Last fall I wrote an article about the fate of dry phosphate fertilizer if applied in the fall or spring.
This article generated this reader question from Ocheyedan, Iowa: "I read your article about dry fertilizer application for MAP and DAP. But you didn't talk about potash. Is it better to apply 0-0-60 in the fall or spring for availability next year?"
We all know the primary reasons for fall application: Our schedules allow more time to apply, the soil is in better condition for trafficking and fertilizer is usually a bit cheaper than in the spring. Retailers also like to move product in the fall -- it removes some of the scheduling pressures that come in the spring, especially if the weather turns wet.
Potash fertilizer (0-0-60) can be applied in fall or spring with similar efficacy. Potash is much more soluble than lime or gypsum, similar in solubility to MAP or DAP, but slightly less soluble than urea or ammonium nitrate.
Potash will move into the soil better than phosphates and there is less risk of it moving off the soil surface in solution or as precipitate potash. Fortunately, potash is not one of the contributors to hypoxia in surface waters. Nevertheless, keeping potash around for the crop is still important.
Potash does have a tendency to fix into the soil matrix (soil particle structure) similar to how phosphate ties up with soil minerals like calcium, iron or aluminum. However, the soil matrix already holds a considerable amount of potassium and it moves back and forth between particles and soil solution much more rapidly than phosphate does.
I asked the spring versus fall question of Robert Mullen, Potash Corp. director of agronomy. "I don't think I could effectively argue that spring is necessarily better than fall for potassium application from an agronomic perspective," said Mullen. "Potassium and phosphorus fertilization is more about improving soil conditions, not necessarily directly fertilizing the crop.
"You are correct though, fall application is more desirous because of field accessibility, time availability, and maybe some cheaper prices," he added in his email response.
As an agronomist, I am a believer in the "keep it for the crop" concept -- put it on just before the crop needs it, when it is most available and then, protect it from loss. Spring applications provide a fresh, more available supply than do fall applications and there is less risk of loss.
Send questions for Dan Davidson to firstname.lastname@example.org
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