What studies/evidence is there that bokashi liquid actually has extra nutrition for plants?

Different analyses have shown that the bokashi liquid contains nitrogen, phosphorous and potassium and that the growth of watercress was better in diluted fermented liquid.

The constitution of the fermented liquid can vary quite a bit, depending on the waste and the provided conditions for fermentation.

Based on the analysis of 10 samples, the liquid includes:  0.12–2.48 % nitrogen, 0.0004–0.49 % phosphorous and 0.01–2.03 % potassium. Another research-based on 7 samples of Bokashi Organko mass showed 3-4.5% nitrogen and 1.5-3% phosphorous. A biological test carried out with watercress has confirmed that it thrives better in diluted fermented liquid than in pure water. In 6 out of 7 tests, the germination index was higher in diluted fermented liquid. A higher index means better germination of seeds and growth of radicles.