I feel like I may have not articulated the question very well. I want to ask: when "they" say that there is (x ammount) of (nutrient y) in 100g of (vegetable z), how do they account for the differences between vegetables grown on one farm as opposed to another?
They use whatever the accepted average would be, based on a variety of samples. Some things will be more accurate than others. For instance, the amount of a certain vitamin or mineral may vary widely. But suppose you are looking at oats and they say that it is this or that percentage starch. That is not going to be nearly so variable as the quantity of some micronutrient.
Same thing of you buy a piece of lumber and it's rated for a certain strength. That will be based on many other tests of the same species of lumber, cut to that dimension. It's not a rating of the actual strength of that piece of wood.
Dan, I don't believe "they" are accounting for any differences between the same veggies grown in different soils. If I read information saying for example that "100g of tomatoes contain 100mg of lycopene" I would take that data with a grain of salt. "they" are not accounting for differences between farms and soil types. There are too many variables in each microclimate where the vegetables are grown, and nutrient levels in vegetables will vary widely. Back to your original question on how the levels are determined, I imagine they take several samples and find an average using fancy lab equipment. The lycopene I mentioned (which tomato plants make) is a compound containing carbon and hydrogen, both readily available in any growing environment. If we think about a tomato containing selenium and the levels found in the tomato, selenium is an element on the periodic table, and tomato plants can't make it. If selenium isn't in the soil, it won't be in the tomato.
"Study books and observe nature; if they do not agree, throw away the books." ~ William A. Albrecht
I always like to remind myself that nutritional science is a long ways from consistent peer-reviewed, widely accepted theories. What we "know" changes (dramatically!) every decade or so, and there are several groups of scientists that believe very different theories co-existing in the world today. Nutritional science in Europe is different than that in America, is different than that in Monsanto's headquarters, is different than that in the American Heart Association's headquarters, etc, etc. Basically: we are still very firmly in the "we don't know what we don't know" stage of nutritional science. That does not make our beliefs wrong, but it certainly does not make them right.
I think the question you asked is a really important one, and one I wish more people would ask about much science (and most science does have good answers for these questions!). But personally, I haven't seen an acceptable level of rigor, ethics, and consistency in nutritional science to believe any of this science as fact yet.
I want to ask: when "they" say that there is (x ammount) of (nutrient y) in 100g of (vegetable z), how do they account for the differences between vegetables grown on one farm as opposed to another?
I would very confidently say the answer to this is no. For example, in America this data comes from the FDA. You can read about their sampling methods here. They are not rigorous, they are subjective:
In so doing, the data base may be designed to consider and select samples based on one or more factors that may impact on nutrient variability, or, preferably, on a combination of such factors. For example, if a data base concentrates on foods selected from one state, the sampling plan might include the collection of samples according to the regions within the state where the food is grown. In addition, because some foods are known to show seasonal variation in total fat content, a relevant data base would likely want to include samples harvested at selected seasons. Furthermore, in some instances, it may be appropriate to sample according to type of processing.
This sampling methodology assumes the distribution of data is a normal distribution, and that the average has meaning.
hau Dan, Any nutritional values you find on line will have been determined by chemical analysis of a given set (representation) of a given vegetable. This means they are shooting for an "average".
The problems are many when trying to use averages to determine nutritional values for any food.
Most of the vegetables found in grocery stores are going to be rather nutritionally poor, this is because of the use of chemical fertilizers that neglect what makes up the nutritional value of any grown food.
Minerals are the key elements that build these values in foods, the more variety of minerals present, the better the nutrition values will be.
These minerals need to be processed by bacteria and fungi so the plants can take in the broken down parts they need and then the cells of the plant build the items it needs to be healthy.
Chemicals can be looked at like simple sugars, they create a lazy "couch potato scenario" bacteria environment which means less nutritional value in the foods.
There is a series of events with rather strict requirements for plants to produce high nutritional values in their fruits.
Even many of the "Organic" foods are lower in nutritional values than vegetables grown in a high mineral content, bio active soil.
The food industry, including those "governing bodies" give out overall averages as determined by actual content added to any supplemental nutritional additions made by the manufacturer.
You will notice that in a grocery store, none of the "fresh" produce has labeling that states the nutritional value.