Source: University of Wisconsin Crop Manager

Without fail every year after planting, questions start popping up about pop-up fertilizer. The questions always occur when there are emergence or germination issues. So before planting gets into full swing, let’s think about seed-placed starter fertilizer.

Why are Fertilizer Salts a Problem? 

Excessive concentrations of fertilizer salts near a germinating seed or seedling root causes injury. The injury is caused when the concentration of ions in the soil is greater than the concentration of ions within the plant cells. The high osmotic pressure created by the fertilizer salts causes water to move out of the plant cells and into the soil. As water moves out of the plant cells, the tissue dessicates and becomes blackened; hence the term fertilizer burn. The result is the eventual death of the plant tissue. 

Some nitrogen (N) fertilizers may cause more seedling and germination injury than expected based on their salt content alone if they liberate ammonia when applied to the soil. Free ammonia is toxic and can move freely through the plant cell wall. Urea, UAN, ammonium thiosulfate and DAP can cause more damage from ammonia toxicity than MAP, ammonium sulfate, and ammonium nitrate. Moderate alkaline soil conditions, either in the bulk soil or caused by reaction of the fertilizer, will promote ammonia production. 

Factors Affecting Fertilizer Burn

Crops vary in their tolerance to salts. Wheat is the least sensitive, followed by corn, forage legumes, soybean and edibe bean (dry or snap) increasing in sensitivity, with vegetables, including sweet corn, being the most sensitive to fertilizer salts. It's suggested that no fertilizer be placed with the seed of super sweet hybrids of sweet corn, soybean, edible beans and peas because of their sensitivity to salts. 

Soil conditions are important for determining why injury may occur in one year and not another. Fertilizer salts diffuse away from the band in moist soils and becomes diluted, reducing the osmotic pressure. Little diffusion takes place in dry soils and the fertilizer remains concentrated with a high osmotic pressure presenting a greater risk to plant injury. 

Soils with low cation exchange capacity (CEC) (coarse-textured with low organic matter content) have a lesser ability to react with the fertilizer compared to high CEC soils (fine-textured) meaning that the concentration of fertilizer salts in the soil solution remains high. Thus, fertilizer burn is a bigger issue on sandy, low organic matter soils particularly in dry springs. 

Soil temperature also plays a role. Roots grow slowly in cold soils; thus, the root is exposed to the higher concentration of fertilizer for a longer period of time. 

Concentration of fertilizer salts is another factor that determines whether or not fertilizer burn occurs. Broadcast fertilizer applications do not often injury seedlings because the fertilizer is dispersed through a large volume of soil. Banded starter fertilizers placed 2 inches to the side and 2 inches below the seed are more likely to cause injury than broadcast applications because banded applications are much more concentrated in a small area near the seed. However, at typical starter fertilizer application rates, fertilizer burn from banded starter fertilizer is unlikely. 

In-furrow (pop-up or seed row) placed fertilizers are typically applied at low rates but their very close proximity to the seed means they are more likely to cause injury than 2-by-2-inch banded applications because there is little opportunity for the root to grow out of the zone of concentrated fertilizer salts before it dies. In general to avoid stand loss from fertilizer injury, no more than 10 pounds per acre of N and potash (K2O) should be applied in-furrow regardless of soil texture. The most suitable fertilizers for in-furrow applications will have: 1) low salt index, 2) high water solubility, 3) no compounds that liberate NH3, and 4) use potassium phosphate instead of KCl as the potassium (K) source. 

Safe Rates of In-Furrow Fertilizer 

Salt index (SI) of a fertilizer is a measure of the salt concentration that fertilizer induces in the soil solution. However, there are many steps in calculating SI and it can be a bit confusing. South Dakota State University developed a Fertilizer Seed Decision Aid spreadsheet and web calculator based on field and greenhouse research. 

The Fertilizer Seed Decision Aid requires users to select the crop to be grown, fertilizer type, seed furrow width, row spacing, tolerated stand loss, soil texture and soil moisture at planting. The tool will then output a maximum rate of fertilizer to apply with the seed. 

The tool is really handy in assessing scenarios. For example, what if the soil was wetter or drier; what if we can accept more or less stand loss. Using the tool you may find that some relatively common practices are a little riskier than you think. A good example of this is using ammonium thiosulfate in seed-placed starters.