Potassium management: Maps showing soil clay type could help Minnesota farmers

By: Leanna Leverich, graduate research assistant, & Dan Kaiser, Extension nutrient management specialist

Soil textural triangle 

When farmers think of clay, they often think of soil texture (clay loam, clay, sandy clay loam, etc.), which is important for making nutrient management and overall field management decisions. However, soil texture classifications only describe how much clay, silt, and sand there is in a soil. They don’t tell us anything about what kind of clay minerals are in the soil. The clay in soil actually contains many different types of clay minerals. Knowing which clay mineral type is dominant in a field could help Minnesota farmers decide how much potassium (K) fertilizer to apply.

Clay minerals: What are they and why do they matter?

Particle size and description 

Clay minerals are more complex than sand and silt particles. First, sand and silt particles sit in the soil like marbles stacked in a jar with the space in between for water and air movement (pore space). Clays are different; they sit like pages of a book, all stacked on top of each other. We consider them “layered,” and these layers are what make clays unique in their interaction with ions, like potassium.

There are two main features of clay layers that impact K:

1. Cation Exchange Capacity (CEC): Differences in the chemical structure of clay layers provides negatively charged sites that can bind ions (potassium, calcium, magnesium, etc.). These charges largely determine a soil’s cation exchange capacity (CEC – see review below).
   
   
2. Interlayer Space: The space between the layers can be large enough of a K ion to get “stuck.” Potassium ions stuck in the interlayer space are often called “Fixed K.” Interlayer space can also be large enough that ions like potassium (K+) can flow in and out of the interlayer.

­Main Clay Types

While there are many types of clay minerals, we are focusing on three clay minerals that are common in Minnesota and the Midwest:

• Kaolinite: low CEC; no interlayer space
• Illite (mica): moderate CEC; interlayer space that can trap (fix) K+
• Smectite: highest CEC; interlayer space that shrinks and swells based on the water content of the soil; can trap or release K

What do we know so far about K and clay type?

Using previous field trial data and recent on-farm trials, we created a large database of soil K test data across Minnesota. Soils from the K trials were tested for K level and for the type and quantity of clay minerals (i.e. Kaolinite, illite, and smectite).

No soil has all smectite or all illite clay. Soils have a mix of many clay minerals but may be dominated by one type of clay mineral. To simply our data, we looked at the ratio of smectite to illite minerals to determine high smectite soils from high illite soils. The ratio (smectite/illite) break point used was 3.5.

Comparing the two clay groups, we saw a difference in critical K level. The critical K level can mean a lot of things to crop producers, but in this case we identify the critical K level as the point where 95% of maximum yield is achieved, which is typically around our medium soil test classification. In illite-dominated soils, the critical K level was lower:

• 

  Potassium deficiency in corn

  High illite soils: critical K level of 85 ppm
  
  
• High smectite soils: critical K level of 106 ppm

What does this mean for farmers?

In higher illite soils, we need a lower soil test K level to maximize yields. So in higher illite soils, we may need to apply less K fertilizer to attain maximum corn and soybean yields since the critical STK level is lower.

Mapping Clay types in Minnesota

Learning that soil test K levels differ by clay type might have growers asking: What type of clay do I have on my farm?

Mineralogy tests are expensive (upwards of $300 per sample) and require sending samples to a special lab that has the capacity to test mineral fractions of the soil. To help Minnesota farmers avoid this cost, we are building mineralogy maps of the state that growers can use.

Clay mineral abundance is not like nutrient levels, which can vary substantially within fields. Rather, clay mineral abundance follows large-scale soil formation and will likely not vary within fields or even farms. With samples from all over the state, we have begun to create useful maps that will show relative abundance of smectite, illite, kaolinite, and other clay minerals (i.e. K feldspars). We will also map useful ratios, like the ratio of smectite to illite.

Watch our progress live

To see our current maps and watch our progress as we build the maps, visit our mapping site: z.umn.edu/MNclayMaps

What’s next for the K guidelines?

Only about half of the samples we have taken have been included in our analysis so far. (The light purple points on the map above have not yet been analyzed.) Once we receive mineralogy data for the remaining samples, we will add those points to our analysis and mapping efforts. Growers can expect to have improved and more accurate maps once these samples are added. As of now, we do not have any changes to the K recommendations for corn and soybean based on mineralogy tests, but growers should expect to see updates in the future.

View the current fertilizer guidelines for corn and soybeans.

Learn more about this research by watching this video recording of Leanna Leverich's full presentation on Reassessing Potassium Fertility Guidelines with Soil Mineralogy at the 2022 Nutrient Management Conference.

Questions about this work can be directed to Leanna Leverich (lever115@umn.edu) or Dan Kaiser (dekaiser@umn.edu).

Thank you to the Minnesota Corn Research and Promotion Council for funding this project and for providing the means to develop these maps for all producers.

---

For the latest nutrient management information, subscribe to the Minnesota Crop News email newsletter, like UMN Extension Nutrient Management on Facebook, follow us on Twitter, and visit our website.

Support for Minnesota Crop News nutrient management blog posts is provided in part by Minnesota's fertilizer tonnage fee through the Agricultural Fertilizer Research & Education Council (AFREC). Learn more at MNsoilfertility.com