Salt Index: A Mini-Refresher Course

Salt Index: A Mini-Refresher CourseOn Jan. 27, the bottom dropped out (temperature-wise) as the jet stream plunged way south, reaching down to south Florida. The official temperature at our Hartwick Post Office was -22º F. I had an early morning doctor’s appointment, and the fluids in our Rogue SUV (other than gasoline) had morphed into something like a blend of Vaseline and bubble gum. The car took its sweet time getting me to my appointment. On the bright side, Mother Nature was helping with snow and ice removal with her gift of super-cold temperatures. Here, frozen water is removed through sublimation, a process in which water molecules are popped loose from ice directly into the gaseous state without having to become a liquid first. 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.

The most familiar salt is sodium chloride (NaCl), or common table salt. This form of salt couples with water molecules, causing a chemical reaction which yields two liquids: hydrochloric acid in solution, and sodium hydroxide, also in solution. 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 concept of salt’s behavior is particularly relevant in discussing how fertilizers perform in such a way as to impact germinating seed, as well as seedlings, and even more mature plants. To try to quantify the effect of saltiness on these tiny crop forms, scientists use the term “fertilizer salt index.”

Most fertilizers are readily soluble, because they are salts. The simplest definition of a salt is that it is the end product of an acid reacting with a base. Once dissolved in soil, most fertilizer materials increase the salt concentration of the soil solution, which in turn increases the soil’s osmotic potential. The greater that osmotic potential, the more difficult it is for seeds or plants to extract from soil the water they need for growth. Essentially what’s happening is that salt is competing with these tiny plants for moisture. Fertilizer materials vary in salt content. A table presented in a University of Illinois on-line Extension publication ( lists some salt index values of common fertilizer materials. These are calculated by comparing the osmotic potential of a given fertilizer to the osmotic potential of an equivalent weight of sodium nitrate added to water. Sodium nitrate, also called Chilean nitrate, is the benchmark for these comparisons, because it is the oldest natural yet chemical fertilizer input used extensively in agriculture.

Sodium nitrate is 100% soluble, and it was the most commonly used nitrogen source when the salt index concept was proposed during the early 1940s. Therefore, sodium nitrate arbitrarily receives the salt index (SI) score of 100. And all other fertilizer inputs are compared to it. For instance, 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.

With a mixture of 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 the fertilizer rate of application or type of formulation that could result in injury. This is because the potential for salt injury depends on additional factors, which may include the type of crop (soybeans being more susceptible than corn), the type of soil (coarse-textured soils being more prone to injury), soil moisture content (more moisture lessens chance of injury) and proximity to the seed or seedling. SI classifies materials relative to each other, showing which are most likely to be a problem.

SI is usually not a problem if fertilizer and plant are 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 this type of placement, seedling injury is minimal. However, when the fertilizer is applied in or near the seed row, salt can cause seed and seedling injury. These band applications hopefully better enable roots to intercept nutrients early in their development. Potassium (K) in band-applied starter fertilizer blends is most beneficial when soil K levels are very low. When band-applying fertilizer, we normally want to limit total nitrogen and potash to 100 lbs./acre. Excess SI symptoms are referred to as seed-burn. The problem with excessive SI was best summed up by sustainable farming advocate agronomist Charles Albrecht, Ph.D., conducting field crops research at University of Missouri during the 1930s: “Band-applied chemical fertilizer offers the corn seedling a very precise target that it can aim to avoid.”

I witnessed an example of what Albrecht was worried about during my own career as agronomy Extension agent during the 1970s. While taking 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 about an inch above the fertilizer. Upon germination, the seedling’s roots spread out, moving downward, encircling but clearly not touching the fertilizer, probably missing it 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 with which that corn seedling, by its actions, clearly begged to differ.

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