The nutritional value of wild foods is very variable.
It depends on soil type, water content and other factors related to the environment in which the plant is growing, and two apparently identical plants growing within a few metres of each other could be vastly different in nutritional content, for example in plant 'A' is nearer to a watercourse than plant 'B', then the water content of 'A' would be higher than 'B', producing a larger root than 'B', but with the same (or possibly less!) nutritional content.
In agriculture, plants are grown in an environment which is modified to provide relatively consistent conditions across a whole field, by irrigation, drainage and use of fertilisers, thus producing equally consistent crops. (Look how consistent in size, colour and quality vegetables in your local supermarket are.)
Th only real discussion I've found on the use of wild foods for effective nutrition is the "Ray Mears Wild Food" DVD and book combination. There are some other sources in scientific papers but they tend to be rather technical.
Hope this helps.
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