Spread a 1- to 2-inch-thick layer of organic matter, such as compost or manure, over the top of your horseradish bed and mix thoroughly with the ground beneath it to a depth of 8 to 10 inches. This should be done as early in spring as the ground can be worked to give the horseradish roots the maximum amount of time to grow.
Dig a trench in your prepared ground and lay your horseradish starts, often called sets, in the trench horizontally or at a 45 degree angle with the top higher than the bottom. One or two sets should produce enough for a home gardener. Use sets about a finger's width in diameter and 12 to 18 inches long. Be sure that the roots are all facing in the same direction.
Cover the sets with 4 to 8 inches of soil, planting more deeply in sandy soil or in hot summer areas. This will give you a slightly elevated mound about 2 feet wide. Water thoroughly.
Once the leaves appear above the ground, fertilize with 10-10-10 fertilizer, either liquid or granular, according to the directions on the package.