New Soybean Potassium Recommendations

Filed Under:

Publication Number: P3434

Thanks to the efforts of MSU Extension soil scientists and graduate students, with backing from the Mississippi Soybean Promotion Board, we have sufficient data to support changes in soybean potassium recommendations.

Soil samples are placed into one of five categories (very low to high) based on extractable potassium level and cation exchange capacity (CEC). From their assigned category, samples are given a fertilizer recommendation.

Beginning in January 2020, soybean groupings were revised to have lower thresholds to trigger a potassium rate recommendation. Table 1 shows the previous categories, and Table 2 shows the revised categories. In addition, soybean recommended rates for K20 fertilizer in the low category changed (Table 3). These changes are intended to help producers optimize fertilizer rates and reach their yield goals.

Samples that are categorized as very low will remain at 120 pounds per acre of K20. All samples that are categorized as low will now receive a recommendation of 90 pounds per acre (previously 60). Samples that are categorized as medium will continue to get a recommendation of 60 pounds per acre. No fertilizer recommendations are given for samples in the high or very high categories.

Here is an example of how this change might impact a sample recommendation: Under previous recommendations, soils testing at 250 pounds per acre extractable K with a CEC of 30 were classified as a high rate and, thus, did not get a potassium recommendation. Data from Dygert (2019; Figure 1) suggests this is not sufficient, and additional K would trigger a yield response. A minimum of 250 pounds per acre of extractable K is required for K to not be a limiting factor in soybean yields.

Additionally, data from research trials conducted from 2011 to 2019 in the Mississippi Delta suggests that, when soil test values are in the responsive range, 80 pounds per acre of K2O are required to maximize soybean yields (Figure 2).

**Table 1. Previous soybean potassium groupings based on soil test extractable K in pounds per acre.**
Category	CEC ≤7	CEC 7–14	CEC 14–25	CEC 25+
Very Low	0–50	0–60	0–70	0–80
Low	51–110	61–140	71–160	81–180
Medium	111–160	141–190	161–210	181–240
High	161–280	191–335	211–370	241–420
Very High	280+	335+	370+	420+

**Table 2. Revised soybean potassium groupings based on soil test extractable K in pounds per acre.**
Category	CEC ≤7	CEC 7–14	CEC 14–25	CEC 25+
Very Low	0–70	0–90	0–120	0–150
Low	71–150	91–190	121–240	151–260
Medium	151–200	191–240	241–290	261–320
High	201–350	241–420	291–510	321–560
Very High	350+	420+	510+	560+

**Table 3. Current and revised soybean recommended fertilizer rates in pounds per acre for K20.**
Category	Current	Revised
Very Low	120	120
Low	60	90
Medium	60	60
High	0	0
Very High	0	0

Image description in text. — Figure 1. Soybean response to potassium application. Data from research
trials conducted between 2011 and 2019 in the Mississippi Delta. Research
conducted by Bobby Golden, PhD, Associate Extension/Research Professor,
MSU Delta Research and Extension Center.

Publication 3434 (POD-03-20)

By Keri Jones, PhD, MSU Extension Soil Testing Laboratory Coordinator, Plant and Soil Sciences.

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