Starter fertilizer can be an important part of a crop fertilization program when managed properly. Nutrients placed in close proximity to the developing plant are readily available for uptake. Early plant development and crop uniformity is encouraged, which can lead to increased yield and/or lower harvest moisture. However, there are potential injury risks associated with starter fertilizer that must be managed.
Virtually all fertilizer materials are salts and they need to salts to become plant Available. When they dissolve in the soil they increase the salt concentration of the soil solution. An increase in salt concentration increases the osmotic potential of the soil solution. The higher the osmotic potential of a solution, the more difficult it is for seeds or plants to extract soil water they need for normal growth. When a fertilizer referred to as a “low salt fertilizer” it is not that fertilizer has less salt, it means the fertilizer has a low index or low salt impact.
Renewed interest in placing fertilizer in or close to the seed row makes it important to remember that an increase in salt concentration in the fertilizer band can cause seed and seedling injury. Placing fertilizer at least two inches away from the seed can usually prevent injury. Excess fertilizer application in a starter band can still produce injury, especially under dry conditions.
The accompanying table shows starter fertilizer application method and rate guidelines from Purdue University. It should be recognized that these are for “typical” growing conditions.
Fertilizer Placement and Rate Guidelines - Corn
2x2 Placement – banded 2” beside and 2” below the seed
Seed-row – applied in furrow, directly on seed.
Although university guidelines in the region don’t directly mention sulfur (S), it is a salt and should be included (N+K2O+S) so that the amount applied does not exceed the limit shown in the table.
Care should be taken when applying fertilizers (urea, MAP, DAP, UAN, ammonium sulfate, ammonium thiosulfate) that produce free NH3 in direct seed contact. Soybeans are especially sensitive, and seed row placement of fertilizer should only be done with extreme caution.
The addition of micronutrients in starters should also be done with caution. Elements like boron, copper, and zinc can be toxic to plants in high concentrations. While the safety tolerance on zinc is quite high, boron and copper can create zones of potentially toxic levels when high rates are concentrated in a band close to the row. This is of greater concern in heavier soils with poor drainage. For example, a common annual broadcast application rate of boron is 0.5 to 1.0 pounds per acre. If that same rate was applied in a 2x2 it would effectively increase the concentration of boron in the band 10 to 30 fold. A 10 pound per acre rate of boron would be toxic to most crops.
A great detailed reference on fertilizer salt index can be found at: https://extension.soils.wisc.edu/wcmc/understanding-salt-index-of-fertilizers-2/