Test the soil first for its pH balance. Contact your county's extension office and ask for recommendations for testing labs. If the pH balance is under 6 or over 7, then it needs to be adjusted before adding any other fertilizer. Add 25 lbs. of agricultural lime per 1,000 square feet to bring the pH balance up, or sulfur to take it downward.
Test the soil for the amount of phosphorus and potassium. You will receive a report back from the lab showing any deficiency and how much phosphorus or potassium to add to the soil. Purchase each mineral supplement separately at a garden or farm store.
Scatter the minerals over the soil and harrow them into the soil when disking for preparation for planting the clover seed. A small backyard gardener can till the fertilizer into the soil or dig it in with a spade and fork.
Continue to test the soil for pH balance, phosphorus and potassium every few years. It is normally not necessary to add more fertilizer each year unless the soil is very poor.