Potassium Fertilization of Cotton Produced on a Low K Soil with Contrasting Tillage Systems

Abstract

Correcting K deficiency in fast-fruiting cotton (Gossypium hirsutum L.) cultivars requires information on growth response to available soil K, soil K2O rates, and supplemental foliar K. Field research was initiated in 1991 on a Memphis silt loam soil (fine-silty, mixed, active, thermic Typic Hapludalf), low in Mehlich I extractable K, to evaluate cotton responses to soil-and foliar-applied K for conventional-tillage (CT) and notillage (NT) production systems in Tennessee. Potassium rates of 0, 30, 60, and 120 lb K2O/acre were soil-applied to the same plots each year. Foliar treatments included: no-foliar K, KNO3, and Ca(NO3)2. KNO3 was applied four times per year to supply 4.4 lb K2O/acre per application. Ca(NO3)2 was applied at 1.4 lb N/acre per application, equalling the N applied in the KNO3 foliar applications. Petiole K concentrations were evaluated for each foliar application. Soil samples (0–6 in.) were collected after harvest from the no-foliar K treatments of each main plot to evaluate Mehlich I extractable soil K. Lint yields were determined by mechanically picking the two center plot rows twice each year. Extractable soil K increased linearly with increased soil K2O rates each year for both tillage systems. Yields in both tillage systems were increased by soil- and foliarapplied K. Regression equations expressing yield as a function of K2O rate were developed for KNO3 and no-foliar K treatments each year for both tillage systems. In 1991 and 1992, the foliar KNO3 treatment increased yields at all soil K2O rates. In 1993, foliar KNO3 increased yields at soil K2O rates up to 94 lb K2O/acre in CT and up to 103 lb K2O/acre in NT. In addition, foliar KNO3 increased yields at soil K2O rates up to 108 lb K2O/acre for the 1994 NT cotton. Relationships found between Mehlich I soil extractable K and soil K2O rates were used to calculate the critical soil extractable K levels for predicting yield response to foliar K. These values ranged from 158 to 161 lb K/acre and correspond to the 160 lb Wacre value used to distinguish between medium and high testing soils in Tennessee. For this soil, foliar K supplemented soil K2O rates and increased yields when extractable K was medium or lower. Moreover, petiole K concentrations indicated plant K was deficient in 1991 and 1992 at all K2O rates and was deficient in 1993 for K2O rates of 60 lb/acre or less. Research Questions Potassium deficiency may limit yields of certain high yielding, fast-maturing cotton varieties. Information is needed on K fertilizer requirements to correct deficiencies on low testing soils. What are the optimum combinations of soil and foliar K applications on a low extractable K soil for conventional- (CT) and no-tillage (NT) cotton? What is the relationship between soil-applied and foliar K for increasing lint yields on a low K soil? Literature Summary Visual K deficiency symptoms in cotton have been observed in the new vegetative growth of fast-fruiting varieties. Deficiencies occur as the boll load is set over a short growth period and when root activity and elongation are reduced. Foliar applying low K rates (4.4 lb/acre K20 per application) four times during the growing season may supplement soil-applied K to increase yields. Study Description Soil and foliar K were applied to CT and NT cotton to evaluate the effect on petiole K and yields of cotton and on extractable soil K. Soil: Memphis silt loam (Typic Hapludalf). Experimental design: Split plot arrangement of treatments in a randomized complete block. Treatments were replicated five times. Main plots: 0, 30,60, and 120 lb/acre soil K20 rates applied as KC1. Sub-plots: foliar treatments; no-foliar K check, KNO3, and Ca(NO3)2. Potassium nitrate was applied at 4.4 lb K20/acre/application and Ca(NO3)2 was applied at 1.4 lb N/acre per application, equivalent to N from KNO3. Foliar treatments were applied at bloom or 2 wk after bloom and on either a nine or 14 d interval for a total of four applications. All foliar treatments were applied in 10 gal water/acre. Petioles were collected and K concentrations determined. Soil samples were collected after harvest from the no-foliar K check sub-plots within each main plot. Lint yields were determined by picking the two center rows twice each year. Applied Question What effect did the soil-applied K20 have on extractable K? Mehlich I extractable K in both CT and NT systems increased linearly with K20 rate during each year of this 4 yr study. What soil-applied K20 rates are required to reduce K deficiency in cotton produced on a soil low in extractable K and to produce yields equivalent to applying both soil and foliar K? Lint yields were increased from both soil- and foliar-applied K in CT and NT systems. In 1991 and 1992, the foliar KNO3 treatment increased yields at all soil K20 rates. In 1993, foliar KNO3 increased yields at soil K2O rates up to 94 lb K2O/acre in CT and up to 103 lb K2O/acre in NT. In addition, foliar KNO3 increased yields at soil K2O rates up to 108 lb K2O/acre for the 1994 NT cotton. Relationships found between Mehlich I soil extractable K and soil K2O rates were used to calculate the critical soil extractable K levels for predicting yield response to foliar K. These values ranged from 158 to 161 lb Wacre and correspond to the 160 lb Wacre value used to distinguish between medium and high testing soils in Tennessee. Where is foliar K's place in cotton production? Based on these data, foliar K may be expected to increase yields on medium and low testing soils, even if fertilized with 120 lb K2O/acre. Response to foliar K is unlikely where Mehlich I extractable K levels exceed 160 lb/acre. What effect did soil-applied K2O have on petiole K concentrations? Petiole K concentmtions indicate plant K was deficient in all K2O treatments in 1991 and 1992. In 1993, petiole K indicated plant K to be deficient when K2O rates of 60 lb/acre or less were applied, but plant K was sufficient at the 120 lb/acre rate. These data support the yield data as to sufficient and deficient K levels.