Fall Fertilizer: Considering P and K Applications

©Debra L Ferguson

There are many benefits to including fall applications of phosphorus (P) and potassium (K) fertilizer into your plans this season. Fall soils tend to be drier, fertilizers are more available, workloads can be spread out over the year, and the nutrients can be incorporated with fall tillage. Fall applied P and K can persist in the soil over winter, remaining available to crops in the spring. 

“Though fall applications of P and K have many practical benefits, growers often ask why they do not often get higher yields with higher application rates of granular P and K,” said Kyle Lilly, certified crop adviser and senior product manager at Compass Minerals Plant Nutrition.

Thinking critically about P and K

Iowa State University studies conducted have reported statistically significant yield responses to increased broadcast P in only one-third of 240 documented trials. With respect to broadcast K, a 2013 analysis of 2,100 studies from the University of Illinois found that broadcast applications gave a significant yield increase only 24% of the time.

Growers are asking some critical questions when it comes to fall-applied P and K fertilizers. Why do less than 50% of P and K fertilizers get taken up by the crop? Where does the fertilizer go? How can growers improve the efficiency of P and K fertilizer?

Where does zinc come in?

Not every field is lacking this key micronutrient, but zinc may be the key to getting the most out of NPK applications in the fall. “Zinc is gaining more attention for good reason – plants are hungry for it,” explained Lilly. University of Illinois research showed that today’s high-performance corn genetics remove up to 27% more zinc than non-rootworm resistant hybrids (Below, 2008).

These situations can especially benefit from zinc fertilization:

  1. Cool, wet soils
  2. Light soils with low organic matter
  3. Soils with high pH

“With zinc being one of the most common micronutrient deficiencies in soils, it’s an important one to consider adding during fall fertilizer applications,” said Lilly.

The advantages of zinc + NPK

“Adequate zinc fertilization may provide relief for inefficient NPK fertilizer uptake and critical zinc deficiencies simultaneously,” said Lilly. Research compiled by the International Zinc Association showed improved yield and nutrient use efficiency of NPK application by up to 25% when zinc is included in the fertility plan.

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“It’s possible that adequate zinc nutrition improves root growth that in turn allows the plant to take up more NPK,” explained Lilly. For example, he said a Compass Minerals Plant Nutrition growth chamber trial demonstrated an increase in corn root growth by 100% compared to a control when zinc fertilizer was applied. The same trial also showed that the zinc fertilizer application resulted in a 36% improvement in plant N uptake, 42% in P, and 75% in K compared to a control.

Next-level zinc application

Applying zinc along with NPK can be done efficiently and effectively with DDP, an innovative dry dispersible powder from Wolf Trax Zinc, which coats fertilizer granules in a blend. “An advantage of this method is that nutrients can be applied using automated and manual powder feeders at the retailer fertilizer blender location,” said Lilly.

This application technology ensures uniform distribution across the field and that zinc is in a form the plant can easily take up.

“While there are no magic bullets for solving fall fertilizer efficiency challenges, the use of zinc as a tool to enhance fertilizer efficiency and protect a grower’s bottom line is promising,” said Lilly.


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