Evaluating the effect of living mulch species as sustainable practice on soil dynamics in perennial orchards in South Africa

Thesis (MScAgric)--Stellenbosch University, 2019.

ENGLISH ABSTRACT: Two plant (weed) species were investigated for their potential as suitable living mulch species in two perennial fruit orchards in two different climatic regions in South Africa. Tradescantia fluminensis Vell. (Tradescantia) is a perennial herb, well established in a ‘Palmer’ navel orange orchard (Citrus sinensis L.) in the Sundays river valley, in the Eastern Cape, while Bromus diandrus (Ripgut) is a winter annual grass species partially established in a ‘Sunkiss’ plum orchard, Villiersdorp, in the Western Cape. A literature review was conducted to determine the effect of cover crops on soil health and fertility and their role in building more sustainable, low input production systems, as well as the role of living mulches in applying these principles directly in the tree row, as opposed to between tree rows as in conventional cover cropping practices in perennial orchards. The effects of Tradescantia and Ripgut on soil-plant dynamics were quantified using soil waterholding capacity (WHC), soil temperature (ST), soil moisture (SM), microbial biomass (MB), weed suppression (WC) and tree root counts and distribution (RC). For the citrus orchard, an orchard containing Tradescantia (TO treatment) was compared to an adjacent orchard with no mulch and conventional chemical weed to maintain a bare orchard floor (BO treatment). For the plum orchard, measurements were taken in the areas containing the Ripgut mulch (RM treatment), as well as in areas with no Ripgut (CC treatment), where chemical methods are used to control weeds. In the citrus orchards, the WHC was higher in the TO treatment in autumn (5.95 %) and winter (9.68 %). The ST fluctuated less in the TO treatment, especially at shallow depths. The microbial respiration (ppm C), calculated MB, as well as estimated potential mineralisable nitrogen (PMN) was higher in the TO treatment. Weed numbers for the TO treatment were 0.67 per m2 compared to 6 per m2 in the BO treatment. The TO treatment generally displayed higher root concentrations at deeper soil levels compared to the BO treatment, probably due to competition from the Tradescantia in the upper soil layers. In the plum orchard, the WHC was significantly higher in the RM treatment for both the autumn (1.87 %) and winter (1.58 %) samples. The SM was higher in the RM treatment at most soil depths, suggesting improved water infiltration throughout the soil profile. The ST fluctuated less in the RM treatment due to the Ripgut’s buffering effect. There was no significant difference between the treatments for microbial respiration, calculated MB and estimated PMN. Weed numbers were significantly lower in the RM treatment (2.17 weeds per m2) compared to the CC treatment (11.17 weeds per m2). Root numbers were higher in the RM treatment, however, the presence of Ripgut roots in the top 20 cm made it difficult to accurately quantify root counts in this region. Both Tradescantia and Ripgut influenced soil health and water dynamics positively, while providing sufficient weed suppression. However, further investigations are recommended to quantify the threats posed by the invasive nature of these plants before commercial implementation.

AFRIKAANSE OPSOMMING: Twee onkruid spesies is ondersoek vir hul potensiaal as gepaste lewende deklae in meerjarige vrugteboorde in twee areas met verskillende klimaatstoestande, in Suid-Afrika. Tradescantia fluminensis Vell. (Tradescantia) is ‘n meerjarige kruid, wat goed gevestig is in ‘n ‘Palmer’ nawel lemoenboord (Citrus sinensis L.) in die Sondagsriviervallei, Oos-Kaap, terwyl Bromus diandrus (Ripgut), ‘n eenjarige wintergras spesie is wat gedeeltelik gevestig is in ‘n ‘Sunkiss’ pruimboord, Villliersdorp, in die Weskaap. ‘n Literatuurstudie het die effek van lewende deklae op grondgesondheid en –vrugbaarheid in die bou van meer volhoubare, lae inset produksie sisteme ondersoek, sowel as die bydra van hierdie beginsels direk in die boomry, in plaas van die konvensionele trekkerrye, in meerjarig boorde. Die effekte van hierdie spesies as lewende deklae op grond-plant-dinamika is ondersoek deur grondgesondheidsmerkers, onkruidbeheer en die moontlike effek van kompetisie op boom wortelverspreiding te gekwantifiseer. Aanwysers was grondwaterhouvermoë (WHC), grondtemperatuur (ST), grondvog (SM), mikrobiese biomassa (MB), onkruid onderdrukking (WC) en wortel hoeveelheid en verspreiding (RC). ‘n Sitrusboord met Tradescantia (TO behandeling) is vergelyk met ‘n naasliggende boord met geen lewende deklae (BO behandeling) nie. In die pruimboord, is Ripgut (RM behandeling) vergelyk met areas sonder Ripgut (CC behandeling) en chemiese onkruidbeheer. In die sitrusboord, was die WHC hoër in die TO behandeling in beide herfs (5.95 %) en winter (9.68 %). Die ST het minder gevarieër in die TO behandeling, veral by vlak grond. Mikrobiese respirasie (ppm C) bepaal mikrobiese biomassa en voorspel ook die moontlike potensieël mineraliseerbare stikstof en was hoër in die TO behandeling, wat dui op hoër grondvrugbaarheid. Onkruidtellings in die TO behandeling was 0.67 per m2 in vergelyking met 6 per m2, in die BO behandeling. Die TO behandeling het hoër wortelkonsentrasies by dieper grondvlakke getoon, in verglyking met die BO behandeling, wat moontlik verklaar word deur kompetisie van die Tradescantia in die boonste grond lae. In die pruimboord, was die waterhouvermoë beduidend hoër in herfs (1.87 %) en winter (1.58 %) in die RM behandeling Die grondvog was ook hoër in die RM behandeling by die meeste grond dieptes. Die hoër waterhouvermoeë en grondvog in die RM behandeling impliseer dat water infiltrasie deur die hele grondprofiel verbeter is deur Ripgut. Die grondtemperatuur het minder gevarieër in die RM behandeling en die Ripgut het die boonste lae van die grond beskerm teen temperatuur uiterstes. Daar was geen beduidende verskille tussen die behandelings in terme van mikrobiese respirasie, berekende mikrobiese biomass en potensieël mineraliseerbare stikstof nie. Onkruid getalle was beduidend minder in die RM- (2.17 plante per m2) vergeleke met die CC behandeling (11.17 plante per m2). Die totale worteltellings was hoër in die RM behandeling, maar die teenwoordigheid van Ripgut wortels in die boonste 20 cm het akkurate kwantifisering van die wortels in hierdie area bemoeilik. Beide Tradescantia en Ripgut het ‘n positiewe invloed op grondgesondheid en –waterdinamika, terwyl dit ook onkruidgroei onderdruk. Verdere studies word voorgestel om die impak van die indringende natuur van hierdie plante te ondersoek voor kommersiële implimentering.

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