Factors Affecting the Persistence of Picloram in Soil¹

The effect of soil type, temperature and moisture on the persistence of 4‐amino‐3,5,6‐trichloropicolinic acid (picloram) was determined using vapor phase chromatographic analyses and plant bioassays. Detectable quantities of picloram were present in Houston clay, Axtell sandy loam, and commercial sand after incubation for 1 year, at 4, 20, and 38 C and at moisture levels of field capacity and 0.1 field capacity from rates as low as 0.25 #g per g (½ lb/A). Movement studies indicated that leaching was an important means of dissipating the herbicide in light soils. Photodecomposition may also be important if the herbicide remains on the soil surface for long periods of time.