Page 255 - Environment: The Science Behind the Stories

180

160 fertilizer use
Fertilizer (millions of metric tons) 120
Total
140

100
80
60

40
20
0
1960 1970 1980 1990 2000 2010
(a) Inorganic fertilizer Year
Figure 9.24 Use of synthetic, inorganic fertilizers has risen
sharply over the past half-century. Today, usage stands at
over 170 million metric tons annually. Data from International Fertilizer
Industry Association.

are generally more susceptible to leaching and runoff than
are organic fertilizers, and they more readily contaminate
groundwater supplies.
Nutrients from inorganic fertilizers can have impacts far
beyond the boundaries of the fields on which they are applied
(Figure 9.25). For instance, nitrogen and phosphorus runoff
from farms and other sources in the Mississippi River
basin spurs phytoplankton blooms in the Gulf of Mexico
and creates an oxygen-depleted “dead zone” that kills fish
and shrimp (see Chapter 15; pp. 428–429). Such eutrophi-
cation (pp. 126–127, 430) occurs at countless river mouths,
lakes, and ponds throughout the world. Moreover, nitrates
(b) Organic fertilizer readily leach through soil and contaminate groundwater, and
components of some nitrogen fertilizers can even volatilize
Figure 9.23 Two main types of fertilizer exist. Inorganic
fertilizer (a) consists of synthetically manufactured granules. (evaporate) into the air. Through these processes, unnatural
Organic fertilizer (b) includes substances such as compost amounts of nitrates and phosphates spread through ecosystems
crawling with earthworms. and can pose human health risks, including cancer and methe-
moglobinemia, or blue-baby syndrome, which can asphyxiate
and kill infants. Human inputs of nitrogen have modified the
nitrogen cycle (pp. 141, 144) and now account for one-half the
contain too few nutrients, crop yields decline. For this reason, total nitrogen flux on Earth.
we go to great lengths to enhance nutrient-limited soils by
adding fertilizer, substances that contain essential nutrients. Sustainable fertilizer use involves
There are two main types of fertilizers (Figure 9.23).
Inorganic fertilizers are mined or synthetically manufactured targeting and monitoring nutrients
mineral supplements. organic fertilizers consist of the remains Sustainable approaches to fertilizing crops target the deliv-
or wastes of organisms and include animal manure; crop resi- ery of nutrients to plant roots and avoid the overapplication
dues; fresh vegetation (green manure); and compost, a mixture of fertilizer. Farmers using drip irrigation systems can add
produced when decomposers break down organic matter such fertilizer to irrigation water, thereby releasing it only above
as food and crop waste in a controlled environment. plant roots. Farmers practicing no-till or conservation tillage
Historically people relied on organic fertilizers to often inject fertilizer along with seeds, concentrating it near
replenish soil nutrients, but during the latter half of the 20th the developing plant. Farmers can also avoid overapplication
century farmers in industrialized nations and nations experi- by regularly monitoring soil nutrient content and applying
encing the Green Revolution embraced inorganic fertilizers fertilizer only when nutrient levels are too low. The Nielsens
(Figure 9.24). The use of inorganic fertilizers has boosted our in Iowa are careful and judicious with their fertilizer use in
global food production, but their application has triggered these ways. They apply fertilizer in spring but not in fall.
254 increasingly severe pollution problems. Inorganic fertilizers After two targeted applications, they test for nitrate levels

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