Answer: You are definitely on the right track with adding organic matter and using a raised bed, since clay tends to be a poorly drained soil and asparagus does best on a moist yet well drained soil rich in organic matter. With regard to adding lime, the only way to answer that is to run some basic soil tests and find out what the existing pH is and add any additional amendments based on the results.
The optimum pH range for asparagus is between 6.5 and 7.0 so you may not need to add lime. If tests show you need to add lime, remember that lime is rather slow-acting, so wait a couple of months and then do another soil test. Your County Extension (745-6767) should be able to help you with the tests and interpreting the results.
With regard to the spacing, it is better not to crowd the plants because they are very large once they start growing. The roots can reach down six feet! Generally the planting rows should be at least four feet apart, so you should be able to put two rows the long way along your wide trench, then plant the crowns about 18 inches apart in each row. When you plant, add soil/compost to the bottom of the entire trench as needed so that the crowns sit about ten inches below the final soil level. Add a few inches of soil/compost on top of the crowns at planting time. Then fill in the rest of the trench over the course of the summer as the plants grow by adding just an inch or two of soil/compost at a time until the hole is filled. If soil storage is a problem, you could fill in the center of the wide trench immediately and just leave two long (deep) planting rows. Then fill in the rows gradually.
Be sure the plants receive adequate water this summer, and keep them well mulched to control weeds. Good luck with your asparagus!
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