EFFECT OF IRRIGATION ON GROWTH AND YIELD OF FORSYTHIA X INTERMEDIA SPECTABILIS AND SOIL-NITROGEN CONTENT ON 2 SITES

Fundamental knowledge concerning irrigation adapted to specific needs of field-grown nursery crops is still missing. In a two year trial (19 94, 1995) Forsythia x intermedia 'Spectabilis' was field-grown with an d without irrigation on two different sites (Sarstedt, Northwest Germa ny: Alfisol derived from loess; Geisenheim, Southwest Germany: Loamy F luvisol in the Rhine valley). Water was applied when soil moisture ten sion exceeded 300 hPa in 25 cm depth. From the soil water characterist ic the amount of irrigation water to reach 100 hPa was calculated, In both years and on both sites yield of fresh matter of irrigated plants was between 78 % and 161 % higher compared to nonirrigated plants. In spite of a similar water and nutrient supply yield differed clearly b etween the two sites: with irrigation 1994 the surplus of the Northwes t German site was 15 % compared to the Southwest German site, and 1995 it was 50 %. Without irrigation the difference between the sites was even bigger. Obviously, on sites with a relatively lower yield level i rrigation can lead to a bigger surplus than on sites with a relatively higher yield level. On both sites the hydraulic gradients calculated from tensiometer readings showed that in months with irrigation percol ation below 75 cm depth did not appear. So, leaching of nitrogen out o f the root zone was prevented with the Irrigation regime applied. With consideration of soil physical characteristics irrigation is environm entally safe with regard to leaching of nitrate. It should taken into account that other nursery crops or different soils, e. g. sandy soils , will lead to results different from this trial.