This study was carried out through two successive seasons (2008 and 2009) in a private orange orchard in Qalubia governorate Egypt in a three years old Navel orange trees budded on sour orange rootstock on clay loam soil (Typic Torriorthents). Planting distance was 2.55* meters apart and flood irrigation was used by river Nile water. Two nitrogen rates were used 150 and 300 g N/tree/y in three forms 100% mineral nitrogen (M.N.) as ammonium nitrate, 100% organic nitrogen (O.N.) as compost and mixed (50% M.N.+50% O.N.) each of these treatments with or without K humate addition (6 kg/feddan). All fertilization treatments were divided into equal five doses added every two months from February to October during the two seasons. The obtained results showed that the highest stem thickness increment percentage was recorded by mixed nitrogen form (50% M.N.+50% O.N.). Mixed nitrogen form with K-humate recorded the highest significant value of plant height increment percentage in the first season, but in the second season, using nitrogen form at 150 g N/tree/y without K-humate recorded the highest significant value. Leaf N content had higher significant values with M.N. treatments. Leaf P content showed insignificant differences among treatments. Mixed N source at 300 g N/tree/y without K-humate recorded a higher significant value of leaf K content, in the second season compared with other treatments. Leaf Ca content showed a higher significant value by organic and mixed N form treatments in the first season. Leaf Mg, Fe, Mn and Zn contents increased significantly with M.N. treatments in the first season. While in the second season leaf Fe content had a higher significant value with O.N. treatments. Leaf Mn content illustrated the lowest significant mean level by using N form at 150 g N/tree/y treatment. While, leaf Zn content recorded a higher significant value with mixed nitrogen source at 150 g N/tree/y with K-humate addition compared with all other treatments. From this results we recommended that the mixed sources of N fertilizer (50% M.N.+50% O.N.) at 150 g N/tree/y is the best N fertilizer, and the application of K-humate or humic acid at commercial rate is not recognized for the young Navel orange trees in clay loam (Typic Torriorthents) soil.
