What are things I should watch for applying potassium with a planter?

Cristie Preston, Ph.D.

Cristie Preston

Cristie Preston, Ph.D.


Senior Agronomist

Agriculture has always been an integral part of Dr. Cristie Preston’s life. She grew up in southwest Virginia and had interest in crop and animal agriculture since an early age. Once she began college, she initially chose to study animal science but switched to soil science. Dr. Preston attributes her decision partly to an influential professor who told her, “You can’t understand animals until you understand what they eat.” She received a Bachelor of Science degree in animal science and a Master of Science degree in crop and soil environmental science from Virginia Tech. Dr. Preston holds a Ph.D. in agronomy, focusing on soil fertility from Kansas State University. While completing her advanced degrees, Dr. Preston conducted more than six years of field and lab research. Dr. Preston has experience in laboratory research measuring volatility loss from urea-based fertilizers. Her field research has focused on phosphorus availability and the interactions with tillage and placement. She also has extensive experience in working with large data sets and analysis. Her main priority is helping growers to identify yield-limiting factors and fix those issues as cost efficiently as possible.

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Q: What are things I should watch for applying potassium with a planter?

A: If you are considering the application of potassium with your planter make sure it is accounted for when determining your “safe” rate of fertilization. For 2×2 applications, keep your total salt (nitrogen plus potassium) rate around 80 to 100 pounds per acre. For in-furrow applications, keep your total salt rate around 5 for coarse textured soil and 8 for heavy textured soils.

Q: How much fertilizer can I apply without doing any damage to the seed or soil structure? Is there any sensitivity of seed with fertilizer that I should I be aware of if I apply fertilizer when planting?

A: When considering any type of starter fertilizer application (whether it is 2×2 or in-furrow with the seed) we have two primary concerns – salt and ammonia. Salt is relatively easy to calculate. It is the sum of nitrogen and potassium that you plan to make per acre. The further away from the seed furrow the higher the salt application rate can be (see answer above for more info). Ammonia is toxic to germinating seeds at relatively low levels, so minimizing the exposure risk is key. Ideally, we would want to avoid urea-based in-furrow products. If urea-based products are going to be applied with the seed, the more acid the solution the better (decreases the chance of producing free ammonia). Also, avoid basic (alkaline) fertilizer materials being applied with the seed. The best example here is MAP (monoammonium phosphate) versus DAP (diammonium phosphate). Monoammonium phosphate creates a slightly acidic pH in solution when it dissolves, and DAP creates a slightly basic pH. DAP has a much higher risk of producing free ammonia than MAP, and is therefore a greater risk to the germinating seed.

Q: How much should I be concerned about salt levels with starter fertilizers causing damage to my crop?

A: Do consider the salt level of the starter material being evaluated. We have a way of determining the salt index of any fertilizer solution. The lower the salt index, the lower the risk of salt injury for the germinating seed. Obviously, rate is still a major consideration, but at equivalent rates, a lower salt index product is safer. Salt index is based upon the source material used to create the fertilizer liquid.