Overfeeding will not do much for your tank except causing a spike in ammonia.
Dirt will keep on feeding your plants for more than a year or so depending on the nutrient demands of your plants and how much light you place over your tank. As pieces of organic matter in your organic potting soil (such as sticks and bark) break down, they eventually become macro fertilizers for your tank. Look for wkndracer's threads on dirt tanks. All of his dirt tanks have grown plants steadily for more than a year without supplemental fertilization.