THERE IS NO more dismal reminder of the slow progress of civilization, even within the most advanced of countries, than the continuing plight of this nation's migrant and seasonal farm workers. For over two decades, innumerable media expos,es have reminded America that the hidden cost of its plentiful and varied food is the degradation of fellow human beings, many of them working and living within easy reach of major cities.

Mechanization has reduced the number of agricultural workers, and many migrants have been displaced by "day-haul" workers from cities, illegal immigrants and contract workers from the Carribean. But exploitation of all these types of field workers continues to be a prevalent condition. Some progress has been made. Lawsits and legislation have tried to secure minimally adequate wages and wipe out the worst sorts of squalor and abuse. Farmworker organizations have won contracts with some major food processors that ensure better pay and benefits. And every now and then -- when a baby dies of dysentery or workers held in bondage are murdered -- some of the most despicable situations are remedied, at least temporarily.

The latest attempt to improve legal protections for field workers is a rewriting of the legislation that governs farm-worker contracts. The legislation is supported by the agricultural industry, which had complained that growers were being subjected to registration requirements -- such as fingerprinting and proof of integrity -- that were designed to crack down on itinerant crew leaders and contractors. But farm-worker groups also support the bill because it makes it clear that liability for abuses -- such as wage skimming, misinformation to workers about pay and other conditions, substandard housing and dangerous transportation -- extends beyond the crew leader, who does the hiring, to growers and agricultural associations, the actual employ- ers.

The legislation -- which was passed by the House without apparent opposition -- is a reasonable compromise of a long-disputed issue, and it deserves quick passage in the Senate when it reconvenes after the election. But there is plenty of history to show that laws are far from self-enforcing, especially when the opposition -- in this case, the growers -- is powerful and the beneficiaries are not.

The Department of Labor, which enforces the law, has cut back its investigative staff, and farm workers claim -- and the record of persistent abuse suggests -- that the department steers away from politically powerful targets and possibly dangerous situations, preferring to make its visits with ample notice to the grower and often failing to look behind the "facts" as the grower presents them.

The surest sort of continuous protection would come from strong worker organizations. But farm workers are difficult to unionize because they are easily intimidated by growers and because they face stiff and growing competition from foreign contract workers and illegal immigrants. Cutbacks in legal- service organizations will also diminish what has probably been the workers' most effective source of help. As long as the public confines its indignation to the occasional shocking expos,e, it is likely that America will continue to harvest shame along with its bountiful produce.