The soil content of available K for 5a, 10a, 20a, 30a, 40a Larix principis-rupprechtii in the northern foot of the Qinling Mountains was analyzed from May to October to determine the seasonal dynamics in content of available K for L. principis-rupprechtii at different ages. The available K content at 0 to 20 cm in different months for different ages of L. principis-rupprechtii was higher than at the soil depth of 20 to 40 cm’ and 40 to 60 cm’. The soil content of available K at soil depth 0 to 20 cm in different months for different tree ages significantly differed (P<0.05). The soil content of effective K and slow-K at 0 to 20 cm in different months for different tree ages was higher than at the soil depth of 20 to 40 cm’ and 40 to 60 cm’. The soil content of effective K and slow-K at soil depth 0 to 20 cm in different months for different tree ages did not differ (P<0.05). At soil depth 0 to 20 cm, the soil content of available K at different tree ages had a parabolic variation, increasing first and then decreasing, whereas that of effective K and slow-K at different tree ages had a decreasing variation; the soil content of effective K and slow-K of 40a L. principis-rupprechtii was higher than at other ages. The soil content of available K and slow-K for different ages of L. principis-rupprechtii without fertilization reached the middle and high levels, so the soil content of effective K was adequate.