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Charles Mitchell Potassium
(K) nutrition of crops on the acid, highly weathered soils typical
of the southeastern United States has always been a concern, especially
for cotton, which is susceptible to K deficiencies. With increasing acreage
and yields of cotton on these soils, new varieties, eradication of the
boll weevil, and new technologies for insect control, K nutrition is of
renewed concern to growers. In
1982, the Two-year Rotation experiments were put into a residual phosphorus
(P) and K mode (no additional P and K were applied to these plots from
1983 through 1998. Cotton has been a principal crop in these experiments
for 46 of the 69 years from 1929 to 1997. Therefore, these experiments
offer an excellent opportunity to study soil K changes with time and re-evaluate
K nutrition of cotton. In
1982 when annual K applications ceased on all treatments except the 60
pounds K2O per acre treatment, the two highest K treatments
were at or above what was considered to be a “high” soil test
K level for cotton at all sites. A “high” soil test is above
an established critical value and is defined as an adequate supply of
that nutrient; no additional application of that nutrient is recommended. There
has been much grower concern that with higher yielding, earlier maturing,
modern varieties, soil test calibration for K on cotton needs adjusting.
However, these data indicate that the sufficiency level approach to critical
K values as used by the Auburn University Soil Testing program in Alabama
and other southern states is still very reliable and accurate. The sandiest
soils (the Dothan and Benndale series) are included in one graph and the
two Lucedale are included in another graph according to current Alabama
soil test calibration. The Decatur soil, which is representative of cotton
producing soils of the Tennessee Valley region, has the highest buffering
capacity and the highest critical soil test K level. No potassium recommendations
are made when the soil test value is above the critical value on the graphs. In
a separate but related study, cotton yields on the two Lucedale soils
and on the Benndale soil were found to be highly significantly related
to soil test K in the 0-8 inch depth, in the 8-16 inch depth, or in the
16-24 inch. However, using soil test K from different depths did not improve
soil test calibration. There
were no significant differences in yield between Nucotn35B and DPL5690
nor any interaction between variety and K fertility at the Prattville
Experiment Field in 1996 and 1997. Both varieties responded to residual
soil K similarly. However, in 1998, Nucotn35B produced higher cotton lint
yields (1,050 pounds per acre) over all K treatments than did DPL5690
(920 pounds per acre), probably a result of less insect damage. Current soil test calibration K values for Alabama soils are still accurate for modern varieties and yields. Plow-layer, soil test K is still a very reliable tool for predicting the need for K fertilization on Alabama soils when other factors are not limiting. There is no apparent need to adjust K fertilization for the new Bollgard® varieties.. |
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