The long term impact of drip irrigation on soil salinity Case Study: Pistachio orchards in Sarvestan plain

The objective of this research was to investigate the distribution and accumulation of salts under drip irrigation irrigated with saline water. To do so, two pistachio orchards in two regions in the Sarvestan plain (Fars Province) were selected. In most cases, soil sampling started 20 cm away from the tree and continued every 30 cm within the wetted area of the dripper perpendicular to the tree row with the last sample 60 cm out of the wetted area. Soil sampling was done to the depth of 150 cm with samples collected from every 30 cm. Soil sampling was done on both sides of the tree. Accordingly, 60 samples were collected for every tree in order to determine salinity condition around the tree accurately. Salinity of soil saturated extract and moisture content of all soil samples were measured according to the standard methods in the laboratory. Results showed that in one of the studied regions (7 dSm-1) accumulation of salinity has occurred minly under the the trees' trunk due to large distance between laterals and row of trees. In the other studied region (10 dSm-1) due to high flow rates of drippers, exceptionally high leaching of salts has occurred in part of the root zone. However, due to inappropriate agricultural practices, accumulation of salinity has occurred between the tree rows and also in the manure pits near the trees. Results also indicated that comparing average root zone salinity of soil saturated extract with salt tolerance of pistachio (measured under no water stress) can not show the the actual reduction in yield due to salt stress imposed on the crop. In the studied regions irrigation with high leaching fraction, reduces salinity of soil saturated extract to such an extent that its comparison with pistachio salt tolerance does not show that crop is exposed to high salinity stress. However, considering long irrigation interval in these regions, plant is exposed to water stress in some periods between irrigations evevts and therefore, soil water osmotic pressure could be increased to such an extent that reduces soil water potential up to the permanent wilting point limit and causes sever drop in crop yield. Therefore, appropriate irrigation scheduling under saline conditions is needed in these and other similar regions in order to reduce the adverse effects of salinity on growth and yield of the crop.