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Pop up fertilizers: What you need to know
Author: Leo Espinoza, Associate Professor & Extension Soil Scientist

Pop up fertilizers are commonly used in some corn growing areas in the country.  The practice consists of applying a few pounds of nutrients with the seed at planting.  Planters are retrofitted with commercially available attachments that accurately deliver the desired volume of liquid fertilizer.  This practice should not be confused with “starter fertilizers” which is the application of fertilizer materials to the side of the seed furrow.

Pop up fertilizers are used to increase seedling vigor and could also be used to apply some of the micronutrients needed for optimum plant growth.  Such micronutrients are applied at considerably lower rates than when applied broadcast.  If I could summarize my experience with pop up fertilizers, I would say that I have seen consistent seedling vigor increases that sometimes translate into a yield increase.  Also, plants receiving pop up fertilizers tend to tassel a few days earlier that those without.

There are a few “Rules of Thumb” when it comes to pop up fertilizers:

  1. Do not apply more than 10 pounds of nutrient per acre.
  2. Avoid using fertilizer with salt index higher than 20.
  3. Rates for sandy soils are typically lower than rates for heavier soils.

During the seed emergence period, nutrient demands by the seedling are low, so applying more than 10 pounds of nutrients may be considered unnecessary, plus results in increased planting time.

Salt index in “fertilizer science” refers to the resultant salt concentration associated with using a specific material.  Sodium nitrate is used comparitively, and it is given a value of 100.  The salt index of some common fertilizer materials include urea with 75; 10-34-0 with 20; and 0-0-60 with around 120.  There are several liquid fertilizer formulations available in the market. Sometimes we see that such formulations, with basically the same amount of nutrients, can differ significantly in price.  The difference in price is associated with the fertilizer source used to make such formulation.  For instance, a common liquid formulation such as 6-24-6 is normally sold at higher price than 7-21-7.  This happens because potassium sulfate is used in 6-24-6 as the potassium source instead of potassium chloride.  The salt index of common 7-21-7 is around 28-29, while the salt index of 6-24-6 is around 11-12.  The use of  fertilizer materials with a salt index higher than 20 increases the risk of seedling injury.

The risk of seedling injury is higher in sandy soils due to the low CEC of such soils that results in more nutrients remaining in solution.  The injury risk increases under dry conditions because of higher concentration (less dissolution) near the seed for extended periods of time.

Using a spraying attachment to apply fertilizer on top of the seed furrow once the seed trench has been packed is probably not very efficient since the nutrients need to move down to reach the seed. Remember that nutrients such as phosphorus do not move much during a season, perhaps just a few inches, so placing the fertilizer 2 inches above the seed may not allow for efficient nutrient use.

Some fertilizers contain polyphosphates such as 10-34-0, while others contain orthophosphates or a combination of both.  So what is the difference?  Well, polyphosphates are basically chains formed with several orthophosphate molecules.  Once a polyphosphate material is applied to the soil it will absorb water and convert to orthophosphate, which is the primary form taken up by plants.  The speed of this conversion will depend on soil moisture and temperature.  In my opinion, under most field conditions plants “may not notice” the difference.

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