Date of Graduation

12-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Crop, Soil & Environmental Sciences (PhD)

Degree Level

Graduate

Department

Crop, Soil & Environmental Sciences

Advisor

Nathan A. Slaton

Committee Member

Larry C. Purcell

Second Committee Member

Trenton L. Roberts

Third Committee Member

Andy Mauromoustakos

Fourth Committee Member

Steve Phillips

Abstract

The potassium (K) requirement of soybean [Glycine max (L.) Merr.] was investigated to determine whether cultivar sensitivity to K deficiency was affected by growth habit (determinate or indeterminate) and how cultivars from each growth habit accumulate and distribute K among plant structures. We also diagnosed K deficiency across reproductive growth stages (R2-6) using trifoliolate leaf- and petiole-K concentrations and at harvest (R8) using seed-K concentration. Soybean responded similarly to K deficiency in terms of yield, selected yield components, and seed-K concentration, regardless of growth habit. The yield loss from K deficiency was greatest on the middle to upper nodes of the indeterminate cultivar and bottom and upper-middle nodes of the determinate cultivar. Seed-K concentration decreased from the bottom to the top nodes of K-deficient plants compared to K-sufficient plants, suggesting K concentration of seeds collected from the upper nodes would be of value for diagnosing K deficiency. We found that both growth habits accumulated maximal K at the R5.5-6.0 stage with the peak accumulation rate at the R3-4 stage. Soybean accumulated 35-45% of the maximum K by the R2 stage and 65-70% by the R4 stage, suggesting that K deficiency of soybean could possibly be corrected by timely fertilization during the early reproductive stages. Trifoliolate leaf- and petiole-K concentrations from the R2 to R5.5 stages were highly correlated with relative soybean yield. The K concentrations in both tissues peaked near the R2 stage and declined linearly at a constant rate with increasing plant age regardless of cultivar, site-year, and K fertility level, indicating that the critical K concentration at each stage beyond the R2 is a negative linear function of time. Mature seed-K concentration across 100 site-years in North America was strongly correlated with relative soybean yield and may seed analysis be an effective post-season tool for diagnosing K deficiency. The threshold of deficient seed-K concentration (<16.5 g K kg-1) accurately predicted that soybean yield would be increased by fertilizer-K at 77% of the sites. These research findings will provide growers with more comprehensive tools to identify fields where K availability to soybean may limit yield.

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