TEN YEARS AGO the world food situation seemed desperate. Officials of 150 governments met in Rome to discuss how to avoid catastrophe. Famine threatened millions in Asia and Africa.

Today the food situation looks better. Grain reserves, depleted a decade ago, have been replenished. Stockpiles of unsold grain are at some of their highest levels in years, and the price of wheat today is $4 a bushel -- far lower than in 1974 after adjustments for inflation. A recent report of the House Foreign Affairs Committee noted "significant areas of improvement" by some parts of the developing world in feeding themselves.

But successes at increasing food production and holding down population growth in individual countries should not delude us. We still face some deeply disturbing trends. The concerns have to do with land, water and the environment -- and ultimately with the cost of producing food.

Every year, the world is losing more than 25 billion tons of topsoil, as a result of poor farming practices and mismanagement. Priceless soil is literally floating out to sea in countries such as the United States, China and India.

Water shortages are also beginning to affect food production prospects. Since World War II, the world's irrigated area has more than doubled, but the flurry of large dam building of the past generation has now subsided and the expansion of irrigation has slowed. With a few exceptions, developing the remaining potential sites will be more difficult, costly and capital intensive. In some areas, irrigated agriculture is threatened by falling water tables.

Finally, there is the loss of farmland itself to cities, highways and other uses. Since mid-century, little new land has been available to plow. In fact, the amount of cropland per person has actually declined by a third since 1950. As a result of this constraint, most of the growth in food production has come from using more petroleum-based fertilizer. World population is projected to expand by 1.5 billion during the remaining 16 years of this century. As it does cropland area per person will continue to shrink and the fertilizer required to satisfy each person's food needs will increase.

This virtually insures that further growth in the world's food supply will not come cheaply.

In addition to these longer range questions, the economics behind the current food surpluses are not as reassuring as they appear at first glance. Stockpiles of food are larger and prices of grain are lower primarily because of a dramatic loss in purchasing power by some of the world's poorest countries. Income per person in Latin America in 1983 was some 10 percent below that of 1980, according to the Inter-American Development Bank. In most African countries he fall in per capita income has been even greater.

Where growth in per capita income has slowed and, in some cases, even reversed itself as a result of debt, high interest rates and world recession, the result has been a reduction in the amount of food consumed. To service their mounting debt, many Third World governments have cut back on food imports.

In effect, economic austerity in many of these countries has rationed the intake of food by millions of people, with the brunt being borne by the neediest and most poorly nourished. Food prices have not risen, but not because food is plentiful. The reason is people lack the means to acquire all the food they need.

This loss of momentum is mirrored in the discouraging food production trends of many (though not all) Third World countries. In the past decade, for example, the 34 countries of sub-Saharan Africa experienced an actual decline in per capita food production. As has been widely reported, there is famine in Ethiopia as well as other parts of the region. Drought, civil wars, high rates of population growth and government mismanagement have all played their part.

Africa, of course, is not the whole world. Optimists point out that gains made in Asian countries show that it is possible for countries to make headway when they have the political will and determination to do so. Bangladesh, for example -- where famine made headlines in 1974 -- has impressively increased production of rice, wheat and other primary foodstuffs and reduced its dependence on expensive imported food.

China, the world's most populous country, has substantially boosted the average calorie intake of its 1 billion people.

Working against such achievements in the long run, however, are a number of trends that point to long-term pressures on food prices and food supplies -- not just in Africa but throughout the world.

Between 1950 and 1973, world output of grain expanded at more than 3 percent a year, faster than population growth. During this period per capita food production climbed by nearly a third. Diets improved worldwide, and life expectancy in the Third World increased from 43 years in 1950 to 53 years by 1973, a remarkable gain of 10 years in less than a generation. Since 1973, however, grain production has expanded at less than 2 percent annually, barely keeping pace with world population growth. Higher oil prices, which caused consumers and foreign countries to economize on the amount of food they bought, and rising costs of gasoline and petroleum-based fertilizer and pesticides, are partly responsible for slowing food production. Unsold grain surpluses caused the U.S. government to encourage farmers to idle croplands where food had been grown in the boom times of the '70s. But more worrisome long-time factors are also at work to put constraints on agricultural production.

