What keeps countries from planting more crops is that after a poor harvest there less food for both people and animals, which leads to a slaughter of work animals then that means you are less able to efficiently grow and harvest corps which leads to less food for people and animals, and that leads to an even larger slaughter of work animals...
That´s only true if countries can´t import farm animals from other countries. Which in the 1860s isn´t true.
You also presuppose that all harvests in Europe somehow fail in the same year. Given the different climates between - say - Britain and Central Europe that strikes me as unlikely.
But I digress, The threat of a grain embargo did work because there was a move by some politicians in Great Britain to recognize the CSA but only smart, cool leadership by Lord Pamlerston {who was very pro Confederacy) kept both Great Britain and France from recognizing the CSA. I have been trying to get research information about the weather and crop harvest levels but I do not have the resources to do so.
http://www.econ.ku.dk/research/publications/pink/2008/0808.pdf
Page 9, figure 5: UK and US wheat production
I´d estimate British domestic wheat production between 10 and 18 million quarters between 1860 and 1865. With a recorded harvest of 18+ million in 1864. With US production around 20+ million during that period.
Page 10, figure 6: UK consumption of wheat and population
UK consumption seems to be between 17 (1861) and 23 (1864/65) million quarters.
(The high consumption in 1864/65 might be a result of the record harvest in 1864? Resulting in lower prices?)
So imports seem to be around 5-7 million quarters of wheat per year?
Plus apparently bad harvests in 1862/63 in Britain?
On the other hand I own a book "Climate History of Central Europe". It records too dry years in 1858, 1859 and 1864 for that region of Europe. Meaning that harvests in those years probably would be below average. However 1860 to 1863 in Central Europe seem to have been perfectly normal years. So exports of wheat from Central Europe are certainly possible during those years.
While the too dry year 1864 in Central Europe is canceled out by a record harvest in Britain.
According to
wikipedia, a quarter is 12.7 kilograms.
So 1 million quarters = 12,700 metric tons.
Which means that wheat imports to Britain were around 64,000 to 89,000 metric tons per year during that period.
I´ve got no data about France or Russia.
Anyway, simply saying that a bad harvest in Britain is somehow representative for the whole of Europe is quite simply wrong.
And the idea that a "grain embargo" could somehow "starve" the European powers into submission is ridiculous. 60000 to 90000 tons (minus imports from European countries) definitely won´t worry Britain.
Wheat prices certainly would have risen. And that´s it.
But by threatening a "grain embargo" public opinion in Europe might have turned against the USA.
While in OTL the "slavery question" played a big role. After all, slavery in the British Empire was abolished on August 1, 1834.
As for Great Britain and France doing anything about an U.S. grain embargo they really can not project force across the Atlantic Ocean, all the U.S. has to do to win is to go in a defensive crouch, build up coastal deferences, build more ships for the navy, and if Great Britain attack, just raise 150,000 troops and take out Toronto and Halifax.
The French did deploy 39,000 troops to Mexico around 1862. Isn´t that a "projection of force across the Atlantic Ocean"? And that was with Mexico being a project of Napoleon III (and not really of the French population).
Don´t you think the British Empire could at least match that?
Now imagine British and French newspapers telling their readers that the "dastardly Yankees" want to blackmail their countries with a grain embargo. Raising prices to make it harder for you to feed your child.
Plus honor, patriotism etc....
I also wonder if the threat of getting cut of from South American saltpeter might influence the USA?