After harvesting and selling a crop of corn, a farmer collects the corn stalks and feeds them to her cows. Why should the farmer add fertilizer containing nitrogen and phosphorus to the field before growing another crop?

The prepare the soil.

Explanation:

Because the plants use the nutrients and minerals that are present in the soil in their process of growth. Feeding the stalks to the cows means that whatever nutrient is in the stalks are mostly going to be used by the cows so in order to be able to grow another crop, the soil needs to be supported with additional nutrients, minerals and elements.

I hope this answer helps.