Increased soil sorption of pendimethalin due to deposition of guayule-derived detritus

Laboratory sorption experiments were conducted to determine the extent to which soil organic matter derived from guayule (Parthenium argentatum A. Gray) residues can affect the sorption of pendimethalin [N-(l-ethylpropyl)-3,4- dimethyl-2,6-dinitrobenzenamine], Soils where guayule had been grown for 0 to 38 mo were collected for sorption experiments from beneath actively growing plants. An additional treatment was collected before planting cotton (Gossypium hirsutum L.) in a field 1 yr after the final harvest of guayule that had been grown for 36 mo was also included. Continuous guayule cultivation resulted in a modest increase in soil organic C from 0.29% (±0.01%) to 0.76% (±0.03%) after 38 mo of production. Pendimethalin sorption to the soil increased with increasing guayule organic deposition as measured by time under cultivation. Soils where guayule was grown for 38 mo had an organic C distribution coefficient, K_OC, 23 times greater than the control soil where guayule had not been grown (389,400 vs. 16,900 L kg^-1). In the treatment before planting cotton, however, the combination of time and cultivation resulted in a reduction of K_OC from 389,400 to 21,500 L kg^-1. The use of pendimethalin in a guayule-cotton rotation requires sufficient time and primary tillage before planting cotton. In addition, the use of pendimethalin during regrowth of guayule from the stump following harvest may require higher application rates to control weeds. Adsorption of pendimethalin to soil using batch equilibrium indicated that soil beneath actively growing guayule has a higher sorption capacity for pendimethalin than soil without guayule.