U.S. investments in Trinidad and Tobago's petrochemical sector have helped push this Caribbean state to the forefront of the world fertilizer trade.

This investment effort has circled back to the United States in the form of U.S. fertilizer imports, nurturing in turn U.S. agricultural production and exports.Trinidad and Tobago government spokesmen say a steady increase in production of ammonia and urea fertilizers over the past five years has made the country the world's second-largest exporter of nitrogenous fertilizers, after the Soviet Union, although exports to the United States have begun to decline.

Leading the development of the fertilizer industry in the English-speaking Caribbean state of 1.1 million people was W.R. Grace & Co. of New York. The sector got a start in the 1950s when Grace established an ammonia plant, Fedchem, in Trinidad, with an average output of 220,000 metric tons a year.

Then Grace followed with a joint venture in 1977 with the government to form Tringen, a venture with production capacity of 400,000 metric tons of ammonia a year.

A third ammonia plant was then set up between the government and Amoco Corp. of Chicago. That Fertrin venture began operations in 1981 with a capacity of 750,000 metric tons a year.

The country's lone urea plant is owned by the government and has a rated capacity of 600,000 metric tons a year.

Summing up these projects, ammonia exports last year totaled 1.35 million metric tons, with shipments of urea set at 425,700 tons, according to Rupert Mends, permanent secretary in the Energy & Natural Resources Ministry.

While shipments to the investing companies and other U.S. importers have been significant, they did not dominate Trinidad and Tobago's fertilizer exports during the last planting season.

During the July 1986 to June 1987 fertilizer year, the country exported 496,000 tons of fertilizer to the United States, down from 543,000 tons in the previous comparable period, according to a spokesman for Washington-based Fertilizer Institute.