Crop Comments: A New Look at Fertilizer Seed Burn
Fertilizer burn is seedling injury caused by high concentrations of salt, nitrogen or ammonia in fertilizer applied too close to the seed or in-furrow. This causes dehydration, yellowing, stunted growth and delayed maturity.
Nature boasts two ways for getting rid of surplus moisture. The most common method is air warmth: warmer air holds more water vapor. But this time of year, nature uses sublimation to get rid of moisture. This phenomenon occurs with the help of super-cold temperatures.
During sublimation, water molecules are popped loose directly from ice into gaseous form without first thawing and becoming liquid. Sublimation occurs at 14º F or lower. This phenomenon (along with snow removal equipment) gets rid of water using physical changes. But salts get rid of water using chemical methods.
A salt is any chemical compound formed from the reaction of an acid with a base. The most familiar salt is sodium chloride (NaCl), aka common table salt. NaCl couples with frozen water molecules, causing a chemical reaction, yielding two liquids: hydrochloric acid and sodium hydroxide. These compounds, dissolved, take up less volume than the ice and the original salt. The remaining ice crystals, in contracting, restructure with a cracking sound.
This salt behavior is particularly relevant in discussing how fertilizers react chemically, impacting germinating seed as well as seedlings and even developed plants. To quantify the effect of saltiness on these tiny crop forms, scientists use the term “fertilizer salt index.”
Most commercial fertilizers, as salts, are readily soluble. Once dissolved in soil, most fertilizer materials increase salt concentration in soil solutions, which then increase soil osmotic potential. The greater the osmotic potential, the more difficult it is for seeds or plants to extract water from soil – water needed for growth. Here, salt is competing with these tiny plants for moisture.
Fertilizer materials vary in salt content. An excellent display of salt index values can be found online at fieldcropnews.com/2025/05/common-fertilizer-ingredients. Salt index (SI) values for common fertilizer ingredients are shown. They’re calculated by comparing the osmotic potential of a given fertilizer to the osmotic potential of an equivalent weight of sodium (Chilean) nitrate added to water. As the oldest natural, yet chemical, fertilizer input used extensively in agriculture, sodium nitrate is the benchmark for these comparisons.
Sodium nitrate (100% water soluble) was the most commonly used nitrate source when the SI concept was proposed around 1940. As the basis of comparison, this compound arbitrarily was assigned the SI score of 100. All other fertilizer inputs are compared to it. Potassium chloride scores 116; ammonium nitrate, 104; ammonium sulfate, 88; urea, 74; potassium nitrate (saltpeter), 70; anhydrous ammonia, 47; potassium sulfate, 43; mono-ammonium phosphate, 26; and potassium phosphate, superphosphate and most rock phosphates all score 8.
Mixed with several components in a fertilizer, the sum of the SI values represents the total SI for that fertilizer. It’s calculated by weighting the individual ingredients’ SIs with the relative proportion of each item in the finished blend. SI does not predict fertilizer application rate, nor application of type of formulation resulting in injury. This is because the potential for salt injury depends on additional factors, including type of crop (soybeans are more susceptible to injury than corn), soil type (coarse-textured soils are more prone to injury), soil moisture content (wetter lessens chance of injury) and proximity to the seed or seedling. SI classifies materials relative to each other, showing which are most likely to cause problems.
SI is usually not a problem if fertilizer and plant are adequately separated by time, distance or both. One example would be placement of starter fertilizer two inches below and two inches to the side of the seed row (“2×2 placement”). With such positioning, seedling injury is minimal.
However, if fertilizer is applied in or near the seed row, salt can injure both seed and seedling. These band applications hopefully enable roots to intercept nutrients early in the plants’ development.
Potassium (K) in band-applied starter fertilizer blends is most beneficial where K levels are very low. When band-applying fertilizer, we should limit total nitrogen and potash to 100 lbs./acre. Excess SI symptoms are referred to as seed burn. Seed burn issues were best summed up by renown German-American sustainable agronomist Charles Albrecht, Ph.D. Albrecht conducted field crops research at University of Missouri during the 1930s. He said, “Band-applied chemical fertilizer offers the corn seedling a very precise target that it can aim to avoid.”
I witnessed an example of what worried Albrecht during my own career as an agronomy Extension agent during the 1970s. During a field crops short course at Cornell, I saw a demonstration illustrating the impact of excess SI: two pieces of plexiglass about 1.5 inches apart were filled with soil. A few granules of corn starter fertilizer were positioned two inches below the top of the soil mass. A kernel of corn was placed an inch above the fertilizer. Germinating, the seedling’s roots spread out, moving downward, encircling – but clearly not touching – the fertilizer band, missing by a half-inch. Below the fertilizer, roots came back together, forming a distinct circle. Some folks think that with fertilizer, if a little is good, a lot will be even better … a concept that corn seedling strongly disagreed with.
by Paris Reidhead
