Persistence of capsicum oleoresin in soil

We studied the persistence of capsicum oleoresin in a sandy loam soil. Five capsicum–soil concentrations (6.00%, 3.00%, 1.50%, 0.75%, and 0.00% wt : vol) were prepared in separate plastic boxes without lids; soils were elevated above wire-screen sub-floors to allow percolation and were exposed outdoors to ambient conditions. Plexiglass shields were placed above the boxes to allow ventilation but to deflect precipitation. During an initial 14-day period, soils were sprinkled once daily with 0.64 ml/cm2 of water to simulate rainfall, then were maintained dry for a final 14 days. Spectrophotometric measurements (282 nm capsaicin wavelength) of independent, dual, oven-dried, 10-g samples from each soil mixture were made for Days 0 (pre-watering), 2, 4, 6, 8, 10, 12, 14, and 28 post mixing. Results showed that: (1) simulated rainfall was an important linear regressor of capsicum oleoresin in a sandy loam soil accounting for 43–75% of spectrophotometric variation, (2) mean λmax absorbance (282 nm) of soil extracts decreased 45% after cumulative sprinkling with 8.96 cm of water (14 days of 0.64 cm/day), then remained essentially unchanged during a subsequent 14 days without simulated rainfall, and (3) regression slopes for the initial 14-day mean λmax absorbance functions for the 4 oleoresin–soil mixtures were heterogeneous (i.e., differed in slope), with greater concentrations showing relatively faster depletion. Thus, capsicum oleoresin persisted in soil for 28 days, with persistence enhanced under arid conditions. Although costs of capsicum oleoresin are prohibitive, these persistence data support the feasibility of developing “in-soil irritant” technology to repel fossorial rodents from prescribed areas—a less expensive, persistent irritant is sought.