1988 and probably a similar proportion of N2 Oemissions, together the equivalent of 0.055 MMT N content.

The above estimates do not take into account the relatively small quantities of other chemical elements embodied in the crops, notably nitrogen, phosphorus, and other minerals taken up from the soil or, in the case of nitrogen, fixed by bacteria. It is of interest, however, that the three major chemical elements in dry plant tissue are carbon, hydrogen, and oxygen, which account for 95 percent of the total mass. Nitrogen accounts for another 2 percent, phosphorus for 0.5 percent, potassium for 1 percent, and sulfur for 0.4 percent. These are the major nutrients that are depleted by harvesting and must be replaced by the addition of fertilizers. The remaining 1 percent of plant mass consists of other mineral elements (Table 2) that are readily available from the soil. The flows of nutrients (nitrogen and phosphorus) in U.S. agriculture are summarized in Figure 2.

In 1988, 133 MMT of grain, vegetables, and oilseeds (mostly soya beans) were exported, not including 11.4 MMT of "feeds and fodders," which are from the processing sector. The remainder was consumed directly or indirectly within the United States. Final consumption of all food products (not including beverages) for 1988 was 186 MMT, plus about 20 MMT for grain-based beverages, alcohol, cotton, wool, and other products. Indirect consumption (as animal feed) accounted for most of the difference between gross production and final consumption.

According to the U.S. Department of Agriculture (USDA) (1992), U.S. livestock in 1988 were fed 119.4 MMT of feed grains and 3.7 MMT of food grains

TABLE 2 Chemical Composition of Plants

Element

Percent by Weight

Oxygen

45

Carbon

44

Hydrogen

6

Nitrogen

2

Potassium

1

Calcium

0.6

Phosphorus

0.5

Sulfur

0.4

Magnesium

0.3

Manganese

0.05

Iron

0.02

Chlorine

0.015

Zinc

0.01

Boron

0.005

Copper

0.001

Molybdenum

0.0001

Total

99.9011



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