The agricultural revolution in the nineth century involved two things：the invention oflabor-saving machinery and the development of scientific agriculture．Labor．saving machinerynaturally appeared first where labor was SCarCe．“In Europe”，said Thomas Jefferson，“the ob．ject is to make the most of their land，labor being abundant；here it is to make the most of ourlabor，land being abundant．”It Was in America，therefore，that the great advances in nine-th-century agricultural machinery first came．At the opening of the century，with the excep-tion of a crude plow，farmers could have carried practically all of the existing agricultural imple-ments(农具)on their backs；by 1860，most of the machinery in use today had been designedin an early form．The most important of the early inventions was the iron plow．As early as 1790 Ch'arles Newbold of New Jersey had been working on the idea of a cast-iron plow and spent hisentire fortune in introducing his invention．The farmers，however，would have none of it，clai-ming that the iron poisoned the soil and made the weeds grow．Nevertheless，many people devot-ed their attention to the plow，until in 1869 James Oliver of South Bend，Indiana，turned out thefirst chilled—steel(冷淬钢)plow． 根据以上内容，回答下列各题。 What is the main topic of the passage?