Vegetable quality cannot be represented by a single indicator. Vitamin C, carotenoids and phenolic compounds are different groups; findings for one group cannot substitute for findings for another.
A particular study on pak choi
Pant and colleagues in 2009 investigated vermicompost extracts on pak choi under organic and chemical fertilizer regimes. The abstract reports effects on growth, mineral nutrition and carotenoids. Phenolic responses varied by fertilization regime and did not increase uniformly in every case.
This study should therefore not be described as showing that vermicompost increases all antioxidants or proves higher vitamin C levels in all vegetables.
What must be specified when claiming better vegetable quality?
A useful report identifies the compound analyzed, method, units and sampling time. Concentrations expressed on a fresh-weight and dry-weight basis should not be compared as though they use the same scale.
When comparing samples, consider variety, harvest age and storage conditions. If these factors differ, not every difference can be attributed to fertilizer.
Applying this to production assessment
Growers can select an objective in advance, such as size, uniformity or a laboratory-tested quality indicator, and then organize an appropriate comparison. Higher yield does not automatically mean every compound increases.
This article discusses how to read crop results. Do not infer human disease prevention or treatment from one chemical indicator in vegetables, or regard it as a quality analysis of a commercial fertilizer product.
Read more: Aerated Vermicompost Tea: What Does Research Say about Effectiveness and Safety?; Vermicompost in Rice Nutrition: Reading a Combined Fertilization Study.