In 1980, Anson R. Bertrand, a senior U.S. Department of Agriculture official, observed that for the United States, "The economic pressure -- to generate export earnings, to strengthen the balance of payments and thus the dollar -- has been transmitted more or less directly to our natural resource base. As a result soil erosion today can be described as epidemic in proportion."

Farmers, in other words, are under pressure to grow more and more food regardless of the impact on the soil that will be needed in future generations.

According to the U.S. Soil Conservation Service, the United States accounts for 1.5 billion tons of the annual global loss of 25 billion tons of topsoil. Each year the Mississippi deposits 300 million tons of topsoil from the U.S. agricultural heartland in the Gulf of Mexico. If the 32 million acres of rapidly eroding cropland, (some 8 percent of the U.S. cropland total) were planted to grass or trees as the Soil Conservation Service recommends, grain surpluses would dwindle dramatically.

This U.S. topsoil loss, though serious enough, pales when compared with erosion rates in other major food producing countries. For example, in India, with the same cropland area as the United States, the Ganges River alone carries 1.5 billion tons of topsoil into the Bay of Bengal each year. The Yellow River Conservancy Commission in Beijing reports that the Yellow River annually transports 1.6 billion tons of soil into the sea.

In the Soviet Union, eroded soils help explain their chronic difficulties on the food front. One scholar of Soviet environmental policies, Thane Gustafson, observes that "50 years of neglect have left a legacy of badly damaged soil." Soviet soil scientists, such as Dr. Vladimir Borovsky, and a few concerned politburo members, such as Mikhail S. Gorbachev, regularly call for an effective national conservation program, but to no avail.

The loss of topsoil may be progressing fastest in Africa, where erosion affects the food prospect of nearly every country from the Mediterranean Coast to the Cape of Good Hope. A 1978 report from the U.S. Embassy in Addis Ababa, Ethiopia, warns of "an environmental nightmare unfolding before our eyes . . . . It is a result of the acts of millions of Ethiopians struggling for survival: scratching the surface of eroded land and eroding it further, cutting down the trees for warmth and fuel and leaving the country denuded . . . . Over one billion -- one billion -- tons of topsoil flow from Ethiopia's highlands each year."

Water is also being depleted at a rapid rate. The southern Great Plains, where much of the U.S. growth in irrigation over the last two decades has occurred, provides a disturbing example. From central Nebraska south through northern Texas, irrigation depends heavily on the Ogallala aquifer, an essentially nonreplenishable underground water reserve.

As the water table in this vast agricultural area falls, the irrigated area is beginning to shrink. Led by a decline of 20 percent in Texas between 1978 and 1982 (the last year for which data are available), irrigated area in the southern Plains states has declined by 7 percent over the four years.

An analogous situation exists in the Soviet southwest, where the excessive diversion of river water for irrigation is reducing the water level of the Aral and Caspian seas. This has many long-term negative consequences, including a diminished fish catch and the gradual retreat of the water line from coastal cities that depend on it for transportation. But given the strong pressures within the Soviet Union to produce more food, the diversion is continuing. This diversion is generating pressure to divert water from rivers that now flow northward, to the semi- arid south.

A second major threat to irrigated agriculture is the often intense competition for water between farming, industry and cities. In the U.S. southwest, the irrigated area is shrinking in states such as Arizona, where Sunbelt migration is swelling cities that are bidding water away from farmers. In Arizona's agriculturally important Maricopa County, which had some 550,000 acres under irrigation in the '50s, the irrigated area has shrunk by more than one-fifth.

