Competition among ports may ebb and flow, but Baltimore remains the preferred port for exporters of tractors, combines and similar agricultural equipment.

A recent shipment offered an excellent example of the reasons for this preference. One of the largest U.S. manufacturers of farm equipment moved 100 combines from its Midwest production plant through Baltimore on the way to Turkmenistan, a remote republic of the former Soviet Union.The example is a good one, according to representatives of the port, for three reasons.

First, Baltimore is closer to Midwest production centers than other East Coast ports. That means inland transport costs are generally lower.

Second, the port caters to a variety of shipping lines that are especially well suited to this kind of cargo. That translates into quicker and easier handling of cumbersome cargo.

And third, Baltimore port executives decided about four years ago to market their port to the farm traffic. Consistent follow-up on that decision has paid off, making the port a preferred place to do business for the nation's largest farm-equipment manufacturers.

The Turkmenistan shipment was also unusual in some respects, according to Lou LoBianco, a senior sales manager with the Maryland Port Administration, the state agency that runs the port.

The shipment was destined for the Port of Helsinki in Finland, and that itself made it out of the ordinary, Mr. LoBianco said. Trans-Atlantic shipments are more commonly destined for the Belgian Port of Antwerp, the French Port of Le Havre or the German Port of Bremerhaven.

Although there are numerous ports far closer to Turkmenistan, Helsinki was used because of its excellent connections to the Russian rail system, which is still operating at a time when many parts of the transport system in the ex- U.S.S.R. are in chaos.

Also unusual was the large size of the shipment. The shipment was soon followed by a second group of 100 combines. A third lot of 100 will complete the series.

Yet another unusual feature was the financing for the deal. Worth $45 million, the purchase was arranged through the U.S. Export-Import Bank, a government agency that promotes the sale of U.S. products to underdeveloped regions of the world. Without Ex-Im financing, it's unlikely the deal would have ever been completed, according to Mr. LoBianco.

The Turkmenistan project does fit the general mold in the sense that the selection of the U.S. port was made by one of four manufacturing companies that dominate the market, said Chris Connor, vice president of marketing and sales for Wallenius Lines North America Inc.

Most shipments of agricultural rolling stock - both exports and imports - are made by just two companies, Mr. Connor said: John Deere & Co. and Case Corp. Case was the shipper to Turkmenistan.

Also important are Ford New Holland and Agco Corp., a holding company that owns well-known brand names such as Allis-Chalmers and White Tractor.

Mr. LoBianco said these companies, like many other large U.S. corporations, have sought in recent years to reduce the number of ports they use. In the early 1980s these companies used almost all East Coast ports to some degree, but this is not the case any longer, he said.

For Baltimore, the strongest competition now comes from the facilities controlled by the Virginia Port Authority and from the ports of Charleston, S.C., and Savannah, Ga.

Mr. LoBianco said, however, that Baltimore is expecting heavier competition

from the Port of Galveston, Texas. Wallenius Lines is introducing a new service there and has close ties with the four big shippers, he noted.

Indeed, "the shippers have agreed to support it" and Wallenius expects the new service to be a success, Mr. Connor said. Ports such as Baltimore and ship lines such as Wallenius concentrate on the big four shippers but don't neglect the smaller shippers either, Mr. LoBianco said.

Baltimore is particularly interested in the Belarus Machinery Co., a manufacturer of farm tractors. Located in Belarus, it is one of the few foreign tractor makers to challenge the North American giants on their own turf.