humans
energy and nutrients
primary
humans
consumers
(animals)
primary
primary
consumers
consumers
nutrients
(animals)
nutrients
(animals)
primary producers
primary producers
primary producers
(plants)
(plants)
(plants)
a) early agriculture
b) urbanizing society
c) industrial agriculture
(mid-19th to mid-20th century)
(mid- to late 20th century)
Figure 7.1. The patterns of nutrient flows change over time. From Magdoff, Lanyon, and Liebhardt (1997).
The first major break in the cycling of nutrients
DIFFERINg FlOW PATTERNS
occurred as cities developed and nutrients began to
Different types of farms may have distinctly different
routinely travel with farm products to feed the growing
nutrient flow patterns. Farms that exclusively grow
urban populations. It is rare for nutrients to travel many
grain or vegetables have a relatively high annual nutri-
miles away from cities and return to the soils on which
ent export (figure 7.2a). Nutrients usually enter the farm
the crops and animals were originally raised (figure
as either commercial fertilizers or various amendments
7.1b,c). Thus, nutrients have accumulated in urban sew-
and leave the farm as plant products. Some cycling of
age and polluted waterways around the world. Even with
nutrients occurs as crop residues are returned to the
the building of many new sewage treatment plants in
soil and decompose. A large nutrient outflow is com-
the 1970s and 1980s, effluent containing nutrients still
mon, however, because a large portion of the crop is
flows into waterways, and sewage sludges are not always
usually exported off the farm. For example, an acre of
handled in an environmentally sound manner.
tomatoes or onions usually contains over 100 pounds of
The trend toward farm specialization, mostly driven
nitrogen, 20 pounds of phosphorus, and 100 pounds of
by economic forces, has resulted in the second break in
potassium. For agronomic crops, the annual export of
nutrient cycling by separating animals from the land
nutrients is about 100 pounds of nitrogen, 6 pounds of
that grows their feed. With specialized animal facilities
phosphorus, and 50 pounds of potassium per acre for
(figure 7.1c), nutrients accumulate in manure while crop
corn grain and about 150 pounds of nitrogen, 20 pounds
farmers purchase large quantities of fertilizers to keep
of phosphorus, and 130 pounds of potassium per acre
their fields from becoming nutrient deficient.
for grass hay.
70
Building SoilS for Better CropS: SuStainaBle Soil ManageMent