Determinate and indeterminate full-season soybeans were grown at sites that represented a wide range of soils with varying tillage systems used in NC soybean production(Table 1 and 2). Two sites, BW and WP, had low CEC and high W/V indicative of sandier soils so more leaching potential was expected at these sites. The HY, TW, PQ, PBRSLT sites had much lower W/Vand higher humic matter which related to higher CEC. The PRSLT and MRS sites had high CEC which is related to their lower W/V indicating finer texture.

At single-year sites before fertilizer application, soil test K levels for each depth (I-A, I-B, I-C) were similar and not significantly different (Table 3). At the long-term sites where fertilizer gradients were previously established, significant differences in soil K existed at the initial sampling as expected (Table 4 and 5). For the single-year sites, initial soil potassium levels (K-I) in the A depth ranged from a low of 30 at BW with the lowest CEC of all sites to a high of 48 at the TW site which had a much higher CEC (Table 3). At the long-term sites, initial soil potassium levels (K-I) in the A depth ranged from 19 to 76 at PRRSLT and from 37to 99at the PRSLT site (Tables 4and 5). At single-year sites after treatment, soil test K in the topsoil (A layer) increased with increasing rate of fertilization (Table 3, Figure 1). At PQ and WP single-year sites, significant movement of K (leaching) into the B depth occurred, especially at higher rates of K application; it should be noted that the sample depth was 4 inches. At the MRS site, varying K among depths after treatment may be partly due to an artifact of sampling depth differences at the time of study initiation (two, 6-inch depths sampled) and after treatment (three, 4-inch depths sampled).

Although no fertilizer was applied for this season to the “0 K” treatments at the long-term sites(PBRSLT, PRSLT), soil K in the topsoil A layer appear much higher at the midseason sampling (AT) (Tables4 and 5).At the PBRSLT at Lewiston, movement of K into the B and C depths under higher fertilization is also evident. It is likely that migration of recently applied K fertilizer and/or crop residue from adjacent plots into the “0 K” occurred and accounts for these soil K-index increases.

Leaf K concentrations at all growth stages for single-year sites are presented in Table 6and for the long-term sites in Tables 7and 8. Significant increases in plant tissue K at V3 (whole plant samples) with increasing fertilization occurred at HY and PBRSLT sites (Tables 6 and 7). At the PQ single-year site and PBRSLT and PRSLT long-term sites at V5, leaf K increased significantly with increasing rate of fertilizer K applied while at TW, WP, and BW, no differences were seen. We did not sample V5 at other sites. At R2, significant increase in leaf K with increasing fertilization occurred at single-year sites HY, PQ, WP, and BW(Table 6, Figure 2) and both PBRSLT and PRSLT long-term sites(Tables 7and 8). At R5where only a limited number of sites were sampled, leaf K% responses to fertilization occurred at both long-term sites and at the HY single-year site.