"which you shall sow": to exclude seed which entered (unintentionally) along with fertilizer or with water, or seed which he sowed and which was blown in by the wind. I might think that also excluded is seed blown in by the help of the wind, (the sower intending this to happen); it is, therefore, written "the seed," (in which the aforementioned instance is subsumed).
If one sustains thorns in a vineyard, R. Eliezer says he is in transgression of kilayim, it being written "which you sow." The sages say: "the seed" excludes the sustaining of thorns in a vineyard.