Sustainable irrigated agriculture entails monitoring water, salt balance and effective depth of root zone. A lysimeter study was based absorption of plant water requirement beyond predefined root depth. Experiment was conducted with three treatments in three replications at the research field of the University of Tehran in Karaj, Iran, in 2007 on soybean including: treatment A at MAD (Maximum Allowable Depletion) of 55%, treatment B at MAD=65% and treatment C also at MAD=65% but with 30% excess irrigation. The results showed in treatment C crop yield not only did not significantly decrease as compared with treatment A but water application efficiency increased by 11.2%. Water extraction of plants at the deeper soil layers in response to longer irrigation intervals can result in increased plant water uptake efficiency.
