The underside of America's rich green farms and forests is colored gold and silver, symbols of nearly $4.2 billion spent on chemicals last year to kill insects and weeds that menaced the greatest agricultural productivity in history.

Few agriculturists question the value of these pesticides and herbicides. Much of the nation's food abundance is tied directly to chemical aids that wipe out pest and weed threats to crops and timber. The chemical aids also make the work of agriculture easier and farmers, in a sense, have become addicted to them.

As the seeming addiction has grown, so has debate about shielding the public from undue exposure to better living through chemistry. Now, as the Reagan administration begins the third year of its effort to lessen regulation on industry, the debate over controlling the chemical genie has accelerated again.

One problem is that most farming experts agree that crop loss to pests is roughly the same today as it was 40 years ago, when American agriculture began the great rush to pesticides. Many insects have simply become immune to them.

Far worse is the fact that most of the pesticides and herbicides are as lethal to man as to a cabbage worm or a clump of witchweed. In infinitesimal amounts, they enter the food chain and the environment. Traces of some are found commonly in the flesh of Americans who rarely go near a farm or forest.

Some can cause cancer. Others can cause birth defects and miscarriages, nerve and brain disorders; an entire range of physical maladies, particularly among farm workers exposed daily to the compounds. They poison soil, forests, streams, lakes, oceans and air.

Central questions have always been: How much is tolerable? How careful must man be? Is the risk worth the benefit?

Alarms have sounded repeatedly since Rachel Carson warned, just 20 years ago, that man seemed bent on slowly poisoning himself and his habitat. Since then, U.S. pesticide-herbicide usage has more than doubled. On farms and in forests last year, nearly four pounds of these toxicants were used for every man, woman and child in the country, enough to fill a railroad train 43 miles long.

This occurs amid developments that have revived public fears about pesticide dangers and reflect on the Environmental Protection Agency's approach to toxic-chemical control and which include:

* In 1981 and 1982, EPA approvals of "emergency" and "special-use" permits mushroomed, allowing use of otherwise banned or unregistered toxic agents on millions of acres of cropland. EPA maintains that it is bound by law to do so. But critics contend, and some industry spokesmen agree, that states and companies are skirting the law, taking advantage of EPA's penchant for less regulation.

* Although EPA claims "streamlining" has improved efficiency, its regulatory budget and staff have been cut sharply during the last two years. Management emphasis is on paperwork reduction and responsiveness to industry. There are strong signs that this has diminished thoroughness and caution while pressuring scientists to reduce registration backlogs with rubber stamps.

* The most controversial emergency case was EPA approval of a South Carolina petition to put DBCP, a sterility- and cancer-causing agent withdrawn from mainland U.S. use in 1979, on about 20,000 acres of peach trees. The DBCP use was stopped by a court order won by environmentalists.

* In Hawaii, at least 15 dairies on Oahu were shut down last year when officials found traces of heptachlor, a pesticide believed to be carcinogenic, in the island's milk supply. Cows had been fed pineapple leaves sprayed with heptachlor, banned in all states but Hawaii in the 1970s. Poisoned milk was taken off the market; the extent of harm to consumers of the milk will not be known for years.

* Concern grew over Temik, a highly toxic pesticide used on various crops in at least 40 states. Fears arose from contamination of water in potato-growing sections of Long Island and Wisconsin, then increased when dangerous concentrations of Temik were found in a Florida orange grove. The product, known chemically as aldicarb, is used widely on citrus there. Despite petitions citing a health threat from Temik, EPA has not suspended it or tightened tolerance levels for foodstuffs.

* In an epilogue to a saga that began during the Carter administration, EPA bowed to congressional pressure and gave three southern states approval for aerial spraying of ferriamicide, another dangerous toxicant, on fire ants. Ferriamicide is made from Mirex, withdrawn from use in 1978 under threat of a ban by EPA. The new approval was withdrawn after the National Audubon Society brought suit in federal court.

* Controversy raged over allegations of a cancer epidemic caused or abetted by Tordon, a herbicide used widely by timber and power companies, railroads and highway departments and the U.S. Forest Service. Lawsuits pending in at least seven states allege Tordon has caused cancer and other illness. The chemical, called Agent White when used as a defoliant in Vietnam, is regarded by EPA as safe, although many scientists disagree.

As these and other instances of pesticide and herbicide contamination were being argued throughout the United States, Congress was grappling with an extension of the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA), the basic law governing chemical registrations and tolerance levels.

Any chance of passage of the legislation in the last Congress collapsed in a fight among industry, states, the administration and environmental and labor groups. A House-passed bill, stronger than industry, the White House or the House Agriculture Committee wanted, died in the Senate.

House and Senate Agriculture committees are expected to try again on FIFRA this year in the new Congress. But before they reach that point, a House subcommittee chaired by Rep. George E. Brown Jr. (D-Calif.) will hold hearings, probably next month, on EPA regulatory policy on cancer and pesticides.

Brown touched off a furor last month when he circulated a lengthy staff study of EPA enforcement activity. It suggested that the administration has relaxed cancer-prevention policy subtly, opening the public to wider exposure to toxic agricultural chemicals.

