The four-field rotation system allowed farmers to restore soil fertility and restore some of the plant nutrients removed with the crops. Turnips first show up in the probate records in England as early as 1638 but were not widely used till about 1750. Fallow land was about 20% of the arable area in England in 1700 before turnips and clover were extensively grown in the 1830s. Guano and nitrates from South America were introduced in the mid-19th century and fallow steadily declined to reach only about 4% in 1900. Ideally, wheat, barley, turnips and clover would be planted in that order in each field in successive years. The turnips helped keep the weeds down and were an excellent forage crop—ruminant animals could eat their tops and roots through a large part of the summer and winters. There was no need to let the soil lie fallow as clover would re-add nitrates (nitrogen-containing salts) back to the soil. The clover made excellent pasture and hay fields as well as green manure when it was ploughed under after one or two years. The addition of clover and turnips allowed more animals to be kept through the winter, which in turn produced more milk, cheese, meat and manure, which maintained soil fertility. This maintains a good amount of crops produced.