I went to look it up to see what I'm doing wrong. As usual, plenty. I had been just adding compost after the harvest. It gets worked in when I pull weeds. I guess I'll start adding some 10-10-10 too though I don't think I'll go as heavy as they say.
This is one of the better write-ups I saw and seems to agree with most. Be careful about letting the fertilizer coming into direct contact with the asparagus plants. It can burn them. Put it away from them and water it in.
Fertilizing Asparagus works hard and needs to be fed twice a year. In the early spring, while the crowns are still dormant, or the budding shoots are still a good two inches (5 cm) under the soil, sprinkle a good balanced fertilizer like a 5-5-5 or a 10-10-10 around the crowns. Use 1 quart (1 l) for every five feet (1.5 m), and sprinkle the fertilizer about 6 inches (15 cm) from the crowns and in all directions. As soon as harvesting of the spears is done, weed the patch carefully, and side-dress with a 10-10-10 fertilizer at the rate of 1 quart (1 l) for every 5 feet (1.5 m). Asparagus plants need to start storing energy back into their crowns and roots after harvest, so allow the spears to grow up and turn into leafy ferns, because that is where the food for the plant is manufactured.
