Estimates for the large wave of Polish immigrants from about 1870s to 1920s are given at about 1.5 million. In addition, many Polish immigrants arrived at the port of Baltimore. The actual numbers of ethnically Polish arrivals at that time is difficult to estimate due to prolonged occupation of Poland by neighboring states, with total loss of its international status. Similar circumstances developed in the following decades: during the Nazi German occupation of Poland in World War II; and further, in the communist period, under the Soviet military and political dominance with re-drawn national borders. During the Partitions of Polish–Lithuanian Commonwealth (1795–1918), the Polish nation was forced to define itself as a disjointed and oppressed minority, within three neighboring empires, in the Austrian Partition, Prussian Partition, and Russian Partition. The Polish diaspora in the United States, however, was founded on a unified national culture and society. Consequently, it assumed the place and moral role of the "fourth province". Poland was largely an agrarian society throughout the Middle Ages and into the 19th century. Polish farmers were mostly peasants, ruled by Polish nobility that owned their land and restricted their political and economic freedoms. Peasants were disallowed from trading, and typically would have to sell their livestock to the nobility, who in turn would function as middlemen in economic life. Commercial farming did not exist, and frequent uprisings by the peasants were suppressed harshly, both by the nobility and the foreign powers occupying Poland. A number of agricultural reforms were introduced in the mid-19th century to Poland, first in German Poland, and later eastern parts of the country. The agricultural technologies originated in Britain and were carried eastward by conversing traders and merchants; Poland gained these secrets in the most developed regions first, and through successful implementation, areas that adopted them boomed. The introduction of a four-crop rotation system tripled the output of Poland's farmlands and created a surplus of agricultural labor in Poland. Prior to this, Polish peasants continued Medieval Era practice of three field rotation, losing one year of productive growing time to replenish soil nutrients. Instead of leaving a field fallow, or without any plants for a season, the introduction of turnips and especially red clover allowed Polish fields to maximize nutrients by green manure. Red clover was especially popular because it fed cattle as grazing land, giving the extra benefit of more robust livestock raising in Poland. Between 1870 and 1914, more than 3.6 million people departed from Polish territories (of whom 2.6 million arrived in the U.S.) Serfdom was abolished in Prussia in 1808, in the Austria–Hungary in 1848 and in the Russian Empire, in 1861.