We will not presently discuss soups with “Mezonot elements,” such as croutons or noodles, which introduce other halachic factors. We are also not referring to chicken soup that includes many more vegetables than pieces of chicken.
The gemara[1] states that the “water of boiled vegetables [has the same beracha] as the vegetables” (i.e., Borei Pri HaAdama). Ostensibly, this should answer your question. However, the Rishonim are bothered by an apparent contradiction, as the gemara[2] also states that the beracha on most fruit juices is Shehakol – not Borei Pri HaEtz, like the fruit themselves. The distinctions between the two cases that various opinions provide are crucial to answering your question.
The Rashba[3] distinguishes between vegetables, which are normally eaten cooked, and pieces of fruit, which are normally eaten intact and raw, not squeezed or cooked. The Rosh[4] distinguishes between the process of cooking and that of squeezing; cooking results in a more qualitative taste.
Another factor is the focus on the vegetables vs. on the broth. The Rosh[5] states that the broth “deserves” HaAdama when it is normal for most people to cook the vegetables in order to eat them. (The Mishna Berura[6] seems to allude to this opinion by requiring the individual to cook the broth with the intention to eat the vegetables in order for it to qualify for HaAdama.) The Rambam[7] takes the distinction in the other direction: If one has in mind when cooking to drink the broth, it is considered important enough to merit HaAdama. The apparent indication of these Rishonim[8] is that when one has in mind to both eat the cooked vegetables and drink the broth, HaAdama is appropriate for both elements. Thus, the classic ruling is that on soup that is based entirely on vegetables that are normally used in making soup, the beracha is HaAdama, even on the broth.[9] Apparently, this was the practice of many poskim.[10]
On the other hand, several classical and contemporary Acharonim advise against this ruling, based on the following opinions and possible distinctions. The Mordechai[11] says that only vegetable broth that is used for dipping warrants HaAdama. The Ra’ah (cited but rejected by the Mishna Berura[12]) and a few others understand the gemara’s statement that water of boiled vegetables has the same beracha as the vegetables as meaning only that the beracha made on the soup’s vegetables covers the broth; if the broth is eaten alone, one recites Shehakol.
This opinion was reason enough for some poskim, including the Kaf HaChayim,[13] to invoke the rule that we avoid “going out on a limb” regarding berachot. The common application of this rule is that of safek berachot l’hakel – to refrain from a beracha when it is unclear whether a beracha is warranted. Its application here is that since Shehakol is valid b’di’eved for all foods, whereas HaAdama is not effective for a food whose beracha should be Shehakol, we recite Shehakol in such a case of doubt.
Important contemporary poskim claim that the broths in today’s vegetable soups often lack a strong enough taste to make them worthy of the beracha of HaAdama.[14] Although many people would consider most vegetable soups they have eaten as full of vegetable taste, these opinions push the direction of practice toward reciting the “safer” beracha, Shehakol, on the broth. When one eats the soup’s vegetables (at least a significant amount of them[15]) along with the broth, the consensus is that HaAdama covers the broth too.[16]
Nevertheless, one whose practice has been to recite HaAdama even on just the broth of vegetable soup is not wrong if he continues to do so. This is the fundamentally stronger opinion, and it is supported by important authorities.