Why just use crops?..Surely a more abiding "message" could be left carved in a rock face or gouged into the earth's pliable soil...hard to imagine that any visiting intelligence, with the ability to traverse space only has tools which are effective on crops...and if the message, if indeed message it is, is so important....why not duplicate it time and time again until we mere homosapiens eventually get it...before shaping up another new and more complex one?
Assuming that a message is being left, it would hardly be wise to disembark from your craft with a spade and a chisel in the face of a possibley hostile welcoming committee. No tools necessary.
Perhaps the act of deciphering them is part of the plan. Learning and expanding your consciousness until you are indeed capable of 'getting it'.
But to have the ability to travel the vast distances between planets(or even solar systems)....and all they can do is flatten a few grain crops on select areas of a planet already struggling to feed all it's inhabitants is a strange blimmin' message to leave no matter what the meaning....but if all they have are planks on bits of string(they imparted this sacred knowledge to a select few drunk blokes at Uni, who were getting bored of being stuck indoors playing World Of Warcraft till the wee small hours)....for flattening crops and/or chisels for carving into rock....makes me wonder....where do they put the coal in their interstellar ships?