Quantification of Plant Available Potassium (K) at Various Depths in Soils of the Texas High Plains.

Most Texas soils are reported to be close to or above the critical value of potassium (K) for optimum cotton production; however, the frequency and severity of K deficiency symptoms in cotton on these highly productive soils has been increasing over the past decade. In order to meet the demands of modern cotton varieties, the soil must be able to replenish available K in a timely manner. The inability of a soil to do so may be a possible explanation for the yield response to increasing rates of knife injected K fertilizer. It is also possible that current ammonium based extraction solutions may be overestimating plant available K in soils containing certain 2:1 clay minerals. If this is the case and consequently K fertilizer is not being applied to a soil that cannot replenish K quickly enough to meet the demands of the crop, deficiency symptoms, decreased lint yield and fiber quality, reduced drought and disease tolerance are possible. A study was conducted to determine the levels of K in selected soils using two different extraction methods. Soil samples were collected at the 0 - 15cm, 15 - 30 cm, and 30 - 60 cm soil depths from a cotton growing area in Lamesa, Texas. Mehlich III and H3A extraction methods were used to determine the soil K levels. Mehlich III extraction resulted in high K levels ranging from 200 to 500 mg/kg, while the H3A extraction resulted in K levels ranging from 73 to 336 mg/kg. A potassium fixation test based on the incubation method revealed 40-60% of the applied K was fixed in the soil. The soil texture was sandy loam to sandy clay loam with sand content ranging from 60-80% and clay ranging from 12-25%. Clay type identifcation will be conducted to determine clay mineralogy in the soils.