Plant-Soil-Water relations and implications for the management of irrigation and fertilization in 'Conference' pear orchards in a temperate climate

Belgium and the Netherlands are the main production area for ‘Conference’ pear due to its temperate climate which suits ‘Conference’ pear. In 2012 total production in Belgium and the Netherlands accounted for 52% of total ‘Conference’ pear production. Since 2002 there was a sharp increase in ‘Conference’ pear acreage in Belgium and the Netherlands. Encouraged by a higher financial benefit for pear fruit, growers shifted from apple growing to pear growing. Intensive training systems, often combined with root pruning to control tree vigor, increases drought susceptibility of the tree and pushes fruit growers to the installation of irrigation systems. Drip irrigation is often combined with fertigation. Fertigation is a tool to simplify split application of fertilization which has been reported to enhance fertilization efficiency in various fruit crops. Among all nutrients N is most frequently dispensed using fertigation. Main objective of the PhD was to reveal possible optimization of irrigation and fertigation practices in ‘Conference’ pear. Firstly the need for irrigation in pear trees (Pyrus communis L. cv. ‘Conference’) under low evaporative demand conditions was studied in three different orchards. The experiment showed that a Ψsoil of -60 kPa during shoot growth has no effect on fruit yield but lower Ψsoil values induced a decline in both fruit size and total yield in contradiction to higher thresholds proposed in environments with a higher evaporative demand (Naor, 2001). Just as for arid environments (Marsal et al. 2000, 2002; Naor, 2001; O’Connel and Goodwin, 2007; Ramos et al., 2000), a Ψstem below -1.5 MPa was related to lower fruit yield in high fruit size classes. Lower Ψsoil and Ψstem values were observed in root pruned trees compared to not root pruned trees in the same irrigation treatment, however without yield decline. Secondly, in search of methodologies to schedule the irrigation, an approach to use a soil water balance model for irrigation scheduling in fruit orchards was developed. The algorithm permitted to calculate average soil water content in the root zone on a daily basis considering the specific preconditions in fruit orchards being drip irrigation and the interaction between tree root zone and grass strip between the tree rows. Another possibility of irrigation scheduling is the use of continuous plant based measurements. An experiment was set up to detect possible water stress in a pear tree orchard. Thermal dissipation probes were used to detect differences in sap flux density (Jp) between different irrigation treatments. Detection of Jp differences under low evaporative conditions was possible after applying moderate water stress. Next to the soil water balance and continuous plant based measurements soil moisture sensors or Ψsoil sensors can be used to schedule irrigation. The Ψsoil output of the ‘Watermark’ granular matrix sensor was compared to gravimetric moisture measurements and a reasonable correlation was observed between both. Only at high Ψsoil values just after recent wetting events a discrepancy between sensor output and moisture measurement was observed. To come to optimal installation guidelines for Watermark Ψsoil sensors and other soil moisture sensors better insight in the water extraction pattern of ‘Conference’ pear tree is a requisite. The water extraction pattern of the ‘Conference’ pear trees was acquired by a calculation of Ψsoil in three experimental plots with a numerical model. A reasonable accordance between calculated and measured Ψsoil was observed with R² = 0.56 and RMSE = 13.4 kPa over 1320 observations. Furthermore the sensitivity of the calculation to the selected root distribution was shown. In search for the optimal N fertigation three different fertigation doses were discussed. Fertigation with 25 to 50 kg N resulted in a 20 % higher fruit yield in two of the three orchards independently from the irrigation regime. N fertigation was related to fruit color in two of the three orchards. Leaf mineral N analysis after the fertigation event related to mineral N content in the fruit and to fruit color. Water stress was also observed to influence TSS however in one of the orchards. The relations between water satus, nitrogen status and fruit quality were elaboraed in a broader survey in 9 commercial orchards. Overall the conclusions of the PhD contribute to a better understanding of the response of ‘Conference’ pear to altered irrigation and fertilization doses. This way irrigation and fertigation guidelines could be outlined. Furthermore insights in the calculation of water movement in the pear tree root zone and insights in methodologies for irrigation scheduling allow application of these guidelines in ‘Conference’ pear orchards.