EFFECT OF GYPSUM AND MINERAL NITROGEN FERTILIZATION ON NITROGEN FORMS DISTRIBUTION THROUGH AGRO-SYSTEM AND YIELD OF POTATO GROWN ON A CLAY SOIL UNDER TILE DRAINAGE SYSTEM CONDITIONS

    A field experiment was carried out on an alluvial clay soil of a pilot field at Talla area, Minufiya Governorate , Egypt during two successive growth winter seasons  2011 / 2012 and 2012 / 2013 to study the effect of agricultural gypsum application  and mineral -N fertilizer ( ammonium nitrate , 33 % N ) solely or in combination under different distances of tile drain on some soil properties and yield of  potato plants (Solanum  tuberosum L.) Scotland cv . Nieta. Agricultural gypsum was applied at rates of 0 and 3 ton/fed. as a protection dose. Nitrogen fertilizer was added at rates of  0 and 500 kg / fed. ( 165 kg N / fed. ) , which equals 100 % of the recommended dose. The field was provided by tile drains spaced at 30 m with 1.5 m depth. The content of nitrogen forms ( NH₄-N and NO₃-N ) distribution through different soil depths, ground and drainage waters  in relation to time were  studied. Water table depth after the first and second irrigations was measured  above and midway of the laterals. Ground and drainage waters were analyzed. Tubers and shoots of potato contents  of  NH₄-N and  NO₃-N were determined. The experiment was carried out in a  split split - plot design with three replicates .
    The obtained data show that ground water depth was increased with increasing the period after irrigation especially after the first irrigation. The greatest increase of ground water depth was found above the laterals. Also , this depth was increased with gypsum application. After the first and second irrigation and harvesting stage, the soil content (mg/kg) from available NH₄-N   and NO­₃-N was decreased with the increase of soil depth. The greatest decrease of soil content of available N was found above the  laterals in the soil treated by gypsum and unfertilized by N especially at harvesting stage. With the different treatments, the soil content of NH₄-N was higher than of NO₃-N. On the other hand, ground and drainage water content (mg/l) of NO₃-N were higher than those of NH₄-N with all treatments under study. Also , this content was increased with both gypsum and N fertilization applications. Soil salinity values were relatively affected by drain spacing treatments. In surface layer , the soil salinity value was 1.35 and 1.54 dSm^-1 for above and midway (15 m) drain spacing.
   Results indicate that the highest tubers yield of potato plants was found in the soil treated by gypsum and fertilized by N above the laterals (16.39 ton / feddan). The highest content (%) of the two N forms in both shoots and tubers were found in the plants grown on above the laterals in the soil treated by gypsum and N fertilizer.