Environmental Engineering Reference
In-Depth Information
Box 11.1
Moisture Content of Harvested Crops
There are obviously large differences between the water content of oilseeds (often less
than 5%), staple grains (in excess of 10%), tubers (more than 70%), green forages
(around 85%), and vegetables, with many cultivars having more than 90% water (Watt
and Merrill 1963 Misra and Hesse 1983; Barreveld 1989; NRC 1996; Bath et al. 1997).
To prevent losses during transportation and storage, cereals should not be marketed
with a water content exceeding 14.5%, but highly variable weather conditions mean
that many cereal crops are harvested when their moisture content is closer to 20%
than to 15%, while others enter commercial exchange with a water content just above
10%. All FAO production statistics refer to fresh (i eld) weight, and hence there can
be no universally valid specii c conversion factors. A good approximation would be to
use an average moisture content of 12% for grains and oilseeds; 75% for tubers, roots,
sugar beets, and sugarcane; 85% for fruits and green fodder crops (alfalfa, clover); and
90% for vegetables.
When using these shares the most likely error would be to overestimate the actual
total of dry phytomass by at least 5%, as the typical moisture content for marketed
cereals may be 13% rather than 12% and white potatoes, the most important tuber
crop, contain 80% water (sweet potatoes and cassava have about 70%). When these
approximate conversion ratios are applied to the harvest in the year 2000, cereals
dominate the dry phytomass, at about 1.8 Gt, and they are also the leading harvest
in all countries except some tropical nations. Among the major food producers they
account for between 70% of the total in China and 55% in the United States. Sugar
crops come second in the global ranking (nearly 400 Mt), followed by tubers and roots
(175 Mt), oil crops (about 100 Mt), and fruits and vegetables (each around 70 Mt).
sions are fairly accurate: their total global mass in the year 2000 was about 2.7 Gt,
a bit less than half the fresh weight (box 11.1). For comparison, Wirsenius (2000)
ended up with a very similar total of 2.54 Gt of dry phytomass as an average for
1994-1996.
My approximate reconstructions of past agricultural harvests (all in dry weight)
show them rising from about 400 Mt in 1900 to 600 Mt in 1925, 800 Mt in 1950,
1.7 Gt in 1975, and 2.7 Gt in 2000, or a nearly sevenfold increase in 100 years,
most of it (as already noted) due to increased crop yields. Between 1900 and 1950,
wheat yields were up only by about 20% in the United States and 38% in France.
Larger U.S. harvests (up 47% by 1950 compared to 1900) came largely because of
the steadily expanding area planted to wheat (Dalrymple 1988). Signii cant yield
gains began only during the late 1950s, when the U.S. area planted to short and
semidwarf cultivars surpassed 40% of all wheat i elds and when sharply rising fertil-
izers inputs made it possible to realize more of their inherent yield potential.
