Many believe phosphorous is the main ingredient. I believe nitrogen is even more important. Nitrogen is essential for early growth. It is helps plants take up phosphorous. It produces root growth that helps early establishment. You should have 20-30 lbs actual nitrogen. Then add phosphorous at 20-30 lbs per acre. This is for most of Ontario. Potash in a starter is optional. Since the corn plant does not need potash until later in its life, potash can be broadcast.
Zinc is becoming more limited on many Ontario soils. This is because Ontario soils are naturally low in zinc and growers are not returning zinc to the soil. Corn needs zinc early in its life. Adding one to two pounds per acre of zinc could be in your starter.
Magnesium is required on some lighter soils. Adding five to 10 pounds per acre of magnesium can help on lighter soil.
You have to watch the safe rates of starter fertilizer. Generally you do not want to add more than 60 pounds per acre of nitrogen or 80 pounds per acre of nitrogen plus potash in a 2X2 band. If the band is closer to the seed in either depth or width, reduce the rates. If urea is the nitrogen source, use less.
There is a relatively new nitrogen product called ESN nitrogen. This is a polymer coated urea. The polymer breaks down with moisture and heat. This makes the product much safer in a starter fertilizer. Many growers could put their total nitrogen requirement in a 2X2 band if ESN is the source of nitrogen. Check with a knowledgeable person before you do this.
The safe rates of starter fertilizer presume that the 2X2 band is indeed two inches to the side and two inches below the seed. It also presumes that there are no other factors that could affect seed burn. High rates of potash plus urea broadcast especially on light soils could increases the probability of seed burn.
The safe rates are set as guidelines. Even with these rates you can still have seed burn that could reduce yield. Dry conditions after planting can increase the probability of seed burn. BF