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Potassium Sources and Availability in a Deep, Sandy Soil of East Texas¹

ABSTRACT

Soil properties were assessed that may have contributed to the failure of Coastal bermudagrass [Cynodon dactylon (L. Pers.)] to respond to application of K on Darco fine sand for 5 consecutive years. The depth to which plant roots absorbed K was assessed by field observation and by determination of K levels in control and treatment plots. Potassium minerals were determined by chemical methods to assess potential nutrient sources.

Potassium removal was indicated to at least 160 cm by soil solution content and to 235 cm by exchangeable K concentration and Gapon selectivity values. Depth of bermudagrass rooting was in agreement with chemical indicators of K removal.

Mica and interstratified mica in the fine silt and clay fractions correlated best with exchangeable K (r² = 0.97) suggesting a direct influence of the finer micaceous materials in Darco soils upon K retained in an exchangeable form and possibly upon K release. Inclusion of feldspar-K with mica-K reduced the r² value from 0.97 to 0.83 suggesting that K-feldspar does not contribute appreciable K to exchangeable form. Feldspar was present mostly in the sand and two coarser silt fractions which expose little surface area to weathering forces.

The large rooting volume of Coastal bermudagrass in Darco soil in conjunction with the mica and appreciable interstratified mica present may explain why no plant response was obtained to K application.

¹ Contribution from Texas Agric. Exp. Stn., College Station and Overton. Presented before Div. S-9, Soil Sci. Soc. of Am., Chicago, Ill. 12 Nov. 1975.

² Graduate Assistant and Professor of Soil Mineralogy, Texas A&M Univ., College Station, TX, 77843, and Associate Professor of Soil Chemistry, Texas A&M Univ. Agric. Res. and Ext. Ctr., Corpus Christi, respectively.

Received for publication May 27, 1975. Accepted for publication January 23, 1976.