But from 18240 to 1870, British farming became very profitable once more, despite the repeal of the corn laws in 1846. Machinery, applied to agriculture, drastically cut labor costs. In 1853 the "Cross kill reaper" was perfected in England, and at about the same time the "McCormick reaper" began to be imported from America. Moreover, Liebig's work on fertilizers had practical effect. Manufacture of super-phosphate of lime was begun in England (1846), use of nitrate of soda grew, and guano from Peru was imported in swiftly increasing amounts. With the advance of chemical and mechanical agriculture, the crops of British landlords rose while their labor costs dropped. They did very well until the 1870's, when a combination of diseases among their animals and cheap food imports on steamships from overseas brought depression.