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Issue No. 5, Article 7/May 7, 2010

How Much Salt Is in the Fertilizer?

This growing season has been unique in the many things farmers have been able to accomplish in such a short time, in part thanks to good weather. Many of last fall's operations were pushed into spring, and most farmers have been able to get them done and plant on time. This is certainly a good change, especially compared to the previous two springs. Although planting is progressing very well and many farmers are probably done planting, the number one question I have been asked this week is about fertilizer salt index.

Most fertilizer materials are readily soluble because they are salts. Once they are dissolved in the soil, they increase the salt concentration of the soil solution, which in turn increases the solution's osmotic potential. The greater the osmotic potential, the more difficult it is for the seeds or plants to extract the soil water they need for growth.

Materials vary in amounts of salt content. Table 1 shows some salt index values of common fertilizer materials, calculated by comparing the osmotic potential of a given fertilizer to the osmotic potential produced by an equivalent weight of sodium nitrate added to water.

Why sodium nitrate? Simply because it is 100% soluble and it was a commonly used nitrogen (N) source when the salt index concept was proposed back in the early 1940s. The salt index of sodium nitrate is defined as 100. When you have a mixture of nutrients in a fertilizer, the sum of the salt index values ("partial salt index" in the table) represents the total salt index of that fertilizer. Something important to keep in mind is that a salt index does not predict the amount of 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 are more susceptible than corn), the type of soil (coarse-textured soils are more prone to salt injury), the soil's moisture content (more moisture means less chance for injury), and proximity to the seed or seedling (discussed more below). The salt index does, however, classify materials relative to each other and shows which are most likely to be a problem.

Salt injury from fertilizers is typically not a problem if fertilizer and plant are separated by time, distance, or both. One example would be placement of starter fertilizer 2 inches below and 2 inches to the side of the seed row (2 x 2 placement). With this type of placement the risk of seedling injury is virtually nonexistent. However, when the fertilizer is applied in or near the seed row, salt can cause seed and seedling injury. This type of application has different names, but most common are "pop-up," "in-furrow," and "seed row."

These applications are typically made to increase the possibility that roots will intercept nutrients early in development. Studies have shown that whenever there is a response to starter, the effect is the same whether the fertilizer is placed with the seed or in some other way, such as 2 x 2 placement. Often the largest response to starter is observed for N followed by phosphorus (P), and then potassium (K).

Potassium in starter normally produces advantages when soils are very low in K. When applying fertilizer in the seed row it is critical to apply as little salt as possible. The usual recommendation is to apply no more than 10 lb of N plus K2O per acre. This recommendation is based on K2O from potassium chloride. If the source of K is potassium phosphate, you can afford to apply a little more because potassium phosphate has a lower salt index. Nonetheless, seed-row applications do not allow as much fertilizer, and they are riskier than 2 x 2 placement or other types of starter applications.

If you are convinced that placement with the seed (seed-row application) is the way to go, I suggest trying to use fertilizer sources and rates that minimize the chance for injury. In addition to salt injury, some N compounds (such as UAN, urea, and ammonium thiosulfate) produce free ammonia, which can cause poor germination or seedling death. The best fertilizers for seed-row application have a low salt index, N compounds that do not produce free ammonia, and potassium phosphate rather than potassium chloride as the K source.--Fabián G. Fernández

Fabián Fernández

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