Svetla Kostadinova Agricultural University, Faculty of Agronomy, Plovdiv, Bulgaria

Zivko Todorov Agricultural University, Faculty of Agronomy, Plovdiv, Bulgaria

Ivan Velinov Agricultural University, Faculty of Agronomy, Plovdiv, Bulgaria

The pot experiment was conducted in a greenhouse to study the effect of nitrogen nutrition of 0, 200, 400, 600 and 800 mg N.kg-1 soil on the concentration of nitrogen, phosphorus and potassium in sorghum plants. The plants were analyzed in 4-5 leaves growth stage by dividing the leaves, stems and roots. It was established that the sorghum plants formed the largest amount of dry biomass when grown at N600 level. Higher levels (N600 and N800) increased the nitrogen content of stems and roots and phosphorus in all plant parts, but they significantly reduced the nitrogen concentration in the leaves. The concentration of potassium in the leaves and stems increased in parallel with the levels of nitrogen from 3.64% K2O and 4.02% K2O at N0 to 4.42% K2O and 5.03% K2O at N800, respectively. The nitrogen level very strongly positively correlated with the nitrogen concentration of roots (r = 0.927**) and the potassium concentration of leaves (r = 0.993**) and stems (r = 0.985**). The relationship between nitrogen fertilization and the nitrogen concentration of leaves was negative (r = - 0.535*). The positive and proven relationship was established between the nitrogen level and the phosphorus concentration of plant organs.