How to Fertilize Through an In-Ground Sprinkler

Start the garden with the best possible nutrition at planting. Dig 3 to 4 inches of organic compost into the top 3 to 4 inches of soil to loosen and nourish the foundation. Be careful to dig around the established sprinkler lines at this time, and plant flowers or vegetables between the sprinkler lines.

Mix starter 6-24-24 or 5-10-5 granular fertilizer into the soil after the amendment for good starting nutrition. Water the site with the in-ground sprinklers for 30 minutes after amending to settle the soil and fertilizer.

Feed flowers and vegetables again at midseason. Dig a 2-inch-deep trench to one side of each plant, or in a circle around each plant. Keep the trench at least 6 inches from the stem of each plant to avoid fertilizer burn. Sprinkle fertilizer into the trench and cover it with soil, then water the plants as normal. The watering dissolves the granular fertilizer to move nutrition into the soil.

Feed flowers and vegetables on a biweekly basis with water-soluble plant food or fertilizer. Mix the plant food or fertilizer with water according to the manufacturer's directions, and water the plants with the mixture. Water-soluble plant food is safe on foliage and soaks into the soil to nourish plants without soil cultivation.