Shrinking cropland a growing worry

Since 1950 US food production has increased by 50 percent despite a decrease in the amount of land under cultivation.

Those working for agricultural land preservation say that this trend -- conversion of prime farmland to homes, parking lots, condominiums, and lawns -- must be reversed or the time will come when the United States will not be able to support itself agriculturally or export food to other nations.

A year ago agricultural economists at the University of Wisconsin at Madison studied the question, ''Can US farmers continue to produce more food on less land or will the nation soon discover that it does not have enough cropland to meet growing domestic and foreign demand for food?''

According to Richard Barrows, one of the researchers, no definitive answer was reached. Several studies estimate that the US will be forced to utilize all of its existing and most of its potential cropland to meet high food demands as early as 1985. But some economists argue that the forces of supply and demand will take care of the situation.

One reason for lack of consensus among researchers, policymakers, farmers, and others on the need to preserve farmland is the number of factors involved. These include:

* Land quality and quantity. Currently, the US has about 413 million acres of cropland and 127 million acres of potential cropland. The amount of cropland lost annually is a small percentage: about 0.6 percent. But once the land is developed, there is little chance of reconversion. Much of this is prime agricultural land. It can be replaced by other, more marginal land, but usually at a cost of increased soil erosion.

* Climate. Many climatologists say the last 40 years in North America have been unusually favorable to agriculture. Even during this favorable period crop failures have averaged 35 million acres annually. Climatologists say that weather-related crop losses many rise in the near future when the nation's weather returns to previous, less favorable patterns.

* Productivity. Fertilizers, pesticides, irrigation, and mechanization all require large amounts of energy. Modern high-yield crops have been tailored for this high-energy environment. But rising energy costs are making energy-intensive agriculture less and less economical. Genetic engineering and other techniques are being applied to this problem. Without some major breakthroughs, crop yields could actually drop. The US has already lost about one-third of the topsoil on its productive farmland. The US Soil Conservation Service estimates the nation's rate of topsoil loss is twice what can be sustained without decreasing fertility. As a result, more cropland may be needed than recent studies suggest.

* Agricultural exports. Food exports increasingly are being looked upon as one solution to the multibillion-dollar US trade deficit. As a result, pressure on US cropland will continue to come from overseas demands for food. Developing countries now account for less than half of US exports, but several studies predict this will increase as third world food production falls further behind food demands. World population growth is expected to result in high demand for US crops.