Why should you use starters fertilizer in your vegetable garden? CSU Extension Master Gardeners Notes 711 explains why.
In setting out transplants, starter solutions often promote early growth. Because transplants have been hardened-off (growth slowed to prepare the plant for movement to the exposed, windy, outdoor environment), the nitrogen in the starter solution gives the signal to resume active growth. Since phosphorus is less available in cold soils, phosphate may also be helpful in spring and before soils have thoroughly warmed.
A starter fertilizer is any water-soluble fertilizer added to the irrigation water. Common examples include MiracleGro, Peters, Schultz Plant Food, Fertilome Root Simulator and Plant Starter Solution, etc. They generally contain ammonium nitrate since it is readily usable by the plant. Some products claim that vitamins or hormones promote plant growth. These claims are not supported by research findings.