Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on him) said: 

It seems to be the case that this share is a kind of trade goods, because those who buy shares in land intend to trade them and make money. Hence they have to pay zakaah on them every year, by calculating their value and then paying zakaah.  If he bought a share of thirty thousand, and after one year has passed this share is worth sixty thousand, then he must pay zakaah on sixty thousand. If after one year has passed the thirty thousand is now worth only ten thousand, then he only has to pay zakaah on ten thousand. Based on this, the questioner should pay the zakaah for each year that he did not pay zakaah. But if these shares have not been sold until now, then if they are sold he should pay the zakaah. But no one should take this matter lightly, rather he should sell them for whatever price he can get, and then pay the zakaah.