Posted: 07/09/2014 at 09:39
Not sure if you'll be looking in again, but I'll answer it anyway.
There is absolutely no reason to be concerned about the nutrients lasting in your SP compost.....on the contrary, too many nutrients will spoil your plants by producing too much over-winter growth.
Whilst on the subject, I would recommend that you use only 'seed' compost to sow your seed, as this has a lower nutrient content. I know some will claim that they use multi-purpose compost with success, but others are unaware that it is responsible for their plants becoming leggy & weak.
Do remember Lyn's advice about using deep pots...SPs are very deep rooted. Sweet pea growing tubes are widely available.
Btw, you may find this thread of some interest: http://www.gardenersworld.com/forum/the-potting-shed/growing-sweet-peas/6143-56.html