I find that parsnips grow really well in my allotments where the ground is quite heavy and sticky but does dry out in the summer months. If you don't have the same level of success maybe try using the following mix which is popular with those growing for exhibition. It is really just a good quality potting compost. Just bore some holes 3-4" in diamater with a pinch bar or similar, fill with compost (firmed with a stick to prevent air pockets) sow 3 - 4 seeds per station and thin down to one later.
16 ltrs peat
16 ltrs sand
16 ltrs loam(or just sterilised riddled top soil)
7 ozs Chempak potting base
4ozs lime
16 ltrs peat
16 ltrs sand
16 ltrs loam(or just sterilised riddled top soil)
7 ozs Chempak potting base
4ozs lime
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