It is the relative advantage that matters in production, not how fertile land is or the total that can be produced. The product with the comparative advantage should be exported while the comparative disadvantaged product should be bought. The reason for the comparative advantage stems from giving up less resources than another country to produce the product. The comparative advantage depends on the trading partners, as this is a comparative, also known as a relative, statistic rather than an absolute one.
Even a product that is heavily produced in a country, the product can be at a comparative disadvantage due to its demand and ability to supply it. Preventing trade, through any means of imposing barriers on competition, means that country chooses to create more waste than necessary. The natural price of the product will increase, but not the natural value which means that the produces would gain at the expense of everyone else. There are those that lose and get hurt by altering production due to more trade, but the country as a whole benefits.
This book is not an easy read. Parts of it have extreme clarity, but others require not just an interpretation of implications made, but also an understanding of Ricardo's contemporary situation. Not a mathematically intense book, but data is used to prove points made. Some chapters are dedicated to clarifying ideas to the most minute detail. Ricardo is a rare type of author, because in some parts of the book, he admits to have considered a problem in the wrong way, and then to elucidates the more appropriate explanation. ( )

In a way Ricardo's economic theory is akin to Aristotle's physical theory, it feels a bit difficult to get a grip of it since it's presented without any mathematical or graphic aids. Nevertheless it's easy to see why this was a groundbreaking work, Ricardo's economic acumen and the explicitness of his arguments are nothing short of brilliant.

Second edition.