You should fertilize corn when planting and late in the growing season.
More Info: Assuming the soil pH is unknown, when initially planting corn till into 100 square feet of soil 3-4 pounds of a commercial fertilizer such as 12-12-12. As soon as the plant has reached a height of 16-18” high side dress the plant with additional fertilizer at a rate of one tablespoon per hill. Fertilize the soil not the plant and water deeply.
Optimal Soil pH for Corn
The best way to tell exactly how much fertilizer and/or soil amendments you will need is to have your soil tested. Corn prefers a soil ph between 6.0-6.8.
How to Lower Soil pH
You can lower your soil pH with the addition of acidic material such as organic mulches, sphagnum peat, aluminum or iron sulfate, acidifying nitrogen, or elemental sulfur.
How to Raise Soil pH
You can raise the pH of your soil with the addition of wood ash, hydrated lime, and limestone.
Refer to How to Change Your Soil pH, from the Iowa University Extension Service for recommendations on pounds per square foot of amendment in relation to desired soil condition outcomes.
 Lerner , Rosie , and Michael N Dana. “Growing Sweet Corn.” Department of Horticulture Purdue University Extension. Purdue University , n.d. Web. 9 May 2011. <www.hort.purdue.edu/ext/HO-98.pdf>.
 Jett, John W.. “Sweet Corn Production.” West Virginia University Cooperative Extension Service. West Virginia University, n.d. Web. 9 May 2011. www.wvu.edu/~agexten/hortcult/homegard/sweetcorn.htm
 ” How To Change Your Soil’s pH | Horticulture and Home Pest News .” Integrated Pest Management at Iowa State University. N.p., n.d. Web. 9 May 2011. <http://www.ipm.iastate.edu/ipm/hortnews/1994/4-6-1994/ph.html>.