Success with under the seed placement requires the insertion of a nozzle directed away from the subsoil shank that will spray the fertilizer with some pressure. Depth of placement below the seed is also important. Placement of fertilizer at greater depths may result in slower uptake of the nutrients. Placement of a starter fertilizer in direct contact with the seed is not recommended because of potential fertilizer burn. Optimum nutrient combinations and rates for starter fertilizers are currently being investigated.
Most studies agree that some nitrogen and phosphorus are needed.
Use the low rate of phosphorus on soils testing very high in phosphorus and the higher rates on soils testing medium to low in phosphorus. Addition of potassium has had varied results. Most positive responses have been on highly leachable soils when potassium was not sidedressed. Micronutrients or secondary elements should be added on the basis of past plant tissue analyses and soil tests where applicable. The most commonly needed secondary or micronutrients for corn are sulfur, zinc, and boron.