to sub-plots and replicated three times. Soil water content (SWC) was recorded in 10 cm increments to a depth of 1 m using a capacitive probe (Diviner 2000). SWC was recorded weekly during the growing season and monthly during the fallow period. Water balances, cumulative evapotranspiration (ΣET), SWC, water use efficiency (WUE) and rainwater use efficiency (RUE) were calculated at the end of the 2013 and 2014 growing seasons. Soil cores were taken at four depth increments: 0-10, 10-20, 20-30 and 30-45 cm. Coarse fragments (>2.0 mm), particle size distribution, water stable aggregate percentage and water retention capacity were determined. Soil microstructure was determined using Dexter's (2004) S-index. Infiltration rate and unsaturated hydraulic conductivity were determined in situ during the 2013/4 fallow period, using a minidisk infiltrometer. Bulk density was also measured in situ, during the fallow, with a calibrated Troxler surface gamma-neutron gauge. ANOVA and Fisher post hoc tests were done to determine statistical significant differences and effects at P < 0.05. The main aims of this study were to investigate to what extent different tillage practices and crop rotation systems influence selected soil physical properties, the water balance and the resultant water use efficiency of wheat and canola, 7-8 years after introduction of the treatment combinations. No statistical differences due to tillage, crop rotation or their combined effect were detected in the soil physical properties or the water balance and grain yield results. However, there was a definite trend for no-tillage performing better in both components of the study. The percentage water stable aggregates were marginally higher under conventional tillage (78.3 %) than no-tillage (76.9 %) and decreased with depth despite clay increase with depth. Bulk density increased significantly with depth while the no-till soils (1464.9 kg.m-3) averaged higher than the conventional tillage treatments (1440.4 kg.m-3). The highest bulk densities were recorded in a rotation that was left fallow during the growing season, before measurements were taken. No-till had a higher infiltration rate (39.9 mm.h-1) and minimally higher unsaturated hydraulic conductivity (4.3 mm.h-1) compared to conventional tillage (29.6 mm.h-1 and 4.1 mm.h-1, respectively). Soil water retention also did not differ significantly between tillage methods, although water retention at high matrix potentials (-10 kPa i.e. field capacity and -50 kPa) were higher in the no-till samples, while water retention was higher in the conventional tillage soil samples at all the lower matrix potentials (-200 kPa to -1500 kPa) analysed. Water retention significantly decreased with depth. Soil microstructure, quantitatively measured with Dexter's (2004) S-index, showed no significant difference between tillage treatments although S-index results were higher for the no-till soils. The soil water content and soil water usage (ΣET) were not significantly affected by tillage although it was generally lower in the conventional tilled soil. It is concluded that higher soil water content leads to higher availability of water for evapotranspiration and thus higher water usage. These results also correlated well with the grain yield, WUE and RUE results that were on average all higher in the no-till plots. Wheat planted after lupins resulted in the highest grain yield (3741.2 kg.ha-1), while the no-till treatment of wheat planted after lupins also gave the highest grain yield (3811.3 kg.ha-1) of all treatments. The yield of canola planted after wheat was marginally lower in conventional tilled treatments. The study found no-tillage had marginally improved soil microstructure and soil physical properties, thus promoting higher soil water content in the soil leading to higher water usage and consequently to higher grain yields, rainwater and water use efficiencies.

AFRIKAANSE OPSOMMING: Bewaringslandbou het om die wêreld heen 'n baie belangrike bestuurs strategie geword met navorsing wat toon dat afname in grondversteuring neig om grondfisiese eienskappe te verbeter en dus daarom ‘n positiewe effek op, onder andere, die grond se water stoor kapasiteit en water gebruik van gewasse het. In 'n water-skaars land soos Suid-Afrika, is die doeltreffende gebruik van water deur gewasse onder droëland van kritiese belang. Identifisering van bestuurspraktyke wat die waterverbruiksdoeltreffendheid (WUE) verhoog is nodig om praktyke te ontwikkel wat die maksimum produksie onder seisoenale reënval verseker. Die grondwater balans is 'n nuttige middel om die doeltreffendheid van die verskillende behandelings te evalueer. Die studie was 'n komponent proef (2013 en 2014) in 'n langtermyn navorsing program wat die effek van grondbewerking en wisselbou op die kwaliteit van grond by die Tygerhoek