"Do you sow it?" meaning: do you cause it to grow? He is saying this to them as a question. "Or are We the sowers?" meaning: you are not the ones who sow it, but rather We are the sowers who cause it to grow.
﴿أَأَنتُمْ تَزْرَعُونَهُ﴾ أَي تنبتونه يَقُوله لَهُم على الِاسْتِفْهَام ﴿أَمْ نَحْنُ الزَّارِعُونَ﴾ أَي: لَسْتُم الَّذين تزرعونه، وَلَكِن نَحن الزارعون المنبتون
