David Franzen, NDSU Department of Soil Science
Amitava Chatterjee, NDSU Department of Soil Science
Manbir Rakkar, NDSU Department of Soil Science
John Breker, NDSU Department of Soil Science

The objective of this project was to investigate the yield response of potassium (K) in corn in North Dakota. The original K fertilizer recommendations for corn in North Dakota were borrowed from other states, because soil test K levels in North Dakota were mostly in the high recommendation range, requiring little K. Export of K from our soils due to a change from a wheat state to corn and soybeans, containing many times more K in grain than wheat, has resulted in much lower K soil tests.

The study was conducted on 29 sites in farmer fields in Cass, Richland, Barnes and Sargent counties in North Dakota. The K rates used were 0, 30, 60, 90, 120, and 150 pounds K2O supplied by potassium chloride fertilizer, commonly known as 0-0-60 potash. Having selected most sites having K soil test levels under 150 ppm, we expected corn yield to increase up to a certain K rate, and then select a K soil test critical level based on the highest K soil test with a yield increase to application, and a rate of K based on what K rate resulted in maximum yield. However, only half of the sites reacted as we initially expected.

On further evaluation, we found that whether or not a site responded had much to do with clay chemistry- either smectite clay or an illite clay, both of which are found in greatest abundance in most of North Dakota. We separated sites into those with a smectite/illite ratio of greater or less than 3.5. Sites with greater a smectite/illite ratio greater than 3.5 required a higher critical level (200 ppm) than sites with a smectite/illite ratio less than 3.5 (150 ppm). In addition, we found that rates greater than 200 pounds 0-0-60 resulted in a yield decrease compared to an optimum potash rate of 150 to 200 pounds 0-0-60 depending on the site. In addition, yield increases to K fertilization were less than expected, perhaps due to the relatively high potassium feldspar mineral content of many soils in eastern North Dakota, where the K rate trials were conducted.

In order to produce recommendations that are easy to use and understandable to farmers and their consultants and suppliers, we needed a map of North Dakota clay chemistry. To produce such a map, surface soil samples from two to three major soil groups from each county of the state were collected in early May 2017, and sent off for analysis of clay type and potassium feldspar content.

Smectite clays supply K to the soil solution from their structural interlayers when the soil is moist, but when the soil is dry, smectite clays collapse and draw in K into the interlayers, causing what is called in soil science ‘fixation’. When the soil re-wets, the K again can be released into the soil solution. Fixation is a problem in drier growing seasons. Illite clays supply K to the soil solution whether the soil is wet or dry, thus the lower required critical K level in soils with greater illite presence.

Specific K fertilizer recommendations for corn have been revised and incorporated into revised Extension circulars and on relevant websites.