adjust NPK to suit onions

I have quite a lot of potato fertiliser NPK 13:13:20 which i would like to use up this year, by using on my onion bed as well, if possible.

I know onions want more Phosphate than there is in that mix, so is there anything I can add to make it better for them, or just use as is 

Superphosphate is about 20% , if you prefer organic then bonemeal at 7% has the highest phosphate content of the common organics (but i have seen higher figures quoted for bonemeal)

Superphosphate is about 20% , if you prefer organic then bonemeal at 7% has the highest phosphate content of the common organics (but i have seen higher figures quoted for bonemeal)

Depends on whether you count total phosphate or discount either the soluble or insoluble part (I can't remember which), IIRC. From looking at a packet of bonemeal in Wilkos and it had two boxes, the one with 'UK regs' at the top said it was 14% P, the RoI regs box said 7%.