Why were turnips and clover grown as a part of a farmer's crop after 1730

Have you heard of the four field system? Viscount Townshend successfully introduced a new method of crop rotation on his farms. He divided his fields up into four different types of produce with wheat in the first field, clover (or ryegrass) in the second, oats or barley in the third and, in the fourth, turnips or swedes. The turnips were used as fodder to feed livestock in winter. Clover and ryegrass were grazed by livestock. Using this system, he found that he could grow more crops and get a better yield from the land. 
 If a crop was not rotated, then the nutrient level in the field would go down with time. The yield of the crop from the field decreased. Using the four field system, the land could not only be "rested", but also could be improved by growing other crops. Clover and turnips grown in a field after wheat, barley or oats, naturally replaced nutrients into the soil. None of the fields had to be taken out of use whilst they recovered. Also, where animals grazed on the clover and turnip fields, eating the crop, their droppings helped to manure the soil.