Effect of Long-term Lime and Potassium Applications on Quantity-Intensity (Q/I) Relationships in Sandy Soil

The effects of long-term lime and K applications on quantity-intensity (Q/I) relationships were investigated on the Ap and B21t horizons of a Kalmia soil (a fine-loamy over sandy or sandy-skeletal, siliceous thermic (Typic Hapludults) from the Delaware Coastal Plain. The predominant mineral suite of the <2faa clay fraction was mica, vermiculite, and chloritized vermiculite. Soil pH and exchangeable bases increased with depth and with lime additions. The equilibrium potassium activity ratio (AR*e) decreased with profile depth due to greater K fixation by specific sites for K in the B21t horizon. The AR\ decreased in the Ap horizon and increased in the B21t horizon with lime additions. The magnitude of ARe [> 0.01 (moles/ liter)] suggests that K adsorption in the Ap horizon occurred on planar positions while adsorption at specific sites was predominant in the B2H horizon [> 0.006 (moles/liter)^]. The parameter AK°, which measures labile K, became more negative with increased lime and K additions, indicating a greater K release into soil solution. While the quantity of K extracted by NH.OAc compared favorably to AK° in the Ap horizon, it exceeded A£° in the B21t horizon, suggesting K exchange involving specific sites in the B21t horizon. The number of specific sites (Kz) increased with K fertilization and with soil depth. The decreased K, with increased lime additions could 1 Contribution from the Dep. of Plant Science, Univ. of Delaware, Newark, DE 19711. Published with the approval of the Director of the Delaware Agric. Exp. Stn. as Misc. Paper no. 907. Contribution no. 114 of the Dep. of Plant Science. Received 28 July 1980. Approved 10 Mar. .1981. "Assistant Professor and Associate Professor, respectively. be ascribed to increased neutralization of hydroxyaluminum interlayer material, resulting in an increase in interlayer "islands." The potential buffer capacity (PBC) parameter increased with lime additions due to increased pH-dependent cation exchange capacity (CEC). Additional Index Words: K exchange, Coastal Plain soils, Debye-Hiitkel theory, specific K sites, activity ratios. Sparks, D. L., and W. C. Liebhardt. 1981. Effect of long-term lime and potassium applications on quantity—intensity (Q/I) relationships in sandy soil. Soil Sci. Soc. Am. J. 45:786-790. F A GREATER understanding of the fertility status of agricultural soils, it is important to study K(Ca-Mg) equilibria relationships. While researchers have noted a lack of crop response to K additions on Atlantic Coastal Plain soils (Liebhardt et al., 1976; Yuan et al., 1976; Sparks, 1980; Sparks et al., 1980; Woodruff and Parks, 1980), little is known of the quantity-intensity (Q/I) relationships of these soils. The latter (Beckett 1964a) relates the change in exchangeable K (quantity) to the ratio of [aK] /[aca + «Mg]^ (intensity). With Q/I information, sounder predictions could be made concerning the K supplying power of Atlantic Coastal Plain soils and how lime additions affect K availability in these soils. Several indices of K availability in soils have been proposed. The quantity SPARKS & LIEBHARDT: LIME AND POTASSIUM: QUANTITY-INTENSITY (Q/I) RELATIONSHIPS IN SANDY SOIL 787 may be presented as a measure of the free energy of exchange of K in the soil by Ca and hence as a measure of the availability of K (Woodruff, 1955). Beckett (1964a) introduced the Q/I concept to aid in predicting the K status of soils. Theory of Q/I Determinations A typical Q/I curve is shown in Fig. 1 with these parameters: AK = quantity by which the soil gains or loses K in reaching equilibrium or the Quantity (Q) factor, AR = activity ratio for K or the intensity (I) factor, A^° = labile or exchangeable K, ARe = equilibrium activity ratio for K, Kx = specific K sites, and PBC = potential buffering capacity. The potassium intensity factor (AR) is computed from the measured concentrations of Ca, Mg, K, and Na corrected to the appropriate activities by application of extended Debye-Hiickel theory as follows from Eq. 1 through Eq. 3 (Moore, 1972): log 0-* = — [1] 1 + a/3 V^ where a± — the mean activity coefficient of the electrolyte, a = constant (0.5042), z = valency of cation, z~ = valency of anion, aft = assumed to be 1, and / — ionic strength, where I = i/2 SiCiZi and [2] where C{ = concentration of ion t and Zj = valency of ion t. The activity ratio (AR) was calculated as AR = CK (<rKCl) [3]