Posted: 21/03/2014 at 01:32
Vermiculite is good as a covering for seeds sown indoors, but I wouldn't use it outside. It's so light it'll blow away. Besides, it's way too expensive to use outside and not in the least necessary. I also wouldn't recommend peat. It's an environmental no-no, contains no nutrients and is expensive. Organic matter is a very good idea but you'll have plenty in the mushroom compost. (As a bonus, you may get a crop of mushrooms too!)
A box of bonemeal for slow-release fertiliser, and blood,fish and bone for faster acting, both good and fairly cheap. Maybe too a box of sulphate of potash as a booster. There are instructions on dosage on the packets - don't be tempted to overdo it. Your mix of sandy loam and mushroom compost will be fine without peat or vermiculite - don't waste your money on them.
Don't be in too much of a hurry to sow if you don't have a greenhouse, though I admit the temptation is to crack on asap. If you wait until light levels and temperatures are higher, they'll grow stronger and will soon catch up. Fleece is great for protection, but if outdoors make sure you peg it down firmly.
Best of luck and happy vegging!