Cotton Response to Residual Fertilizer Potassium on Vermiculitic Soil: Organic Matter and Sodium Effects

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Economic return from K addition to soils with a large K‐fixation capacity is largely determined by the residual benefit. To quantify the residual benefit on a vermiculitic soil, wheat ( Triticum aestivum L.) was grown in 1988 and cotton ( Gossypium hirsutum L.) in 1989 and 1990 in treatments that had received previous fertilizer‐K inputs of 0, 360, 720, and 1440 kg ha −1 from 1985 to 1987. Wheat yield and K uptake were not affected by previous K input levels. In contrast, lint yield of cotton with the highest residual‐K rate increased by 240 to 250 kg ha −1 in 1989 and 1990 (a 22–23% increase) compared with the control without previous K addition. In all years, lint yield was closely correlated with solution‐phase K + in surface soil although a slow K‐fixation reaction decreased this soil K pool by 23% from 1987 to 1990. This reduction occurred despite net inputs of 85 and 130 kg K ha −1 to cotton in 1989 and 1990, respectively, that were uniformly applied as manure or mineral fertilizer‐K main plots over the existing previous K‐rate treatments, which were subplots. Manure application resulted in a significant increase in soil organic C and reduced the slow fixation of K from the labile pool, compared with equivalent K inputs from mineral fertilizer. Use of moderately saline groundwater for irrigation in the drought years of 1989 and 1990 increased extractable Na from 0.07 cmol kg −1 in 1987 to 1.24 cmol kg −1 in 1990. With increased soil Na, there was a shift in the lint yield response to solution‐phase K + such that the critical level for maximum yield was reduced. These results demonstrated a large cotton yield response to residual fertilizer K on vermiculitic soils, and this residual benefit was greater with organic‐matter addition. Where Na has been leached from the root zone, a moderate increase in soil Na may reduce the total K requirement due to a Na‐K substitution that is known to maintain cotton yields at lower levels of K supply.