In this article:
- Salt Index definition
- How salt injury occurs
- Conditions favoring salt injury
- Products prone to causing salt injury
- Avoiding salt injury
- Link to an article with further information plus instructions for calculating the Salt Index
Fertilizer salt index is a measure of salt concentration induced in a soil solution. Salt index is a numerical value expressed as a ratio in which the selected fertilizer product is compared to the same weight of sodium nitrate (NaNO3), where sodium nitrate is assigned a value of 100. Sodium nitrate is used for comparison because it was widely available when the salt index was developed, and because it is 100 percent soluble in water.
Salt injury to plant tissue is caused when the salt is in close proximity to germinating seed or to growing plant tissue. The presence of the salt creates an osmotic imbalance in which water flows from regions of higher relative water concentration in the plant tissue to regions of lower relative water concentration, where the fertilizer salt is located. This loss of water from the plant tissue causes desiccation, often referred to as fertilizer burn. Salt injury can occur with soil applied fertilizer or with foliar fertilizer applications.
Salt index of a fertilizer product and crop injury are generally of relatively little concern, except under these conditions:
- High salt index fertilizers are applied to foliage or in the seed furrow, or high amounts or concentrations of fertilizer products are applied.
- For foliar applications, leaf tissue is young and tender or plants have been growing under cloudy, moist, cool conditions.
- Susceptible crops are being grown. (Vegetables, sweet corn, etc.)
- For planter applications, soils are dry, preventing normal dilution of product through the soil and allowing high concentrations to remain near seed and seedling tissue.
- Weather conditions are hot and dry.
In general, nitrogen and nitrogen-plus-sulfur products should be watched for salt index concerns. Ammonium thiosulfate has among the highest salt indexes and foliar or in-furrow applications should be managed to avoid problems. Potassium chloride products also have high salt indexes. The old rule of thumb is that you should not apply a total of more than 10 lb./acre of N + K2O products alone or in combination as a seed furrow application. Also, for these seed furrow applications, products with salt indexes greater than 20 should not be used. Consider 2x2 planter applications or soil broadcast applications.
Salt injury from foliar applications can be reduced by avoiding application of products having a high salt index, avoiding application of high rates of fertilizers, diluting the fertilizer with water (increasing the total application volume), and avoiding fertilizer applications during periods with high daily temperatures and/or high relative humidity.
Some of the information in this article came from a 2001 article by Dr. John J. Mortvedt. The article features tables with salt index values, and offers instruction on how to calculate salt index for fertilizer combinations. This article is reproduced on the
Spectrum Analytic website.