Republicans on Brown's subcommittee objected to the study and demanded new hearings. Dr. John A. Todhunter, assistant EPA administrator for pesticides and toxic substances, called it "a political diatribe." To clear the air, Brown has asked an assortment of experts to review and comment on his cancer chapter before his hearings are held.

If all of that seems unclear and confusing, the chemicals-and-agriculture issue has rarely been anything else. For years, farm chemical regulation, such as it was, was in the hands of the Department of Agriculture, a conflicting role for an agency created to put the farmer first.

When President Nixon created the Environmental Protection Agency in 1970, regulatory functions were transferred from the Agriculture Department into its hands. The agency's performance since then has been marked by constant controversy: natural contention between profit-oriented industry and environmentalists, criticism from all sides, congressional investigations, and periodic signal changes from Capitol Hill.

Under FIFRA, the EPA shares regulatory duties with the states. The agency is supposed to set the basic standards and do the basic scientific review and oversight of old and new products. The states, although free to set even stricter limits on chemical usage, handle most of the enforcement and administration of "emergency" exceptions to the law.

The phenomenal growth of the agricultural chemical industry and farmers' increasing reliance on chemical solutions to nature's problems have put extra pressure on the EPA not to upset a juicy apple cart. The industry, represented by the 230-member National Agricultural Chemicals Association (NACA), constantly stresses the importance of pesticides to a well-fed and prosperous nation. The Chemical Specialties Manufacturers Association (CSMA), with more than 400 members, plays a similar tune for the nonagricultural bug killers.

But there is overwhelming irony in all of this. Experts quibble about the numbers, but American crop loss to pests and weeds today is roughly the same as it was when the great rush to pesticides began after World War II. NACA agrees that crop destruction, even with the chemicals, runs about 30 percent annually. Others do not put it that high.

Studies by David Pimentel of the New York State College of Agriculture and Life Sciences indicate a 33-percent annual crop loss to pests, even though use of chemicals has increased about tenfold since the early 1940s. He and others conclude that production losses have been offset by use of higher-yielding seed varieties, more fertilizers and other improved farm-management techniques.

The late Robert van den Bosch, a noted agricultural researcher, looked at the numbers and became a fervent critic of the system. The bugs, he said, simply had immunized themselves against lethal chemicals. In his 1978 book, "The Pesticide Conspiracy," he wrote:

"A major 'reward' of our elevenfold increase in insecticide use has been a doubling of the bug problem. This reflects incredibly bad technology and extremely poor economics--unless, of course, one is selling insecticides."

NACA publications stress the abundance made possible by weed and bug killers, claiming yield gains for most crops and raising the specter of famine and plague in a world without pesticides. NACA is abetted by a federal and state agricultural research network that often receives study funds from industry and often concludes that pesticides, rather than natural approaches, are the farmers' deliverance.

For its part, the production-oriented Department of Agriculture does little to encourage farmers to change their chemical ways, although it has given increasing attention to farming by integrated pest management (IPM), a system mixing pesticide-herbicide use with nonchemical approaches that require keen managerial skills to ensure success.

Agriculture Secretary John R. Block has told Congress that USDA research in many instances has applications to nonchemical, that is, organic farming. But the department appears to have dug in its heels against promoting organic techniques. The department's only full-time specialist on organics was reduced to part-time duty, then dismissed last year as part of budget-cutting.

When members of Congress, headed by Rep. James Weaver (D-Ore.), attempted last year to push the Agriculture Department toward a new look at farming without heavy infusions of chemicals, they were voted down. Block, who once described organic farming research as "a dead end," led administration opposition to a bill to promote nonchemical-farming research at USDA, and it was defeated on the House floor.

The department is hooked in other ways. Its U.S. Forest Service will spend as much as $30 million this fiscal year for chemical warfare on bugs and underbrush that threaten publicly owned timber. At the same time, the Forest Service this year cut more than $2 million, about 20 percent, from a small-scale IPM research program.

In this atmosphere, once extension scientists have documented the need for pesticides and herbicides, industry takes over. Products are promoted heavily in journals read by most farmers. For good reason, many of the product names, hawked in colorful, macho ads, suggest force: Pounce, Ambush, Roundup, Lasso and Bravo; Bladex, Counter, Amaze and Split-Shot. Others conjure mystery and science: Ferriamicide, Temik, Paraquat, Dibromochloropropane, Tordon.

Once caught on these and other chemicals, farmers break the habit only with difficulty. Today's tight economics give a farmer little room for experimenting without chemicals. The risk of ruinous infestation is too great, and the larger-scale farming of the 1960s and 1970s tends to reinforce the farmer's reliance on pesticides and herbicides.

Luther Shaw, a NACA official in Washington, puts it this way: "Our food is not grown; it is manufactured, in a sense. As we got into industrialization and mechanization of agriculture, we entered what I call the chemicalization of American agriculture."

There is another side to the addiction idea. Work done at the Rodale organic farming research center in Pennsylvania has shown that the soil itself reacts adversely to quick withdrawal from chemicals. Rodale researchers have found that from three to five years are required to regenerate cropland to a rich, productive status without synthetic boosters.

Most farmers, who operate on razor-thin margins and must produce in volume, cannot wait that long. A barrel of chemical support is only a phone call away.