*2.3. Sugar Beet Analyses*

#### 2.3.1. Nitrogen and Phosphorus Content in Plant Materials

Nitrogen and phosphorus contained in plant tissues were determined by mineralization with a mixture of sulfuric acid, hydrogen peroxide, and selenium. A portion of the analysed sample was oxidized with hydrogen peroxide in concentrated sulfuric acid. After decomposition of the hydrogen peroxide and distillation of the water, the mineralization was completed by boiling with sulfuric acid under the catalytic action of selenium. The resulting solution was analysed using a San plus System SKALAR analyser (Skalar Analytical B.V., Breda, The Netherlands).

#### 2.3.2. The Contents of K, Ca, Mg, and Na in Plant Materials

The contents of K, Ca, Mg, and Na in plant tissues were determined by oxidation with hydrogen peroxide in a concentrated nitric acid medium in a closed system with a controlled temperature rise, using a Milestone microwave digestion system (Milestone Inc., Sorisole, Italy). The final analysis was carried out using a ICP–OES Trace Scan device (Thermo Jarrel Ash, Trace Scan, Franklin, TN, USA).
