*2.3. Estimation of Plant NPK*

The plant N content was measured at 20, 60, and 120 days using the standard procedure described by Watson et al. [24]. A digestion tube was filled with the plant-dried sample (1.0 g). Then, in a digestion block, 15 mL of concentrated H2SO4 and 1 g digestion mixture (K2SO4 + CuSO4 @ 9:1) were combined, and the tubes were heated for 2 h at 450 ◦C. After heating, the color of the solution was changed from transparent to yellowishgreen, visible in the digestion tubes. A distillate unit produced the required volume for the distillation process. Then, the material was placed in a receiver containing 4% boric acid (25 mL). After that, a few drops of the indicator were added; the purple color then changed to golden yellow through the distillation process. The subsequent distillates were then titrated with 0.1 N H2SO4, resulting in purple as an endpoint from a golden yellow shade [24].

The following spectrophotometer and the spectrophotometric vanadium phosphomolybdate processes were used to estimate the P percent in the plant following the standard protocol of a previous study [28]. The spectrophotometer was used to run the standard of P samples. Following this, the P concentration of plant samples was assessed using the yellow color procedure. The digested plant samples were used to distill water, and a coloring reagent was placed in a flask. The flask was then placed at room temperature for about 30–35 min, with the development of color over the subsequent time. The P percent in plant samples was then calculated using just a spectrophotometer at 420 nm using the standard method [29]. The flame photometer procedure was used to measure the K concentration using a method developed by a previous study [30].
