• Starter fertilizer defined
• Salt Index defined and risk assessment
• Ammonia burn and starter fertilizers
• Soil physical properties and soil environments conducive to crop injury

With planting approaching, questions about starter fertilizer applications for both corn and soybeans outside the ranges of what is considered early season crop needs have been numerous. Starter fertilizer is confined to small amounts of fertilizer placed in close proximity to or on the seed to contribute to early season crop needs. This article will review the crop risk associated with the use of high rates of fertilizer for both, in-furrow and 2-inch by 2-inch fertilizer placement.

Fertilizer salt index is a measure of salt concentration induced in a soil solution.Salt index is a numeric ratio of the increase in osmotic pressure produced by a selected fertilizer product compared to that produced by the same weight of sodium nitrate (NaNO3), where sodium nitrate is assigned a relative value of 100.Sodium nitrate is used for comparison because it was widely available when the salt index was developed, and because it is 100 percent soluble in water.Salt injury occurs when the salt is in close proximity to the germinating seed or to growing plant tissue.The presence of the salt creates an osmotic imbalance in which water flows from regions of higher relative water concentration in the plant tissue to regions of lower relative water concentration where the fertilizer is located.This loss of water from the plant tissue causes desiccation injury, often referred to as fertilizer burn.

An additional potential crop injury risk beyond salt injury is free ammonia burn.This ammonia burn should not be confused with anhydrous ammonia burn.Fertilizer sources such as urea, UAN, ammonium thiosulfate, and DAP can liberate ammonia and burn seed or plant tissue. Use of these fertilizer sources on the seed or in a 2”x2” results in release of ammonia when these sources are applied to soils.Use of these sources on the seed should be avoided, while use in a 2”x2” configuration should be limited to rates reflecting only early season crop needs to minimize the risk of injury.Other sources of fertilizer such as MAP, ammonium sulfate and ammonium nitrate do not release ammonia as easily and are of lesser concern, but not completely absolved of this type of injury.

The actual calculation of salt index is somewhat involved and fully explained here: Understanding Salt Index of Fertilizers by Carrie A.M. Laboski of the University of Wisconsin-Madison.A less exact way to calculate salt index is to sum the number of “actual” pounds of N + K₂O + ½S to be applied. As a rule of thumb, you should not apply more than 10 lb./acre of N + K₂O products alone or in combination as a seed furrow application.Also, for these seed furrow applications, products with salt indexes greater than 20 should not be used.While a 2”x2” planter application allows for higher rates of application beyond early season crop needs to a degree, limitations on the amount of fertilizer that can be banded exist and are based on soil texture, CEC, and soil pH.Coarse soil textures (sandy soils) have greater pore space, allowing increased movement of concentrated liquified forms and liberated ammonia away from the band.Lower CEC soils (CEC ≤ 12) have fewer exchange sites to which ammonium and potassium ions can attach, leaving them mobile in the soil.Regardless of the soil pH prior to application, ammonia volatilization and plant injury can occur under higher rates of free ammonia liberating fertilizers such as urea, ATS, UAN, and DAP. Soil pore space empty of water can provide a pathway to the young roots or seed to gases and salt solutions.Any plant roots that grow toward concentrated bands of fertilizer can become burned and that can affect the root’s ability to take up nutrients.If cool wet conditions exist, prolonged contact with fertilizer bands can result in salt injury or ammonia burn, once again affecting nutrient uptake.

Crop injury from salt injury and /or liberation of free ammonia are generally of relatively little concern under normal rates of fertilizer except under these conditions:

• In-Furrow, pop-up, on-the-seed and 2”x2” placement of fertilizer
  • High salt index fertilizers are applied.
  • Higher than early season crop needs of fertilizer are applied.
  • Soils are dry, preventing normal dilution of product through the soil and allowing high concentrations to remain near seed.
  • Weather conditions are hot and dry.
  • Soil pH is >6.8 resulting in liberation of ammonia.The higher the rate applied, the greater the risk.
  • Urea, ammonium thiosulfate or DAP are used as a high percentage of total analysis applied as a 2”x2” band.Caution: DO NOT APPLY THESE SOURCES AS IN-FURROW.
  • Maladjusted or ill-maintained starter equipment that results in < 2”x2” fertilizer placement

Using certain micronutrients with starter fertilizers can negatively impact crop establishment. For example, applying boron directly on the seed can significantly decrease germination.With all other secondary nutrients and micronutrients, add small amounts to the mix to ensure that no antagonisms occur.Antagonisms can be related to the formation of unwanted compounds or competition with macronutrients for plant uptake.

Whether using an in-furrow or a 2”x2” starter application, moderation of the rate applied goes a long way in mitigating the risk of injury to the seed or plant.An additional read on started fertilizers can be found here: NebGuide G316 from University of Nebraska Extension.For assistance on starter fertilizer rates and the potential adverse effects on the crop as an in-furrow or a 2”x2” application, contact your FS Crop Specialist.