The ancients calculated agricultural yield as the ratio of grain produced to the seed that was planted. For them, the constraining factor was the supply of seed grain. But as agriculture spread, and as seed became more abundant and land more scarce, the focus shifted from the productivity of seed to that of land.

From the beginning of agriculture until World War II, land productivity increased very slowly. Following World War II, however, crop yields began to rise rapidly in virtually every industrial country. During the 1960s, introduction of the fertilizer-responsive varieties of dwarf wheats and rices initiated rising yields in many Third World countries. From the end of World War II through 1971, the steady rise in cereal yield per hectare was one of the most predictable trends in the world economy.

Since 1971, however, the increased has slowed. Yields have become much more erratic and less predictable. From 1960 to 1971, the world cereal yield per hectare climbed 24 percent per year. Since then, it has increased at only one-fourth that rate, contributing to an unprecedented rise in the world price of food.

Within the United States, the long term rise peaked in 1972, turning abruptly downward in 1973. The 1977 yield was 6 percent below that in 1972. Per-hectare yields of wheat, barley, oats, rye and rice peaked in 1971, while that of corn leveled off in 1972.

Another agriculturally advanced country that has experienced a yield downturn is France, the leading food producer in Europe. Closely paralleling the U.S. experience, the cereal yield per hectare in France nearly tripled between 1950 and 1973, climbing to 4.3 metric tons per hectare, but after this impressive climb, it turned downyard. During the four years since it has flucuated between 3.4 and 4.2 metric tons per hectare.

Egypt and China, two Third World countries with rather intensive agricultural systems, have also experienced a leveling off in per-hectare cereal yield during the 1970s. Egypt's yield nearly doubled between 1960 and 1971, but it has not increased since. China's yield increase, more modest overall, has not increased at all since 1971. Except for the sharp falloff in the aftermath of the 1958 "Great Leap Forward," China's cereal yield had edged steadily upward throughout most of the 1950-to-1974 period.

The reasons for the interruption of the trends in crop yields vary by country. Within the United States, the return to production of poorer quality cropland previously idled under government programs is undoubtedly the dominant factor. In some developing countries, the new land being cultivated, such as in the interior of Brazil or the outer islands of Indonesia, is inherently less fertile than traditionally cultivated land.

Since the end of World War II, the massive increase in the use of chemical fertilizer has been the principal source of expansion in world food output. But in the agriculturally advanced countries, where fertilizer use is heavy, the response to further increases is diminishing. An additional pound of fertilizer that would once raise grain yields by 15 to 20 pounds per acre now leads to only a few pounds of additional grain.

A third influence is a shrinkage in the fallowed area in dryland farming regions. The reduction of land in fallow - and left idle in alternate years to accumulate moisture - affects particularly the United States and the Soviet Union. As world wheat prices rose during the 1960s, U.S. fallow land dropped from 17 million hectares in 1969 to 13 million hectares in 1974. Pressures to expand food production in the Soviet Union following the massive grain imports of 1972 have reduced the fallow area from 18 million to 12 million hectares.

In tropical and subtropical regions, where shifting cultivation has evolved as a method of restoring fertility, mounting population pressures are forcing shifting cultivators to shorten the rotation cycles. That, in turn, acts to reduce crop yields. A World Bank study of Nigeria reports that "fallow periods under shifting cultivation have become too short to restore fertility in some areas."

Similar pressures on the land are evident in Latin America. According to FAO researchers: "These is abundant evidence in certain regions of Venezuela that, with growing population pressure, the fallow period is becoming increasingly shorter so that soil fertility is not restored before recropping. . . . With the population of modern times, formerly stable shifting-cultivation systems are now in a state of breakdown."

Soil erosion is also taking its toll. A number of soil surveys and studies indicate that the inherent fertility of one fifth or more of the world's cropland is declining. Although the process of soil erosion is not always highly visible, its cumulative effect on land productivity can be graphic. A U.N. study of Latin American agriculture links falling potato harvests to soil erosion in the Peruvian Andes, the region that gave the world the potato.

Collectively, these influences have interrupted the rapid sustained rise in crop yield per hectare of the postwar period. Coming at a time when there is little fertile new land to be brought under the plow, they cast a shadow over the long-term food prospect. They also confirm the U.N. projections of a growing gap in many countries between food demand and food productivity. That tacks us to the bottom line: a promise of future food-price rise that may dwarft those of the recent past.