You are here

New Soybean Potassium Recommendations

Filed Under:
Publication Number: P3434
View as PDF: P3434.pdf

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.

 

Image description in text.
Figure 2. Response curve of relative seed yield influenced by increasing
Mississippi soil test K for 0- to 15-cm soil depths up to the relative seed
yield plateau. Data from Dygert (2019). Varietal and residual soil test K
level effects on soybean leaf K status and yield. MS Thesis. Mississippi
State University.

 


Publication 3434 (POD-03-20)

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

Copyright 2020 by Mississippi State University. All rights reserved. This publication may be copied and distributed without alteration for nonprofit educational purposes provided that credit is given to the Mississippi State University Extension Service.

Produced by Agricultural Communications.

Mississippi State University is an equal opportunity institution. Discrimination in university employment, programs, or activities based on race, color, ethnicity, sex, pregnancy, religion, national origin, disability, age, sexual orientation, genetic information, status as a U.S. veteran, or any other status protected by applicable law is prohibited. Questions about equal opportunity programs or compliance should be directed to the Office of Compliance and Integrity, 56 Morgan Avenue, P.O. 6044, Mississippi State, MS 39762, (662) 325-5839.

Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. GARY B. JACKSON, Director

The Mississippi State University Extension Service is working to ensure all web content is accessible to all users. If you need assistance accessing any of our content, please email the webteam or call 662-325-2262.