Biostimulants are gaining growing importance among preparations used for plant production. They may cause increasing the effectiveness of nutrient uptake. The aim of the study was to assess P and K accumulation by winter rapeseed after the application of a biostimulant under conditions of varied NPK and S fertilization levels. The field experiment was established on Alfisol, and the factors were: Two levels of NPK fertilization (high 180 N, 70 P, 132 K (kg ha−1) or low 144 N, 35 P, 66 K (kg ha−1)); elementary S fertilization (36 or 0 kg ha−1) and application of seaweed biostimulant or without that treatment. Biostimulant caused an increase in P and K accumulation in the shoots of rapeseed during generative development. Application of biostimulant in rapeseed fertilized with lower NPK rates or not fertilized with S increased P and K accumulation in shoots to the level obtained at higher NPK and S rates without that treatment. Increased level of NPK fertilization caused an increase in P and K accumulation in both shoots and roots at flowering and ripening stages. At the fruit development stage, under higher NPK fertilization there was higher uptake of P and K in rapeseed shoots after presowing fertilization with S.

The present study has aimed at enhancing the insufficient knowledge of functional soil enzymes properties influenced by inorganic fertilization and biostimulant application to increase the uptake of nitrogen affecting the winter oilseed rape yield. Field experiments were conducted in Poland (53° N, 18° E) in Alfisol (USDA). In this experiment, the NPK rates applied were as follows: high 180 N, 70 P and 160 K 132 N (kg ha−1) or low 144 N, 35 P and 66 K (kg ha−1); fertilization with elemental S 36 or 0 (kg ha−1); and the seaweed biostimulant Kelpak was applied or there was no such treatment. Due to low NPK fertilization rates, the activity of dehydrogenases, peroxidases, and catalase increased in subsistent generative development stages from flowering to ripening. At the ripening stage, the activity of these enzymes, as well as nitroreductase activity, were inhibited by high NPK fertilizer rates. The seaweed biostimulant application and S fertilization increased N accumulation in plants of oilseed rape in generative development, by 16% and 13%, respectively, as compared with the lack of these treatments. The application of S increased the uptake of nitrogen in shoots and in whole oilseed rape plants only after application of higher rates of NPK.

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