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Fluid Journal : Fall 2010
Table 3. Strip trials locations, years, and summary field information. Soil-Test Values† Planting Corn Soil-Test K Year Site Company Date Hybrid Min Avg Max pH OM ----------ppm---------- 2007 1 Iowa 1 May P34A12 116 214 283 7.4 5.9 2 Bremer 5 May DK C58-13 111 156 229 6.5 3.8 3 Greene 14 May NT 2503HX 124 175 251 6.5 4.6 2008 4 Jasper 19 May Crows 4940 T 71 102 133 5.7 2.7 5 Bremer 15 May DK 61-69 113 180 213 6.6 4.9 6 Iowa 2 May P 33F12 91 130 197 7.0 7.1 7 Washington 8 May DK 61-69 132 220 433 7.4 4.6 8 Washington 8 May DK 58-16 110 223 280 6.9 4.1 †Ammonium-acetate test for K; Min, minimum; Avg., average; Max, maximum; OM, organic matter. Table 2. Potassium fertilization effects on early corn growth and K concentration (V5 to V7 stage) and grain yield for eight strip trials. No Broadcast K Broadcast K Field No K Starter K No Starter Starter Plant Dry Weight (g/plant) 1 11.1 10.7 11.2 10.0 2 4.1c 4.4d 3.5a 3.9b 3 3.5a 3.8a 4.3c 3.9b 4 6.6c 6.3b 6.6c 6.1a 5 15.1c 12.9a 12.0a 11.2a 6 9.9 9.5 8.6 8.6 7 3.9 4.3 4.1 3.9 8 6.4b 5.6a 6.8b 6.4b Plant K Concentration (%) 1 4.3 4.3 4.7 4.6 2 4.1 4.2 4.5 4.3 3 3.6a 4.0b 4.2c 4.7d 4 3.0 3.3 3.2 3.7 5 3.0a 3.2a 3.5b 3.7c 6 2.8a 3.4b 3.6b 4.1d 7 4.1b 3.9a 4.2c 4.2c 8 4.0b 3.9b 3.8b 3.5a Plant K Uptake (mg/plant) 1 465 456 536 461 2 165a 187b 160a 165a 3 127a 153b 182c 183c 4 196 208 205 218 5 455 412 416 411 6 296 331 326 357 7 162 169 173 165 8 260b 222a 266b 230a Grain Yield (bu/acre) 1 174a 185b 195c 190c 2 149 151 149 149 3 133 133 140 141 4 172a 189c 183c 180b 5 200 206 211 204 6 205a 217b 224b 224b 7 222 231 230 230 8 221 222 222 224 Numbers in a row followed by no letter or a similar letter do not differ (P < 0.10) and for unknown reasons, application of broadcast or starter K alone did not affect plant K concentration but application of both decreased it. It is interesting that increases in plant K concentration were observed when fertilization decreased or increased early growth. The previous research with 3-18-18 and 0-0-30 starter fertilizers also showed large plant K concentration increases, even when 0-0-30 seldom increased and sometimes decreased it. Early corn K uptake responses (Table 2) reflected mainly effects on plant growth and, therefore, effects were infrequent and inconsistent across fertilizer and sites. At field 2, only starter K increased K uptake. At field 3, both fertilizers increased K uptake but the increase was largest for broadcast K. At field 8, broadcast K did not affect growth but starter K decreased it. These inconsistent results for starter K are in agreement with results from the other set of small plot trials. The K fertilization effects on corn grain yield were significant in three fields (1, 4, and 6), where both broadcast K and starter K applied alone increased yield (Table 2). In field 1, broadcast K alone increased yield more than starter K, and the increases over the control were 11 bu/A for starter and 21 bu/A for broadcast. In field 4 the effects of broadcast K alone and starter K alone on yield were statistically similar, although the increase seemed greater for starter K than broadcast K (6 bu/A greater). In field 6 the effects of broadcast K alone and starter K alone on yield were also statistically similar, but the increase seemed greater for broadcast K than for starter K (7 bu/A greater). An important result was that starter K applied in addition to broadcast K did not increase yield further at any field. Another important result was that there was no yield response to any K fertilizer at fields where at least some treatments increased early plant growth. The soil test K values for these fields (Table 3) and current interpretations in Iowa explained the yield responses only partially. A small or no yield increase also was expected in field 1 because mean soil test K was borderline between high and very high, but values across the field ranged from low to very high (Table 3). A yield increase was expected in field 4 because mean soil test K was low and values ranged from very low to borderline between low and optimum. A yield increase also was expected in field 6, although smaller than for field 4 because mean soil test K was borderline between low and optimum, although values ranged from very low to high. On the other hand, no statistically significant yield increase was observed in field 2 where a small response was expected because mean soil test K was optimum. The unexpected increase response in field 1 or lack of response in field 2 might be explained by variability of soil test K or soil types within the fields. By soil type. Soil types may affect corn response to K fertilization within a field because of potential differences in soil test K and other properties that may affect crop growth and response to K fertilizer. There were two dominant soil types in seven of the fields, so we analyzed corn response to K fertilizer for each dominant soil. Fertilizer K effects on early corn growth and K uptake by soil type were as inconsistent as for the whole-field analysis, while both fertilizers usually increased early-plant K concentration. Therefore, we show in Table 4, and discuss only results for grain yield, the four sites in which there was a differential yield response to K fertilization across soils. In field 1, a whole-field yield increase was explained by a response only in areas with Zook soil, and responses were similar for broadcast and starter K fertilizers. Soil test K was very high for both soils, being only slightly higher for Zook. Therefore, we believe that the higher response for the Zook soil is explained by properties that increase the likelihood of crop response to K, such as finer texture and poorer drainage. In Field 3 the whole-strip analysis showed no yield response, but the analysis by soil type showed a response for the Webster soil. In areas with Webster soil the broadcast K increased yield by 15 bu/A compared with the control, but starter K did not increase yield. We cannot explain the lack of response to starter K. Although soil test K was similar (optimum) for both soils, other soil properties may explain a yield Fall 2010 The Fluid Journal 14
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