The earth splits to bring forth plant growths. So Allah (swt) mentions another impregnation. First it was the fluid of the male man, which gushes forth that impregnates the woman. Now Allah (swt) mentions a fluid which comes from the sky, that impregnates the earth – giving life of the plants and the trees.
Verify, this life is but a set of harmonic regulations that are governed by one rule prevailed in all the existence; in the upper and down universe.