You also have to take into account the crown to dropout length of the fork. If the Soma fork is longer than your stock fork, it raises the front-end of the bike and reduces the head tube angle and increases the trail. If the Soma is shorter, it lowers the front-end and decreases the trail. If the two forks are the same length then just use one of the online trail calculators to figure out where the fork will you.

ETA: I realized you mentioned what bike you already have so if you have the Soma Cyclocross fork, there is 4mm of difference in length between it and the low-trail fork. That's probably negligible and not worth considering. I think the general rule of thumb is 1cm difference in fork length changes the head angle 1/2 degree.