plow in
To plow the soil so as to mix some other substance thoroughly into it. A noun or pronoun can be used between "plow" and "in." (Usually spelled "plough" in British English.) You plow in this new kind of insecticide so that the plants are naturally resistant to bugs as they grow. Make sure you plow the fertilizer in before you sow the seeds.
plow something in
to work something into soil by plowing. Lay the fertilizer down and plow it in. Plow in the fertilizer as soon as you can.