Salt Index Calculator

Fertilizer salt index measures how much a given fertilizer raises the osmotic pressure of the soil solution around it, relative to a reference material -- a higher salt index means a fertilizer draws more water away from nearby seeds and roots, which can burn or delay germination when placed too close to the seed. This calculator looks up a published salt index value for the fertilizer product you select (values here range from a relatively gentle 27 for MAP up to 116 for muriate of potash) and multiplies it by your application rate to estimate a salt effect at the rate you're actually applying -- the same fertilizer at a heavier rate produces a proportionally larger salt effect. It also reports the Primary Nutrient the product is valued for (N, P₂O₅, or K₂O) and the amount of THAT nutrient applied per acre (application rate times the product's guaranteed percentage of that nutrient) -- for a multi-nutrient blend like MAP (11-52-0), only the dominant 52% P₂O₅ is counted here, not the secondary 11% N, consistent with how salt index tables are conventionally referenced.

It also shows salt index scaled to a 100-lb-nutrient basis, which lets you compare fertilizers on a nutrient-delivered basis rather than a raw-product basis. From the salt effect at your rate AND the product's own salt index, the calculator assigns a qualitative seedling injury risk band (low, moderate, high, or very high) and a seed-placement recommendation that escalates when a heavy application rate pushes the salt effect high, even for a naturally low-salt-index product -- higher-salt-index products (or any product at a heavy enough rate) need more separation from the seed row or should be broadcast rather than banded. These are general agronomic guidelines; actual seed tolerance also depends on soil moisture, texture, and the crop being planted.