How would I formulate a constraint to insure at least 30% of the mix is soybeans?

The Southfork Feed Company makes a feed mix from four ingredients: oats, corn, soybeans, and a vitamin supplement. The company has 300 pounds of oats, 400 pounds of corn, 2100 pounds of soybeans, and 100 pounds of vitamin supplement available for the mix. A pound of oats costs $0.50; a pound of corn costs $1.20, a pound of soybeans costs $0.60, and a pound of vitamin supplement costs $2.00. Use the following decision variables: Let X1 = the number of pounds of oats to put in the mix Let X2 = the number of pounds of corn to put in the mix Let X3 = the number of pounds of soybeans to put in the mix Let X4 = the number of pounds of vitamin supplement to put in the mix