From Farmer to Shrimper: The Dead Zone
(pg 4 of 8)
Why the increase? Largely because of the
changing nature of agriculture in the upper
In the early 1900s, agriculture in the upper
Mississippi watershed consisted of mixed crops of
wheat, oats, barley, rye, alfalfa, and pasture.
Numerous wetlands and small lakes trapped nearly
all the nitrogen that ran off of farm fields.
Since then, Midwestern grain farmers have
switched to corn and soybeans. Corn requires large
amounts of nitrogen fertilizer. Soybeans capture
nitrogen from the air and fix it to the soil.
Furthermore, artificial drainage in farm fields
short circuits the filtering of nitrogen through
groundwater and flushes it directly to surface
waters. Excess soluble nitrate remains in soil and
groundwater, to be flushed out by high
precipitation. Minnesota soil scientist Gyles
Randall has calculated that nitrogen losses from
corn and soybean fields can be 30 to 50 times
higher than from land planted to perennial plant
systems such as hay or grass (Randall
et al., 1997).