51:3 Some authorities maintain that if one ate a complete entity in its entirety, such as a single nut or a single bean, then he would recite a concluding bracha even though it is not the size of an olive. Other authorities disagree, so it would be advisable to avoid a doubtful situation by not eating less than a k’zayis of such things. If one split the item before eating it, this revokes its status as a whole entity and all authorities agree that one would not recite a concluding bracha on less than a k’zayis.
51:4 Everything that a person eats combines to make a k'zayis. For example, if one ate half a k'zayis of something whose bracha is shehakol and half a k'zayis of something that would require al hamichya, or even half a k’zayis of bread, he would recite borei nefashos over the cumulative k’zayis that he ate. Similarly, if one ate half a k'zayis of something whose concluding bracha is “al ho’eitz” and half a k'zayis of something whose concluding bracha is “al hamichya,” or half a k'zayis of bread, he would recite borei nefashos (because there’s not enough of any single item to obligate him in one of the more specific concluding brachos). If one ate half a k'zayis of something whose concluding bracha is “al hamichya” and half a k'zayis of bread, he recites al hamichya. Drinking and eating do not combine with one another in this matter (even when they both would require borei nefashos – MB 210:1).