Answer: Exotic bulbs always need loose, well-drained, organically rich soil, and the time to prepare that is at planting. Digging about now may damage sprouts that are starting or set them back further. It's a "six of one, half a dozen of another" situation. However, six weeks isn't that long of a wait for a bulb to come up, and dahlias prefer warm temperatures, so I'd probably just wait and see. If they don't come up, chalk it up to experience and improve the soil next time. Below is information on soil improvement for garden beds:
To improve your soil, incorporate plenty of compost. In sandy soils, compost improves soil fertility, water and nutrient retention. In clay soils, it improves fertility and drainage. Add a 4-6 inch layer of compost and incorporate it about 12-18 inches deep. You can use manure if it is well-aged (6 months) or you won't be planting until it has lost it's heat and decomposed. Each planting season, add more compost. Incorporate a balanced fertilizer or add organic fertilizers such as fish emulsion, bone meal, and seaweed/kelp. Follow package instructions.
As salts accumulate in clay soils, they impede water penetration. Gypsum is a soil amendment that helps sodium be dispersed and leached beyond the root zone. With our high pH soils, it isn't really possible to reduce the pH significantly or permanently. The addition of organic matter reduces it temporarily on a localized basis. Soil sulfur can do the same. Sulfur also acts the way gypsum does (but uses a different set of chemical reactions), so it isn't necessary to add both.
After planting, add a 1-2 inch layer of mulch. Mulch is great to help retain soil moisture, reduce weeds, and as it breaks down it provides nutrients to the soil. Any organic matter can be used as mulch. Try compost, bark, wood chips, straw, or pine needles. As it breaks down, dig it into your soil and add more. I hope this info is helpful.
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