Melt margarine in a large pot over medium-high heat. Saute onion in melted margarine until tender, about 5 minutes.
Stir potatoes with the onion; season with cumin. Pour chicken broth over the potato mixture; bring to a boil, reduce heat to medium-low, place cover on the pot, and cook until the potatoes are tender, 20 to 30 minutes.
Remove pot from heat. Stir jalapeno peppers and baking soda into the broth. Stream evaporated milk into the broth while stirring.
Mash potatoes in the pot with a potato masher until coarsely smashed; stir. Season the soup with salt and pepper. Return pot to medium-high heat and bring soup to a simmer; reduce heat to medium-low and cook, stirring frequently, until the soup is warmed through, about 15 minutes.
