The main difference between these two agricultural revolutions is that the Second Agricultural Revolution was based mainly on industrialization and the Third Agricultural Revolution was based largely on biotechnology.
The Second Agricultural Revolution occurred largely as a result of the Industrial Revolution. The Industrial Revolution led to the creation of all sorts of machinery. Included among these machines were machines that could help increase farm productivity. For example, during the Second Agricultural Revolution, farmers started to be able to use tractors to pull various types of farm implements such as plows and harvesters. By using these new kinds of machinery, farmers were able to increase their productivity tremendously.
The Third Agricultural Revolution happened in the second half of the 20th century. This was a revolution based on science. During this revolution, researchers used genetic modification techniques to create varieties of plants that were more productive. They also used science to create fertilizers, pesticides, and other chemicals that increased productivity.
In short then, the difference between these revolutions is that the earlier one was based on machinery and the later one was based on scientific advances in things like genetics.