Water scarcities are also emerging in Africa. South Africa, adding 720,000 people each year, is fast running out of new irrigation sites. A 1983 report commissioned by the President's Council in South Africa identified the scarcity of fresh water as a constraint on that country's capacity to support population growth.

Africa may offer a preview of the long-term food crisis that is only now beginning to unfold. Since 1970, grain output per person has fallen about 1 percent per year. By 1982 it had fallen a total of 12 percent. In 1983, a year of record drought throughout much of Africa, the grain harvested per person fell an additional 14 percent for a total drop of 26 percent since 1970. By early 1984, survey teams from the United Nation's Food and Agriculture Organization (FAO) reported that 22 African countries were facing famine, including Angola, Chad, Ethiopia, Ghana, Mali, Senegal, Somalia, Tanzania and Zimbabwe.

The decline in per capita food production in Africa has led to an increase in grain imports from 5 million tons in 1970 to 20 million tons in 1983, a fourfold increase in little over a decade. The FAO teams estimated that in addition to the usual commercial imports, 5 million tons of emergency relief grain will be needed in 1984 to stave off famine in the region. This year, 130 million of Africa's 515 million people will be sustained with grain from abroad. Africa, an agrarian continent, is losing the capacity to feed itself.

The deteriorating food situation in Africa is rooted in three trends: the fastest population growth of any continent in history, widespread soil erosion in the countryside and a persistent failure among most African governments to invest adequately in agriculture.

Lack of investment is largely due to national food price policies that favor cheap food for urban consumers, instead of price supports for farmers that would encourage farmers to invest in their farms and plots. These food policies also encourage the migration of rural young people to the cities, thus exacerbating the problem of feeding urban populations.

Africa's food production has grown 2 percent a year since 1970, which is quite respectable by international standards. But it falls short of the region's population growth of 3 percent. Even more serious, there are no developments under way in either family planning or farming that will arrest this decline in per capita food production in the foreseeable future. Without dramatic new initiatives on both fronts, Africa's gradual slide into crisis will continue.

This negative trend in Africa will unfold elsewhere if population policies and economic priorities are not adjusted. The forces reducing per capita food production in Africa are also gaining strength in the Andean countries, in northeastern Brazil, in Central America and possibly in the Indian subcontinent as well. Despite the impressive gains by Bangladesh in producing more food, population increases have negated most of them.

The issue is not whether the world can produce more food.

Indeed, it is hard to foresee any limits on the amount the world's farmers can produce. The question is at what price farmers will produce it. If the price is too high, it will do little good for the 1.5 billion poorest people.

Against this backdrop, slowing population growth so as to slow growth in demand and delay the rise in costs emerges as the key to reducing hunger.

How population growth affects efforts to eliminate hunger can be seen in agricultural successes as well as failures. If Africa is the failure of the past decade, China is the success story. As of the early '70s, China, like Africa, had made little progress in raising its per capita food production from mid-century levels. But during the decade since then, the grain harvest has climbed from around 440 pounds per person to some 550 pounds, well above the subsistence fare of 360 pounds -- roughly one pound per day. The current level of grain output does not yield an abundance of protein in the diet, but it substantially boosts overall caloric intake and allows some gains in consumption of livestock products, principally pork, poultry and eggs.

China has achieved this food breakthrough by simultaneously pursuing vigorous family planning and food production efforts. More reliance on farm markets in the countryside (as opposed to rigid state planning), energized farmers, and family planning efforts have cut the population growth rate in half.

Although China faces a serious soil erosion problem and its cropland base is projected to shrink by some 5 percent by the end of the century, prospects are good that China will continue to improve diets.

Slowing population growth will not in itself eliminate hunger in food- short countries. But in an age when little new land can be brought under the plow, when water scarcities constrain agricultural growth in many countries and when farmers everywhere face rising operating costs, controlling population growth is more essential than ever. This should not be forgotten just because, temporarily, our granaries are full.