Navorsingsplaas naby Riviersonderend ondersoek. Drie wisselbou stelses: deurlopende koring (WWWW), koring / medic-klawer / koring / medic-klawer (WMcWMc) en koring / canola / koring / lupien (WCWL) insluitend alle opeenvolgings van koring / canola / koring / lupien (WCWL) was ge-allokeer aan hoof persele. Die eerste en laaste letter in die rotasie-kode verwys onderskeidelik na die eerste en laaste gewas geplant in die opeenvolging. Twee bewerkingsmetodes, konvensionele bewerking (CT) en geen-bewerking (NT) is toegeken aan sub-persele en drie keer herhaal. Grondwaterinhoud (SWC) is in 10 cm inkremente gemeet tot op 'n diepte van 1 m deur gebruik te maak van ‘n kapasitansie meter (Diviner 2000). Grondwaterinhoud is weekliks gemeet tydens die groeiseisoen en maandeliks tydens die braak periode. Waterbalanse, kumulatiewe evapotranspirasie (ΣET), grondwaterinhoud, die watervebruiksdoeltreffendheid (WUE) en reënwaterverbruiksdoeltreffendheid (RUE) is bereken teen die einde van the2013 en 2014 groeiseisoene. Grond monsters is geneem by vier diepte inkremente: 0-10, 10-20, 20-30 en 30-45 cm. Growwe fragmente (> 2.0 mm), deeltjiegrootte verspreiding, water stabiele aggregaat persentasie en waterhouvermoë is bepaal. Grond mikro-struktuur is bepaal met behulp van Dexter (2004) se S-indeks. Infiltrasietempo en onversadigde hidrouliese geleivermoë is gedurende die 2013/4 braak tydperk in situ bepaal met behulp van 'n Mini Disk infiltrometer. Bulkdigtheid is ook in situ gemeet gedurende die braak periode met 'n gekalibreerde Troxler oppervlak gamma-neutron meter. ANOVA en Fisher se LSD posthoc-toetse is bereken om te bepaal of daar statisties beduidende verskille en effekte was by P < 0.05. Die belangrikste doelwitte van hierdie studie was om te ondersoek tot watter mate verskillende bewerkingspraktyke en wisselboustelsels ‘n effek het op die gekose grondfisiese eienskappe, die water balans en gevolglik ook die watergebruik doeltreffendheid van koring en canola, 7-8 jaar na instelling van die bewerking en rotasie kombinasies. Geen statistiese verskille as gevolg van bewerking, wisselbou of hul gekombineerde effek is gevind in die grond fisiese eienskappe of die water balans en graanopbrengs resultate. Daar was egter 'n definitiewe tendens vir geen-bewerking wat beter presteer in beide komponente van die studie. Die persentasie water stabiele aggregate was effens hoër onder konvensionele bewerking (78.3%) in vergelyking met geen-bewerking (76.9%) en daar was 'n afname met diepte ten spyte van klei toename met diepte. Bulkdigtheid het aansienlik toegeneem met diepte, terwyl die geen-bewerking (1464.9 kg.m-3) se gemiddeld hoër was as die van konvensionele bewerking (1440.4 kg.m-3). Die hoogste bulkdigtheid is gemeet in 'n rotasie wat braak gelaat was gedurende die groeiseisoen voordat metings aangeteken is. Geen-bewerking het 'n hoër infiltrasietempo (39.9 mm.h-1) en minimaal hoër onversadigde hidrouliese geleivermoë (4.3 mm.h-1) gehad in vergelyking met konvensionele bewerking (29.6 mm.h-1 en 4.1 mm.h-1, onderskeidelik ) .Grondwaterretensie het ook nie beduidend verskil tussen bewerkingsmetodes nie, hoewel waterretensie by hoë matriks potensiale (-10 kPa-veld kapasiteit en -50 kPa) hoër was in die geenbewerking monsters terwyl dit hoër was in die konvensionele bewerking se grondmonsters by al die laer matriks potensiale (-200 kPa tot -1500 kPa) getoets. Waterretensie het aansienlik afgeneem met diepte. Grond mikro-struktuur, kwantitatief gemeet met Dexter (2004) se S-indeks, het geen betekenisvolle verskil tussen bewerking behandelings getoon nie, hoewel dit hoër was in die geen-bewerking gronde. Die grondwaterinhoud en grondwaterverbruik (ΣET) is nie beduidend beïnvloed deur bewerking nie, hoewel dit oor die algemeen laer was in die konvensionele bewerkte behandelings. Daar is tot die gevolgtrekking gekom dat 'n hoër grondwaterinhoud lei tot hoër beskikbaarheid van water vir evapotranspirasie en dus hoër waterverbruik. Hierdie resultate het ook goed gekorreleer met die graanopbrengs, waterverbruiksdoeltreffendheid en reënwaterverbruiksdoeltreffendheid resultate wat op gemiddeld altyd hoër was in geen-bewerking behandelings. Koring geplant na lupiene het gelei tot die hoogste graanopbrengs (3741.2 kg.ha-1), met die geen-bewerking behandeling van koring geplant na lupiene wat die hoogste graanopbrengs (3811.3 kg.ha-1) van alle behandelings gegee het. Canola geplant na koring was effens laer onder konvensionele bewerking. Die studie het bevind dat geen-bewerking grond mikro-struktuur en fisiese eienskappe marginaal verbeter het, wat aanleiding tot ‘n hoër grondwaterinhoud gee. Dit op sy beurt gee weer aanleiding tot tot ‘n hoër waterverbruik en gevolglik hoër graanopbrengste, wat verder die reënwater- en waterverbruiksdoeltreffendheid bevorder.

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