A person who partook of a "Kezayit" of fruit from among the special species (grapes, figs, pomegranates, olives and dates) must recite the Beracha of "Al Ha'etz" after eating.
If a person does not remember the text of this Beracha and does not have access to a Siddur, should he recite "Boreh Nefashot" in place of "Al Ha'etz"?
Hacham Ovadia Yosef addresses this question in his work Yabia Omer (vol. 2, Orah Haim 12:7) and rules that a person in such a case should not recite any Beracha, as "Boreh Nefashot" cannot serve as a substitute for "Al Ha'etz." Nevertheless, he writes, if a person in this situation did recite "Boreh Nefashot," then he has, post facto, fulfilled his obligation. As such, if he then gains access to a Siddur, he should not recite "Al Ha'etz," since he has fulfilled his requirement by reciting "Boreh Nefashot," despite the fact that optimally he should not have done so.
This ruling is predicated on the famous Halachic principle of "Safek Berachot Le'Hakel," which dictates that one should not recite a Beracha in a case where he is unsure whether it is warranted. The question of whether or not "Boreh Nefashot" can serve as a substitute for "Al Ha'etz" is subject to a debate among the Halachic authorities, and therefore optimally one should not recite "Boreh Nefashot" in place of "Al Ha'etz," even if he has no possibility of reciting "Al Ha'etz." But if one did recite "Boreh Nefashot" in place of "Al Ha'etz," then since according to some authorities he has fulfilled his obligation, he should not recite "Al Ha'etz" if he then gains access to a Siddur.
It should be noted that Rabbi Moshe Halevi, in his work Birkat Hashem, disputes this ruling and maintains that according to the accepted Halacha the Beracha of "Boreh Nefashot" can never satisfy one's requirement to recite "Al Ha'etz." In his view, if one recited "Boreh Nefashot" when he was required to recite "Al Ha'etz," he has not fulfilled his obligation and must therefore recite "Al Ha'etz" if the opportunity arises.
In any event, we follow the ruling of Hacham Ovadia Yosef that although one should not recite "Boreh Nefashot" in place of "Al Ha'etz," if one did so he is considered as having fulfilled his requirement, and should not recite "Al Ha'etz" if the opportunity arises.
Summary: If a person who is required to recite "Al Ha'etz" does not remember the text and has no access to a Siddur, he should not recite any Beracha. If he mistakenly recited "Boreh Nefashot" in place of "Al Ha'etz," then he is considered as having fulfilled his requirement and therefore should not recite "Al Ha'etz" even if gains access to a Siddur.
