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

[摘要] ENGLISH ABSTRACT: During a study conducted in a newly established orchard on a gravelly soil inGrabouw, the most effective irrigation schedule for optimum performance, includingroot growth and root distribution, of young apple trees was determined. In order to beprofitable, apple trees in newly established orchards must fill their allocated space assoon as possible. Soil water status and root growth distribution are believed to bemajor determining factors in achieving such a favourable effect.Malus domestica „Bigbucks‟ (a mutation of „Corder Gala‟) with an average size of 1.8metres grafted on MM109 rootstocks were subjected to three different irrigationcycles from December 2016 to May 2017. Treatment one (T1) was a short irrigationcycle, treatment two (T2) was a medium cycle and treatment three (T3) was a longirrigation cycle. Between December 2016 and May 2017, T1 received ca. 10 mm ofwater every 3 to 4 days, T2 received ca. 20 mm water every 7 days and T3 receivedca. 30 mm water every 14 days. Rainfall to an amount of 153 mm also added to thewater supply of the trees.Physical and chemical properties of the soil were determined, followed by theinstallation of irrigation equipment, soil water measuring instruments and rhizotronsfor studying roots in situ several times during the season. Irrigation systems wereequipped with controllers that were operated remotely by cell phones and soil watermeasurements were logged continuously. At the end of the season (May 2017) treeresponse to irrigation treatments was determined by measuring stem circumferenceand shoot growth. Root studies using the soil profile wall method was carried out toevaluate final root distribution after the first season.The evapotranspiration (ET) of each irrigation treatment during the growing seasonwas 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 and568.5 mm for T1, T2, and T3, respectively. All three treatments received a sufficientamount of water during the growing season as the lower ET values of T2 and T3restricted neither vegetative nor root growth of the apple trees.There was no significant difference between the three treatments in terms of shootgrowth and trunk circumference. Rhizotrons were used to determine total root lengthdensities. At the end of the growing season T2 had the highest total root lengthdensity, followed by T3 and T1. The use of rhizotrons to study roots in situ proved tobe successful and cost effective. The rooting index that was determined using theprofile wall method showed that soil conditions were more favourable for the twodriest treatments, T2 and T3, than T1. These two treatments (T2 and T3) hadsignificantly higher rooting densities throughout the soil profile, grew to deeper soillayers at a greater distance from the tree and had a significantly higher meanamount of roots in the clayey textured subsoil than T1. This finding implies thatlonger irrigation cycles produced bigger root systems and that such trees will be lessprone to drought.

[发布机构] Stellenbosch University