As 20th century research on biblical and extra-biblical parables showed, the term “parable” (parabolē) could be applied to a wide range of expressions, from terse aphorisms to riddles seemingly pregnant with significance to extended stories such as we usually think of when encountering the term.
A number of helpful insights came about from the rather intense attention that parables received in the 20th century. One distinction among parables that I have not heard made is one that I’m finding more and more helpful in my own thinking: that is the distinction between parables that explain how something is and parables that encourage a certain kind of action or change in behavior—what we might call explanatory versus exhortative parables.
Had I had this distinction in my mental pocket 20 years ago, I might have (sadly) avoided one of my favorite and most memorable arguments. It was with a professor for whom I was a T.A. and the argument was over the interpretation of the Parable of the Sower. He complained that most of the sermons one hears on this parable end with “So, be good soil!” as if “soil” had or could have any role in generating its own fertility and thus producing fruit, his point being that only God can make us good soil, prepared to receive the word and produce much fruit. I argued that point of sermon is indeed to warn people about allowing their hearts to become hard or choked with the cares of the world. Actual soil is inert, true, but we are not. Neither of us would budge on our interpretation. That professor remains one of my best colleague-friends and an unfailing advocate of my work.
If I had had in mind the distinction between explanatory and exhortative parables I might (might!) have been willing to concede that the parable seems to be far more explanatory that exhortative, although I think it treads the line. Some parables are clearly exhortative (the Good Samaritan ends with, “Go and do likewise”) and some seem to be almost strictly explanatory (the parable of the Net [Mt 13] seems simply to be about the comprehensive nature of the judgement that will happen at the coming of the kingdom and the fate of those judged).
The Parable of the Sower (or Seeds or Soils) on the one hand invites hearers to consider what sort of soil they might be and whether they are producing fruit and if not why not. Yet, on the other hand, assuming that the readers are supposed to identifying on some level with the disciples, then Jesus’ statement that “To you has been given to know the secrets of the kingdom” certainly sounds as if the disciples have received the word and do not have the blind eyes and deaf ears that prevent both repentance and fruit-bearing (typical of bad soil). In that case, particularly when one looks at the whole pericope including the rationale for parables and the allegorical interpretation, the parable seems to be essentially explanatory. Although I still maintain that its explanatory point is not our inability to do anything to become good soil.
 This without even taking into account the not-altogether-clear explanatory point of the parable. Is it simply instructing the disciples (and us) that the word is sometimes productive and sometimes not? Is it mainly explaining why some people seem to receive the word and others not (or seem to receive it but not to the point of producing fruit)? Is it explaining the rejection of Jesus’ message by those of his own time, particularly the religious leaders?