The field experiment was conducted under trickle fertigation at the experimental field of Nuclear Research Center for Agriculture and Medicine in four replications. The plot size was 3×10 meter with three irrigation lines (one meter apart from each other). The injections of fertilizers were down by the means of a single fertigator pump. Soil moisture monitoring, and irrigation scheduling, were down by the means of neutron moisture gauge. The fence separated 15 plants and three drippers in the middle row of each fence were blocked and plants received 32P labelled phosphorus through 20 litter’s plastic containers. The specific activity of the labeled fertilizer was 10.43 MBqg-1P-1. The total amount of labeled fertilizer divided into 36 units and applied with each irrigation. Fruits were harvested 6 times. In each replication, nine soil samples were collected (at three different depths) and each sampling procedure was repeated twice (20 cm apart from each other perpendicular to the irrigation line). Determination of the activity (dps) of the soil and plant materials was done by radioassay technique using appropriate detector (Beta Counter). The results showed, 44.1 percent of total phosphorus in the root, derived from labelled fertilizer and there were no significant difference (%5 level) for other plant organs. %23.3 of phosphorus fertilizer used by tomato plant, that refers to fertilizer use efficiency under current design condition. Phosphorus fertilizer use efficiency for fruit, leaf, stem and root were %11.7, %7.8, %2.2 and %1.6 respectively. The highest amount of phosphorus absorption was measured during the first and second harvest time (%65). %61.5 of phosphorus fertilizer was leached up to 60 cm depth and %15.2 was considered as out of tracing field.
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