For a rewarding potato crop, the plants must make a rapid, healthy start.
There are a couple of good ways to add the extra nutrients at planting time to supplement the natural fertility of your soil. Choose a balanced commercial fertilizer, such as 5-10-10 or 10-10-10, or use compost or organic fertilizer.
Broadcast Your Fertilizer
Broadcasting is an easy way to spread the fertilizer over your planting area. Use about a 12-quart pail of 5-10-10 per 1,000 square feet of garden soil. Walk over the area, scattering the fertilizer uniformly by hand, then work it into the top three to four inches of soil with a rake or tiller.
Alternately, wait until planting time. Before you place the seed pieces in the row, put down a small handful or two of compost every 10 or 12 inches, along with a small handful of superphosphate (0-20-0) or bonemeal (2-11-0). Both these fertilizers are high in phosphorus, which helps potato roots develop quickly.
The Best Place for Fertilizer
Next, cover the fertilizer with a couple of inches of soil. Looking down the row after this step you'll see small mounds every foot or so. When you plant each seed piece, put it at the edge of the fertilizer but not directly over it. Research has shown that the best placement for fertilizer is two inches to the side and slightly below the seed piece. While you don't have to be precise about this, it's vital to keep the seed pieces from coming into contact with any commercial fertilizer, such as superphosphate or 10-10-10 if you use it. The fertilizer will burn the tender new roots that come in contact with it.
Don't use fresh manure as a fertilizer on your potato patch, because it often contains scab-causing organisms. Old, thoroughly composted or decomposed barnyard manures are usually fine, though.