Examining links between soil fertility and various nutrient management regimes on yield and quality of sweet potato (Ipomoea batatas)

Mahanjana, Aviwe (2018-03)

Thesis (MSc)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: Food producers are faced with the task of meeting growing global food demands that follow a surging and increasingly wealthy population. This pressure is compounded by a global movement towards sustainability that necessitates agriculturists, specifically, crop growers, to ensure that intensive food production does not strain the essential resources that future generations will need in order to feed themselves. In other words, crop production must be done in such a way that crop yields are either maintained over time or enhanced through good management practices that prioritise two essential resources: the soil and the environment. Maintaining or enhancing soil fertility is the single most important way to help ensure sustainable crop yields. One of the ways to accomplish the maintenance of soil fertility and minimize damage to the environment, without compromising crop yields is to replace or reduce inorganic fertilisers use through the inclusion of organic and more sustainable soil conditioners like compost and co-applications of organic and inorganic fertilisers. There are few comparative studies that assess the impact of chemical fertilisers and numerous sustainable and organic soil nutrient management systems within the African context. The African context is characterised by a dire need for nutritious crops like sweet potato (Ipomoea batatas (L.) Lam) and small-scale farming operations. In keeping with this, a study was conducted to determine and compare the effects of inorganic NPK fertiliser and four alternative soil nutrient management regimes on soil fertility. The study also investigated the effects of these different soil nutrient management regimes on sweet potato growth, yield and nutrition. Experiments took place at the Welgavellen Experimental Farm in Stellenbosch between December 2016 and May 2017. A randomised complete block design was used where each treatment was represented in each block to give four replicates. The treatments were an inorganic fertiliser applied at the recommended rate of 100 kgN.ha-1 in an NPK formulation (100% CM); a co-application of biochar applied at a rate of 2% and inorganic fertiliser applied at 50 kgN.ha-1, half the recommended rate (2% BC + 50% CM); a compost comprised of 30% food waste and 70% dairy cow manure applied as a 5 cm layer (FWC); a commercial organic fertiliser applied in the same formulation as the inorganic fertiliser and also applied at 100 kgN.ha-1 (COF) and manure-derived anaerobic digestate (MAD) applied as a liquid fertiliser at a rate of 17 L per plant. The control was unfertilised soil. Both the organic and inorganic fertilisers were applied in a split fashion with half applied at planting and the rest 6 weeks after transplanting. ANOVA and the Tukey HSD were used to analyse the data. Canopy coverage was recorded 6 weeks after transplanting and was observed to be most extensive in the integrated 2% BC + 50% CM treatment, followed by the FWC treatment (73.67 % and 65%, respectively). These were significantly higher (P < 0.05) than the 100% CM, COF and MAD treatments, which only produced 41.08%, 39.92% and 39.2% canopy coverage, respectively. Although, the FWC and 2% BC + 50% CM treatments also resulted in the high fresh and dry shoot biomass at harvest (16 weeks after transplanting), it was not significantly different to the rest of the treatments. The integrated treatment produced the significantly higher fresh tuber yields (26.41 t.ha-1), followed by the 100% CM treatment 19.02 t.ha-1 and FWC 14.55 t.ha-1. Yields were lowest in the COF and MAD treatments. The same trend was observed for marketable yields. The longest tubers were observed under the 2% BC + 50% CM treatment. Tuber diameter, harvest index were unaffected. Sweet potato moisture, crude fiber and starch content were unaffected and did not differ significantly (P > 0.05). However, protein was significantly different (P < 0.05) from the unfertilised control and the MAD treatment. The inorganic treatment resulted in a 2.12% protein content in tubers. Biochar trailed behind by a relatively large margin and resulted in tubers with a 1.45% protein content. Soil cation exchange capacity, pH and acidity were most improved under the 2% BC + 50% CM and FWC treatments, while the highest acidity and lowest pHs and CEC’s were observed under the remaining regimes. Mineral content was richest in soils amended with FWC, however, the highest NH4+ and NO3- levels were observed under the control. Soil fungal and aerobic microbe populations were unaffected by fertiliser regime. COF and MAD performed very poorly as soil conditioners and crop yield enhancers and proved to be inadequate organic alternatives to inorganic soil nutrient management systems. In contrast, 2% BC + 50% CM produced higher yields than the inorganic fertiliser despite the 50% reduction in NPK. Soil FWC enhanced yield satisfactorily, albeit to a slightly lower extent than 100% CM. Both alternative regimes are viable sustainable alternatives to completely inorganic soil nutrient management regimes. However, the high nutrient content and imbalance in FWC may result in yield penalties if certain cations like Na+ compete with cations like K+ and Ca2+.

