Is zakat due on the entire current value of the stored goods, or on the difference in value between the past year and the current year?

Zakat is obligatory on trade goods every year if their value reaches the equivalent of 85 grams of gold or 595 grams of silver. Zakat is to be paid on the current value of the goods, and the obligatory amount of Zakat is a quarter of a tenth (two and a half percent).