The research was taken place in Grdarash field the experimental farm of the College of Agriculture Engineering Science, Salahaddin University Erbil . Spilt-spilt plot design was used in the field with different NPK-fertilizer applications [ Nitrogen ( 0 , 40 , 80, 160 ) , Phosphorus ( 0 , 80 , 160 ) and Potassium ( 0 , 50 , 100 ) Kg ha -1 ] .The amount of Macro and Micro nutrients are excessive which used in Onion crop production . Nutrient balance impacted the crop production and quality. This study aimed to know the influence of the different rates of Nitrogen, Phosphorus &Potassium on the onion-yield to establish Nutrient Indexes and critical points concentrations for the elements (N P K S) , by using foliar analysis , so we can obtain the conditions of the elements-concentration in the leaf of the crop ( balance , imbalance , deficiencies , excessive ). The results show that there was high Significant interaction between (N P K) elements and Onion bulb yield. The best combination between Potassium and Nitrogen elements was (K50 N40) between Potassium and Phosphorus was (K50 P80). The treatment K1 P2 N 1 (K 50 , P 160 & N 40 Kg ha-1 ) has the highest Significant Onion-yield and recorded the best interaction between the ( Nitrogen , Phosphorus , Potassium ) elements .The critical concentration points for the Nutrients ( N P K S ) were ( 2.65 , 0.10 , 2.14 , 1.36 ) % respectively . The N-indexes, P-indexes, K- indexes and S-indexes ranged from (- 20.18 to 21.38 ) , ( - 27.30 to 17.07 ) , ( - 20.61 to 14.55 ) , ( - 11..22 to 23.11 ) respectively .