Can you sacrifice a land to pay for a spell's additional cost after announcing the spell?

When you cast a spell, you follow a series of steps defined in CR 601.2. After announcing the spell and making choices (targets, modes, etc.), you reach the payment step where you pay all costs — including any additional costs like sacrificing a permanent.

Specifically, CR 601.2b covers choosing the modes and optional/additional costs you'll pay, and CR 601.2g is where you actually pay all costs simultaneously. Sacrificing a land as an additional cost happens at this payment step (601.2g), which is still part of casting the spell — the spell isn't yet on the stack until all costs are paid.

It's important to note that the land you sacrifice must be one you control at the time of payment. You cannot sacrifice a land that has already left the battlefield or one your opponent controls. The sacrifice is mandatory if the cost requires it — you cannot cast the spell without paying the full cost (CR 601.2h).

Concrete example: You cast Harrow, which has an additional cost of sacrificing a land. You announce Harrow, then during the payment step (601.2g) you sacrifice a Forest you control to pay the additional cost, and also pay the 2G mana cost. Only after all costs are paid does Harrow go on the stack.

No player can respond between your announcement and your payment of costs — priority is not passed until the spell is fully on the stack (CR 117.3a), so opponents cannot destroy your land to prevent you from sacrificing it as a cost.