The infiltration capacity of a soil follows the Horton's exponential model, f = c, During and experiment, the initial infiltration capacity was observed to be 200 mm/h. After a long time, the infiltration capacity was reduced to 25 mm/h. If the infiltration capacity after 1 hour was 90 mm/h, the value of the decay rate constant, k (in 11', up to two decimal places) is____