AFRKAANSE OPSOMMING: Voedselprodusente word gekonfronteer met die taak om groeiende globale voedselbehoeftes te ontmoet wat gepaard gaan met 'n groeiende en toenemend ryk bevolking. Hierdie druk word vererger deur 'n wêreldwye beweging na volhoubaarheid wat landbouers, spesifiek gewasprodusente, noodsaak om te verseker dat intensiewe voedselproduksie nie die noodsaaklike hulpbronne wat toekomstige geslagte benodig om hulself te voed, benadeel nie. Met ander woorde, oesproduksie moet so gedoen word dat oesopbrengste oor die jare gehandhaaf word of verbeter word deur goeie bestuurspraktyke wat twee noodsaaklike hulpbronne prioritiriseer: die grond en die omgewing. Bevordering of verbetering van grondvrugbaarheid is die enkele belangrikste manier om volhoubare gewasopbrengste te verseker. Een van die maniere om die vrugbaarheid van grond te bewerkstellig, sonder om opbrengste nadelig te beïnvloed is om anorganiese bemestingstowwe te vervang of te verlaag deur die insluiting van organiese en volhoubare grondverbeterings soos kompos en mede-toedienings van organiese en anorganiese bemestingstowwe. Daar is min wye vergelykende studies wat die impak van chemiese bemestingstowwe en volhoubare en organiese grond nutriëntbestuurstelsels binne die Afrika-konteks assesseer. Die Afrika-scenario word gekenmerk deur 'n ernstige behoefte aan voedsame gewasse soos patats (Ipomoea batatas (L.) Lam) en kleinskaalse boerderybedrywighede. In ooreenstemming hiermee is 'n studie gedoen om die effekte van anorganiese NPK- kunsmis en vier alternatiewe grondvoedingsbestuursregimes op grondvrugbaarheid, soetpatat-groei, opbrengs en voeding te bepaal en te vergelyk. Die studie het op die Welgavellen-proefplaas in Stellenbosch plaasgevind tussen die Desember 2016 en Mei 2017. 'n Gekontroleerde volledige blokontwerp is gebruik waar elke behandeling in elke blok verteenwoordig was om vier replikate te gee. Die behandelings was 'n anorganiese bemesting toegedien teen die aanbevole dosis van 100 kg N.ha-1 in 'n NPK-formulering (100% CM); 'n mede-toepassing van ‘biochar’ toegedien teen 'n koers van 2% en anorganiese bemesting toegedien teen 50 kg N.ha-1, die helfte van die aanbevole dosis (2% BC + 50% CM); Kos afval kompos toegepas as 'n 5 cm laag (FWC); 'n kommersiële organiese bemesting wat in dieselfde formulering as die anorganiese kunsmis toegepas word, is ook toegedien teen 100 kg N.ha-1 (COF) en mis-afkomstige anaërobiese verteer (MAD) wat as 'n vloeibare kunsmis toegedien word teen 'n koers van 17 L per plant. Die beheer was onbevrugte grond. Beide organiese en anorganiese bemestingstowwe is op 'n gesplete wyse toegedien. ANOVA en die Tukey HSD is gebruik om die data te analiseer. Die plantbedekking is 6 weke na uitplant aangeteken en is waargeneem as die mees omvattende in die geïntegreerde 2% BC + 50% CM behandeling, gevolg deur die FWC behandeling (onderskeidelik 73,67% en 65%). Dit was aansienlik hoër (P <0,05) as die 100% CM, COF en MAD behandelings, wat onderskeidelik slegs 41.08%, 39.92% en 39.2% canopy dekking behaal het. Alhoewel die FWC en 2% BC + 50% CM behandelings ook op die oes (16 weke na uitplanting) hoë vars en droë loot biomassa tot gevolg gehad het, was dit nie beduidend teenoor die res van die behandelings nie. Die geïntegreerde behandeling het die grootste vars knol opbrengs (26.41 t.ha-1) opgelewer, gevolg deur die 100% CM behandeling 19.02 t.ha-1 en FWC 14.55 t.ha-1. Opbrengs was laagste in die COF en MAD behandelings. Dieselfde tendens is waargeneem vir bemarkbare opbrengs. Die langste knolle is waargeneem onder die 2% BC + 50% CM behandeling. Tuber deursnee, oes-indeks was onveranders. Soet aartappel vog, ruwe vesel en stysel inhoud was nie beïnvloed nie en het nie beduidend verskil nie (P> 0.05). Proteïene was egter aansienlik anders (P <0.05) van die onbevrugte beheer en die MAD-behandeling. Die anorganiese behandeling het gelei tot 'n 2,12% proteïeninhoud in knolle. Biochar het agtergebly met 'n relatief groot marge en het tot knolle met 'n 1,45% proteïeninhoud gelei. Grond kationuitruilingskapasiteit (KUK), pH en suurheid was optimaal onder die 2% BC + 50% CM en FWC behandelings, terwyl die hoogste suur en laagste pH en KUK onder die oorblywende regimes waargeneem word. Minerale inhoud was die rykste in gronde wat met FWC gewysig is, maar die hoogste NH4+ en NO3- vlakke is onder die beheer waargeneem. Grondskimmel- en aërobiese mikrobe bevolkings is nie beïnvloed deur kunsmisbeheer nie. COF en MAD het baie swak gedoen as grond ‘opkikkers’ en gewasopbrengsverbeteraars en blyk onvoldoende organiese alternatiewe vir anorganiese grondvoedingsbestuurstelsels te wees. In teenstelling hiermee het 2% BC + 50% CM hoër opbrengste as die anorganiese kunsmis geproduseer ten spyte van die 50% -verlaging in NPK. Grond FWC het opbrengste bevredigend verbeter, al is dit ietwat minder as die 100% CM behandeling. Beide alternatiewe regimes is lewensvatbare volhoubare alternatiewe vir volledige anorganiese grondvoedingsbestuursregimes. Die hoë nutriëntinhoud en wanbalans in FWC kan egter opbrengsstraf tot gevolg hê indien sekere katione soos Na+ kompeteer met katione soos K+ en Ca2+.

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