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
The third experimental greenhouse applied a high nitrogen fertilizer without magnesium salt, and produced 7 kg of cucumber.
 Since nothing else was applied to the greenhouse except water, the higher yield of the first greenhouse must be due to magnesium salt