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

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Abstract
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.

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