field keystone correction

例句：

1. 1. The farmer uses the field keystone to measure the growth of his crops.
   农民使用田野上的石柱来测量他的庄稼的生长情况。
2. 2. The farmers have set up a new field keystone to improve their yield.
   农民们已经设立了一个新的田间标杆来提高他们的产量。
3. 3. The farmer used the field keystone to mark the boundaries of his fields.
   农民用田野上的石柱来标记他的田地边界。