Environmental Engineering Reference
In-Depth Information
Crops and Their Residues
Food harvests in temperate and subtropical latitudes of the Old World have been
traditionally dominated by half a dozen major cereal grains (wheat, rice, barley, rye,
oats, buckwheat), leguminous grains (beans, peas, lentils, soybeans), and oil seeds
(olives, rapeseed, linseed, sunl owers, peanuts), supplemented by a large variety of
leafy and root vegetables, fruits, and nuts. Premodern diets in the Americas shared
a heavy reliance on corn and beans, and three of the hemisphere's major crops,
potatoes, tomatoes, and peppers, became worldwide favorites within a few centuries
after their introduction to Europe, Asia, and Africa. The large-scale production of
sugar crops (sugarcane and sugar beets) is of an even more recent origin.
Crop harvests are reported in terms of fresh phytomass, whose water content
varies from less than 10% for some seeds to more than 90% for many vegetables;
staple grains are marketed with a moisture content of less than 15%, while tubers
have more than 70%. Depending on the time and method of harvesting, water
content varies appreciably even for the same cultivar of staple grains. The moisture
content of cereal grains at harvest ranges between 11% and 25%, with optima
between 14% and 17%. The long-term storage of grains requires that their moisture
be reduced to less than 13.5%, but actual water shares are often 1%-2% above or
below that mean.
Virtually any food phytomass can also serve as feed for some domesticated species
(particularly omnivorous pigs), while healthy ruminant nutrition requires cellulosic
roughage, which has been always provided by cereal straws. Concentrate feed (cereal
and leguminous grains) was traditionally used only sparingly, during periods of
heavy draft work, and became a norm in animal feeding only with the emergence
of a more intensive cropping that created food surpluses and hence allowed a sig-
nii cant proportion of farmland to be devoted to forage crops (mixed hay, alfalfa,
clover, vetch) or feed grain (now most notably corn and soybeans) and tuber (potato,
cassava, turnip) crops.
A complete quantii cation of crop phytomass productivity must also account for
all belowground growth and for the residual phytomass that is either removed from
i elds, to be used as bedding and feed (then often recycled) or as raw material
(removed from short-term carbon cycling), or directly returned to soil (plowed in).
Shares of belowground phytomass range from less than 10% for many vegetables
to between 20% and 25% for cereal grains, about 35% for corn, and well over
50% for root crops (about 60% for potatoes). The largest amount of residual
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