This can either relate to giving a plant extra nutrient by providing it with organic material (I bought some Good Grow liquid the other day so I can fertilise my carrots) or relates to the process of fertilisation which is part of sexual reproduction.

Adding nutrients to the soil to promote growth and healthy plants - fertiliser, either artificial or natural substance that adds nutrients to the soil. Also the act of introducing pollen to produce seed.

provide with fertilizers or add nutrients to; "We should fertilize soil if we want to grow healthy plants"

make fertile or productive; "The course fertilized her imagination"