You are given the following premise:
(1) The demand for corn is increasing
(2) Corn and soybeans are substitute goods
(3) Corn and soybeans compete for the same inputs of production
Based on the rules of demand and supply, we know that (1) causes the price of corn to increase. (Demand goes up, price goes up.)
Based on the rules of cross elasticity of demand, we know that the increase in price of a good will increase the demand for its substitutes. (Price of Sprite goes up, people want 7-Up instead.) Therefore, we know from (2) that as the price of corn goes up, the demand for its substitute, soybeans, also goes up. Also, the price of soybeans increases as its demand increases.
Finally, we know from (3) that farmers face a question of whether to replant their soybean fields as corn instead. If they plant more corn, they can plant less soybean. In other words, more corn production will entail an opportunity cost equal to the value of soybean production foregone.
So the big question is this: Is the payoff for more corn production, i.e. the increase in corn prices, worth the opportunity cost of foregone soybean production, i.e. the increase in soybean prices? Both corn and soybean prices increased, so to answer this question, we need to figure out which of the two increased more.
I can't answer that question with absolute certainty, because I have to make educated guesses about three quantities: the price elasticity in demand of corn, the price elasticity in demand of soybeans, and the cross elasticity in demand between the two. I would guess that the price elasticity in corn and soybeans are about the same, and that the cross elasticity between corn and soybeans is less than 1, because there are multiple substitutes for corn. Based on these guesses, I would predict that the price of soybeans would increase less than the price of corn increased, therefore farmers would plant a more corn and less soybean, decreasing the supply of soybeans.