There has been a lot of discussion about the term “salt index” and what it means with regard to crop safety for fertilizers.

When synthetic fertilizers were first becoming prominent in the marketplace one of the concerns was the crop safety that each product provided, and how that related to where a product should or shouldn’t be placed.

The term “salt index” was used to help describe the relative safety of fertilizer products — both liquid and dry. Over the years, the term salt index has been used for a variety of things — some that make sense, and some that were, perhaps, not technically accurate.

In order to understand salt index, it’s important to understand what is meant by the term “salt.” A salt is any chemical compound that is composed of a positively charged ion and a negatively charged ion. When most of us hear the word “salt” we tend to think of sodium chloride, or table salt. Sodium chloride is a salt but it’s not a common component of fertilizers.

The question is often asked about how much salt fertilizers have. In strict chemical terms, fertilizers ARE salts. One of the more recognizable fertilizer formulas is K-Cl, or potassium chloride. That compound is 0-0-60 potash. The potassium component is a positive ion and the chloride component is a negative ion. That fertilizer, along with all others, are salts.

Selecting a Fertilizer

Why was the concept of salt index developed? The original intent was to develop a scale or index of the potential for a fertilizer to cause crop injury. The actual numbers reported can be measured values using electrical conductance tests, or can be calculated values based on product components.

It’s easy to see how different analysis methods can give different index values, so comparing the salt index of various products is problematic unless the products were all measured (not calculated) using the exact same methods.

When describing the potential for fertilizer injury, is the salt-index number of any value? Not as much as it once was. Some literature suggests fertilizers with salt indexes above 20 should not be applied near the seed of sensitive crops.

Commodity fertilizer products such as potash or DAP are well known to cause crop injury when placed too close to a sensitive seed. Some liquid fertilizers, such as 10-34-0 or 6-18-6, can be applied in furrow to certain crops but with significant rate restrictions. Many newer technology products are safe for in-furrow application to many crops, including some products that have salt index values higher than 20.

If salt index isn’t a good predictor of fertilizer injury to many crops, what should be considered when selecting a fertilizer?

First and foremost, crop safety and performance of any product should be the focus of any discussion. Product crop safety and performance claims should be backed by research and field experiences rather than justified by a laboratory value.

When selecting fertilizer products and application placement it’s important to use the best agronomic practices for the product, crop, and row spacing. For example, corn and soybeans may have different limitations on what rates products can be applied in furrow or as a foliar spray.

In addition to the product itself, there are several environmental conditions that need to be taken into account when determining crop safety risks. Soil environmental conditions play a large role in crop response to fertilizer products — with colder, dryer soil conditions having a higher potential for adverse crop response when compared to a warmer, moist soil.

Foliar applications have additional issues to consider with regard to crop safety and performance. Crop growth stage is a very important factor in the safety and performance of foliar fertilizer applications. Tank-mix partners and surfactants may also play a role in safety and performance.

Salt Index Takeaways

When tank mixing with crop protection products it’s important to READ AND FOLLOW LABEL DIRECTIONS of the pesticides. Pay special attention to tank-mix restrictions and compatibility testing instructions on the pesticide label.

Here are some quick takeaways about the salt index

  • Don’t worry about absolute numbers. Methodology, test conditions and the products tested all influence the index value is reported. Also, don’t get caught up in salt index comparisons with other products.
  • Do consider the safety, flexibility and performance of products, and the research plus field experiences that prove performance.
  • Do select and apply fertilizers based on sound agronomic practices. Consider what crops, application methods, tank-mix partners and environmental conditions are present when making fertilizer decisions.

No-Till Farmer's Focusing on Smarter, Sustainable Fertilizing Strategies series is brought to you by AgroLiquid.

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