Improved Recommendations for Potassium Management in Missouri Claypan Soils Affected by Drought Stress

Peter Motavalli, Kelly Nelson, and Manjula Nathan---University of Missouri

[Adapted from field day presentation, August 1, 2002, Greenley Agronomy Center, Novelty, Missouri]

Increasing incidences of potassium (K) deficiency in soybean have been observed in Missouri and other Midwestern states due to several climatic and management-induced factors including: 1) the decrease in soil K availability due to drought conditions and soil compaction, 2) reductions in applied K fertilizer and the frequency of soil testing by producers due to decreasing commodity prices, and 3) higher corn yields and increasing soybean acreage in rotation with corn has increased K fertilizer requirements. The objective of this research was to examine the agronomic benefits of applying pre-plant or foliar-applied K fertilizer at different times during the growing season to soybeans as part of a post-emergence weed management program.

A field trial was established in 2001 on a cooperator’s farm southeast of Novelty, MO on a claypan soil in which K deficiency had been observed the previous year. Initial soil test K was low to medium averaging 148 lbs K/acre (std. dev. = 19 lbs K/acre). Four rates of broadcast pre-plant K fertilizer and three additional rates of post-emergence foliar-applied K fertilizer were applied at three growth stages of soybean (i.e. V4, R1-R2, and R3-R4 growth stages).

Visual K deficiency symptoms were observed during the growing season in the check plots compared to plots receiving pre-plant fertilizer K (as K₂SO₄) (Fig. 1A-C).


Fig. 1A. Effect of Pre-Plant K on Soybeans



Fig. 1B-C. K deficient soybeans

Figure. 1 (A-C). Visual K deficiency symptoms in soybeans on a farmer's field near Novelty,
Missouri during the 2001 season.

Application of pre-plant fertilizer K significantly increased soybean yields approximately 20 bu/acre compared to the check treatment up to an application of 250 lbs K/acre (Fig. 2). In addition, foliar application of potassium sulfate increased soybean yields compared to a foliar-applied magnesium sulfate check (S0) at three different growth stages during the season (Fig. 2). Based on visual ratings of leaves, no significant salt injury was observed due to the foliar fertilizer application up to a rate of 32 lbs K/acre (data not shown). Leaf tissue samples taken before each foliar application are currently being evaluated to confirm the observed K response.

These initial research results suggest that foliar K applications in soybeans may be a possible management tool to mitigate reduced yields caused by K deficiency. However, additional testing of this approach is necessary at other locations and under different climatic conditions in Missouri to confirm these results and evaluate the economic benefits.

A special thanks is extended to the Potash & Phosphate Institute (PPI) for their support of this project.