The same number of cucumber seedlings were planted in the two experimental greenhouses. Magnesium salt was applied in the first greenhouse, but not in the second. The first one produced 10 kilograms of cucumbers and the second one produced 5 kilograms. Since nothing else was applied to the greenhouse except water, the higher yield of the first greenhouse must be due to magnesium salt.
The statement 'There was a small amount of magnesium salt in the soil of both experimental greenhouses' weakens the above argument most.