How can the presence or absence of natural resources and arable land affect a nation's economy?

The economy is strong if the country exports a lot: it then gets money from other countries. If a country has natural resources (think: diamonds for example!), it will be rich and have a strong economy.

The economy is weak if the country has to import stuff and spend money on it! especially if it's the necessary things: the country has no choice but to import food if they can't produce it, for this reason for example the food items in the north of Canada are every expensive.

Generally, exporting is good for economy and importing bad for it.