"Obviously the immigrant was attracted by the cheap lands of the frontier, and even the native farmer felt their influence strongly." (1)

Between the years 1845 and 1920 the United States saw the greatest influx of immigrants in its history.  By 1860 more than one out of every eight Americans was foreign-born, and in cities like Chicago and St. Louis the immigrant population outnumbered the native-born two to one. (2) 

In the Farm Field and Fireside collection, you will often find reference to these new Americans, primarily as seasonal workers and day laborers.  Farmers wanted to attract the large foreign-born populations away from the city and to the country, where they could be put to work on farms.  This was not only considered to be advantageous for the farmer, who could count on immigrants working longer hours for less pay, but advantageous to the country as a whole, since it was believed that dispersing the immigrant population throughout the countryside would solve the problem of the so-called "idle" foreigners crowding the cities and jockeying for political power. (3)

"Farmers and Immigration," Prairie Farmer, February 2, 1905

"Farm Labor From Abroad," Prairie Farmer, February 7, 1907.

"Importing Aliens for Farm Labor," Prairie Farmer, September 5, 1907.

You might try searching for these words first: immigration, emigration, immigrant, emigrant.  However, immigration and its cognates are often used in the Farm, Field, and Fireside collection to refer to the exodus West, and not necessarily the influx of foreign-born people into the United States.  So, in addition to searching for the above terms, also try using the language of the times.  Aliens and foreigners, for example, are also good search terms.

Next, try combining the terms above with words like labor, farm or industry using Boolean logic.  By searching for aliens labor, you are likely to cut your results in half.