Mother Nature recently helped remove snow and ice in most of the Northeast. She did so by shoving outdoor thermometer readings down into single digit Fahrenheit readings. Other than some higher altitude sites, I hadn’t heard of any sub-zero (F) readings.
In the second week of February 2024, the bottom dropped out, temperature-wise, as the southern branch of the northern jet stream plunged way south. Fortunately, Mother Nature helped remove snow and ice by causing sublimation through super-cold temperatures.
In sublimation, water molecules are popped loose from ice directly into gaseous form without first thawing and becoming liquid. Sublimation occurs at +14º F or lower. This phenomenon (and snow removal equipment) get rid of water using physical changes. But salts get rid of water using chemical methods.
Our most familiar salt is sodium chloride (NaCl), common table salt. NaCl couples with water molecules, causing a chemical reaction yielding two liquids: hydrochloric acid in solution and sodium hydroxide, dissolved. These compounds, in solution, 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, 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. The simplest definition of salt is that it is the end product of an acid reacting with a base. Once dissolved in soil, most fertilizer materials increase salt concentration of soil solutions, which then increase soil’s osmotic potential. The greater the osmotic potential, the more difficult it is for seeds or plants to extract water needed for growth from the soil. Here, salt is competing with these tiny plants for moisture.
Fertilizer materials vary in salt content. Tables located online at tinyurl.com/44ma6vyw list salt index (SI) values of common fertilizer materials. They’re calculated by comparing the osmotic potential of a given fertilizer to the osmotic potential of an equivalent weight of sodium nitrate added to water. As the oldest natural yet chemical fertilizer input used extensively in agriculture, sodium nitrate (also called Chilean nitrate) is the benchmark for these comparisons.
Sodium nitrate – 100% water-soluble – was the most commonly used nitrogen source when the SI concept was proposed during early 1940s. This compound arbitrarily receives the salt 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 weighing the individual ingredients’ SIs with the relative proportion of each item in the finished blend. SI does not predict fertilizer application rate, nor application or type of formulation resulting in injury. This is because the potential for salt injury depends on additional factors, including type of crop (soybeans more susceptible than corn), soil type (coarse-textured soils 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 cause problems.
The most visible form of salt injury I ever encountered dealt with slugs, which are extremely salt-sensitive. I learned about slugs’ salt vulnerability from radio pastor Chuck Smith. Salt is mentioned at least 40 times in the Bible – all those times very favorably. But Rev. Smith talked about how as a boy he learned to get rid of garden-variety slugs by sprinkling table salt on them.
Talk about salt injury: these granules’ crystals sucked far more than their mass in moisture from the slimy creatures, making them shrivel up and die in seconds. Guess who became a slug salter!
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 placement, seedling injury is minimal. However, when 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 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 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 renowned German-American sustainable agronomist Charles Albrecht, Ph.D. Albrecht conducted field crops research at University of Missouri during the 1930s. Quoting him: “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, 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 clearly took issue.
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