Agriculture Reference
In-Depth Information
Studies have indicated that 40 to 60 percent of the yields for crops harvested in
England and the United States are due primarily to fertilizer application.
6
In most
areas of the tropics with a history of settled agriculture, the portion of grain yields
due to fertilizers is probably similar. Areas of the tropics that are being cleared and
put under cultivation for the first time are much more demanding. Fields cleared in
the Amazon basin of Peru were cultivated for 15 years. Over 90 percent of the crop
yields were attributed to chemical fertilizer application. In other words, without chemi-
cal fertilizers yields would have been reduced by 90 percent.
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In this chapter fertilizer is considered as anything added to the soil to increase crop
production. Farmers in the United States frequently use the word fertilizer only for inor-
ganic, or chemical, additives. Many other countries consider anything added to the soil
as fertilizer.
For clarity we will use some more specific terms. Chemical fertilizers are com-
pounds containing growth-promoting nutrients for plants that have been manufactured.
Examples are urea, ammonium nitrate, and superphosphate. They may be applied as a
gas (anhydrous ammonia), liquid (ammonium sulfate), or dry granules. Green manure
is a term applied to green plants that are cut or plowed under before flowering for the
purpose of adding both organic matter and nutrients to the soil for a crop to be planted.
Examples are rye, crotalaria, and forage soybeans. Compost is a material made by
decomposing organic materials. It is similar to green manure, but it has been decom-
posed, screened, and dried. Any plant or animal waste can be composted. Examples
are sewage sludge, pig manure, and plant residues. Organic fertilizers may be either
composted materials or certain chemical fertilizers. The concept is that they must
occur in nature and do not come from factories. Examples are sodium nitrate (saltpeter),
seaweed, and earthworm casts.
Dry chemical fertilizers are the most common form of nutrients applied to crops
around the world. About 13 elements are considered essential for plant growth. The
list of essential elements for plants in Table 10.3 shows that humans need more
elements than plants. In fact, some of the elements, like arsenic, are poisons! The
key factor is quantity. Like table salt, a very small quantity is beneficial, but more
is deadly.
The nutrient element needed in largest quantities by plants is nitrogen used to form
proteins, chlorophyll, and DNA. Most chemical nitrogen fertilizers are manufactured
using natural gas. Their prices are dictated by the price of natural gas on the
world market.
A hectare of maize producing 9500 kg of grain will typically require about 125 kg
N/ha for adequate plant growth. However, plants cannot use the pure form of most
elements. In the case of nitrogen, gaseous N
2
is inert and cannot be used by plants or
humans as a nutrient. The most common form of nitrogen taken up by plants is
either as ammonium (NH
4
) or nitrate (NO
3
2
). A hectare of maize producing 9500 kg
of grain will typically require about 125 kg N/ha for adequate plant growth. If we
want to apply 125 kg N/ha in the form of ammonium nitrate (34 percent N), we will
need to apply 367 kg of ammonium nitrate fertilizer per hectare.
The second most important element for plant nutrition, in terms of quantity needed,
is potassium. Potassium functions as a catalyst, permitting most plant enzymes to
