http://revach.net/article.php?id=2224

Section: Parshas HaShavua   Category: Quick Vort
Parshas Behar: Yovel - When All The Jews Are In Eretz Yisroel

שֵׁשׁ שָׁנִים תִּזְרַע שָׂדֶךָ, וְשֵׁשׁ שָׁנִים תִּזְמֹר כַּרְמֶךָ

"Six years you shall sow your field, and six years you shall prune your vineyard."

לֹא תִזְרָעוּ--וְלֹא תִקְצְרוּ אֶת-סְפִיחֶיהָ, וְלֹא תִבְצְרוּ אֶת-נְזִרֶיהָ

"Yovel shall that fiftieth year be unto you; you shall not sow, neither reap that which grow of itself in it, nor gather the grapes etc. 

Concerning Shmittah the pasuk uses the singular - תִּזְרַע שָׂדֶךָ

תִּזְמֹר כַּרְמֶךָ

However by  Yovel  the pasuk uses the plural

ֹ לֹא תִזְרָעוּ--וְלֹא תִקְצְרוּ

The gemara states that the laws of Yovel only apply when most Jews live in Eretz Yisroel. However, Shmittah applies even when only a minority lives there. This is why the pasuk uses the plural by Yovel - to underscore that there must be a majority living in Eretz Yisroel for Yovel to apply: and the singular by Shmittah to tells us that Shmittah applies even with a minority of Jews there.