The effect of irrigation scheduling on the performance of young apple trees in newly established orchards

Stofberg, Aline (2018-03)

Thesis (MScAgric)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: During a study conducted in a newly established orchard on a gravelly soil in Grabouw, the most effective irrigation schedule for optimum performance, including root growth and root distribution, of young apple trees was determined. In order to be profitable, apple trees in newly established orchards must fill their allocated space as soon as possible. Soil water status and root growth distribution are believed to be major determining factors in achieving such a favourable effect. Malus domestica „Bigbucks‟ (a mutation of „Corder Gala‟) with an average size of 1.8 metres grafted on MM109 rootstocks were subjected to three different irrigation cycles from December 2016 to May 2017. Treatment one (T1) was a short irrigation cycle, treatment two (T2) was a medium cycle and treatment three (T3) was a long irrigation cycle. Between December 2016 and May 2017, T1 received ca. 10 mm of water every 3 to 4 days, T2 received ca. 20 mm water every 7 days and T3 received ca. 30 mm water every 14 days. Rainfall to an amount of 153 mm also added to the water supply of the trees. Physical and chemical properties of the soil were determined, followed by the installation of irrigation equipment, soil water measuring instruments and rhizotrons for studying roots in situ several times during the season. Irrigation systems were equipped with controllers that were operated remotely by cell phones and soil water measurements were logged continuously. At the end of the season (May 2017) tree response to irrigation treatments was determined by measuring stem circumference and shoot growth. Root studies using the soil profile wall method was carried out to evaluate final root distribution after the first season. The evapotranspiration (ET) of each irrigation treatment during the growing season was calculated using the root-zone water balance equation as described by Hillel (2004). The ET at the end of the growing season was 644.3 mm, 580.1 mm and 568.5 mm for T1, T2, and T3, respectively. All three treatments received a sufficient amount of water during the growing season as the lower ET values of T2 and T3 restricted neither vegetative nor root growth of the apple trees. There was no significant difference between the three treatments in terms of shoot growth and trunk circumference. Rhizotrons were used to determine total root length densities. At the end of the growing season T2 had the highest total root length density, followed by T3 and T1. The use of rhizotrons to study roots in situ proved to be successful and cost effective. The rooting index that was determined using the profile wall method showed that soil conditions were more favourable for the two driest treatments, T2 and T3, than T1. These two treatments (T2 and T3) had significantly higher rooting densities throughout the soil profile, grew to deeper soil layers at a greater distance from the tree and had a significantly higher mean amount of roots in the clayey textured subsoil than T1. This finding implies that longer irrigation cycles produced bigger root systems and that such trees will be less prone to drought.

AFRIKAANSE OPSOMMING: Gedurende 'n studie wat in 'n nuutgevestigde boord op 'n gruiserige grond in Grabouw gedoen is, is die mees effektiewe besproeiingskedulering vir die optimale prestasie, insluitende wortelgroei en -verspreiding, vir jong appelbome, bepaal. Appelboome in nuutgevestigde boorde moet so gou as moontlik hulle geallokeerde spasie vul om winsgewend te wees. Die hoeveelheid grondwater en die wortelverspreiding van bome word as belangrike bepalende faktore geag om so „n gunstige effek te bewerkstellig. Drie verskillende besproeiingsiklusse is toegepas op Malus domestica 'Bigbucks' ('n mutasie van 'Corder Gala') met „n gemiddelde grootte van 1.8 meter wat geënt is op MM109 onderstamme vanaf Desember 2016 tot Mei 2017. Behandeling een (B1) was 'n kort besproeiingsiklus, behandeling twee (B2) 'n medium besproeiingsiklus en behandeling drie (B3) was 'n lang besproeiingsiklus. Vanaf Desember 2016 tot einde Mei 2017 het B1 ongeveer 10 mm water elke 3 tot 4 dae ontvang, B2 het ongeveer 20 mm elke 7 dae ontvang en B3 het ongeveer 30 mm elke 14 dae ontvang. Reënval van 153 mm het ook bygedra tot die watervoorsiening van die bome. Die fisiese en chemiese eienskappe van die grond is bepaal, gevolg deur die installering van die besproeiingstoerusting, sensors om grondwater te meet en rhizotrons om wortels in situ verskeie kere gedurende die seisoen te bestudeer. Die besproeiingstelsel is toegerus met 'n beheerstelsel wat met 'n selfoon aan- en afgeskakel is en grondwatermetings is deurlopend afgelaai. Aan die einde van die seisoen (Mei 2017) is die bome se groei bepaal deur die stamomtrekke en lootgroei van die bome te meet. Wortelstudies, met behulp van die profielwandmetode, is gebruik om die wortelverspreiding na die eerste seisoen te evalueer. Die evapotranspirasie (ET) gedurende die groeiseisoen van elke besproeiingsbehandeling is bepaal deur die waterbalans-vergelyking van die wortelsone (Hillel, 2004) te gebruik. Die ET aan die einde van die groeiseisoen was 644.3 mm, 580.1 mm en 568.5 mm vir B1, B2, en B3 onderskeidelik. Al drie behandelings het voldoende water ontvang aangesien die laer ET waardes van B2 en B3 nie die vegetatiewe- of wortelgroei van die appelbome beperk het nie. Daar was geen betekenisvolle verskil tussen die drie behandelings in terme van lootlengte en stam-omtrek nie. Die rhizotrons is gebruik om die totale digtheid van die wortellengte te bepaal. Aan die einde van die groeiseisoen het B2 die hoogste totale wortellengte-digtheid gehad, gevolg deur B3 en B1. Die gebruik van hierdie metode om wortels in situ te bestudeer was suksesvol en koste-effektief. Die wortelindeks, wat bepaal is met behulp van die profielwandmetode, het getoon dat die grondtoestande gunstiger was vir die twee droogste behandelings, B2 en B3, as vir die natter behandeling, B1. Hierdie twee droër behandelings (B2 en B3) se worteldigthede was betekenisvol hoër regdeur die grondprofiel, hul wortels het tot in dieper grondlae en verder vanaf die bome gegroei en hulle het betekenisvol meer wortels in die kleierige ondergrond as B1 gehad. Hierdie bevindinge impliseer dat langer besproeiingsiklusse groter wortelstelsels mee gebring het en dat sulke bome minder vatbaar is tydens droogte toestande.

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