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New Potassium Recommendations for North Dakota Crops (08/31/17)

During the past 3 years my graduate students and I have worked on the yield response of corn to fertilizer potassium (K).

New Potassium Recommendations for North Dakota Crops

During the past 3 years my graduate students and I have worked on the yield response of corn to fertilizer potassium (K). What I thought would be an easy task quickly revealed that our understanding of K in this state relative to corn was lacking. The K soil test only predicted whether a site responded or not about half the time. We began to investigate what other people had found in different parts of the US and the world, and found that clay type, potassium bearing minerals, soil moisture and other factors influenced response of crops to K fertilizer. In North Dakota, our soils have between 2 and 10% of the total mineral material as potassium feldspar (Figure 1). This modifies our K response, because opposed to what textbooks try to tell us, there is a rapid equilibrium between potassium feldspar and the soil solution that can be measured in hours and days, rather than in lifetimes as some used to suggest.


More importantly, the clay type makes a difference in K availability. We have three dominant clay types in North Dakota; smectite (shrinking-swelling 2:1 clays), illite (not-so shrinking-swelling 2:1 clays), and kaolinite (non-swelling 1:1 clays), with a very small amount of chlorite (2:1:1 non-swelling clays). The K availability of kaolinite is totally dependent on CEC, but K availability of smectites and illites have nothing to do with CEC. In illites, the K is released from surface charge and interlayer K whether the soil is wet or dry. In smectites, K is released from surface charge and interlayer K only if the soil is moist. When the soil dries, in our area typically in late June, July, usually August, K is drawn back into the interlayer and becomes temporarily retained and plant-unavailable. Therefore, a soil with a large dominance of smectite requires a greater K availability initially than a soil with significant illite.

                In our work, the sites we worked at - about 30 sites from Cass, Richland, Sargent and Barnes counties- were a mix of soils with smectite and illite. The results of the study were that sites with a smectite/illite ratio greater than 3.5 required 200 ppm K in the soil test before there was no response to K in dry years. Soils with less than 3.5 required only 150 ppm K in the soil in wet or dry years.

Because a recommendation to the grower is based on clay type without any indication what are the clay types within their neighborhoods, it would be very confusing, and very expensive for growers to determine their smectite/illite ratios, a survey of the state was conducted during this past spring. In each county not sufficiently sampled during the K rate study, soils within two to three major soil groups in each county were sampled at the 0-6 inch depth in spring 2017. The soils have only recently been completely analyzed for potassium feldspar content and clay species in the clay fraction of the soils, along with other minerals. The map in Figure 2 provides the areas within North Dakota that correspond to those recommendation critical levels. Fields near the boundaries of the two classes should error on the side of higher ratios given our summer tendency for dryness in the state.


The map in Figure 2 will be used to revise current K recommendations for corn, sugar beet and alfalfa, with the new recommendations officially printed and circulated in their respective Extension Circular revisions within the next 60 days. For now, this article should be a helpful guide as fall fertilizer plans for 2018 crops are being developed.

In addition, we found that K fertilizer applications greater than 200 pounds 0-0-60 per acre in a given season resulted in lower corn yields than the 150 or 200 pounds 0-0-60 rate, so 0-0-60 rates should be capped at 200 pounds per acre 0-0-60 for a single application. If a grower chooses to build soil test levels, application to soybean or other crops to obtain the higher total rotational rate would be advised, rather than apply large amounts of potash before corn or any other crop. We do not know why the yield response tails off with rate, but this phenomenon has been seen in other states as well.

Finally, soils that are sandy loam or coarser, with less than 2.5% organic matter will not be able to be built to higher levels than about 100 ppm K, if that. It is better to fertilize these low organic matter sandy soils every year, rather than apply buildup K rates.

Dave Franzen

NDSU Extension Soil Specialist



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