Yes, it's a horrible sentence but correct. The writer should be shot! The structure of this sentence is: reason - main point - consequences. I suggest that you cut up the sentence into different clauses. Put each clause on a new line (including the relative clause).

The subject of the main clause is 'This agricultural revolution', and it has two verbs, both of them 'increased'. It increased nitrogen content and increased number and quality of animals. This main clause has both a 'cause', which comes before it, and a 'result', which comes after it. It's a hideous sentence.

Since nitrogen rich crops such as alfalfa, clover or sainfoin began to be planted, onto previous barren fields. After a previous fallow year. The nitrogen content of the soil for subsequent crops, not only increased grain yields, but also increased the number and quality of farm animals, thus increasing the supply of meat and animal products for food and also increasing the fertility of the soil by increasing the supply of animal manure for fertilizers . I will leave the grammar points and the linguistic points to teachers on italki. Instead I will attempt to rewrite the paragraph for you.