The Washington Post is a weighty newspaper.

Last Tuesday (not a particularly big day, poundage-wise,) it took 225 tons of paper to make it.

The Federal Register and Congressional Record required another 35 tons of newsprint, and the federal government used 175 more tons of paper for memos, copying letters, envelopes, notebooks and sundry red tape. The government also gobbled up 10,000 new heavy cardbaord boxed.

People in Washington also used tons of paper towels, disposable diapers, breakfast food from paper cartons, toilet paper, paper plates and cups and TV dinners packaged in paper.

Finally, there were milk cartons, tens of thousands of them. This country is to milk cartons what Saudi Arabia is to oil. It produces 400,000 tons of them a year for use at home, and ships abroad another 400,000 tons from which they are made.

Most Americans seldom give more than a passing thought to this blizzard of paper perhaps because it is such an everyday convenience. Paper is as pervasive as plastic and steel. It touches ourlives every waking hour. Without it, society would quickly break down so totally do we depend on it to communicate.

Yet the system that brings it to us affects the environment, the labor force, the transportation system, the national economy and the politics of states and regions. Production of paper and pulp products is a $50 billion a year industry, one of the 10 largest in the country. It is such a factor in the economy that the government watches the price of the paper used in heavy cardboard boxes to chart changes in the cost of living.

At the center of this system are the giant international pulp and paper companies - the "multipulps." By almost any measure, these companies are rich, powerful and important.

The sales of 11 paper producing companies exceeded $1 billion apiece last year, and those of 34 others were more than $100 million each. These companies run on enormous amounts of capital, energy and natural resources.

It costs about $220 million today to construct the simplest kinds of mill, one that turns out ordinary brown paper for bags and wrapping. A plant to make the specially treated stiff paper for milk cartons costs more than $400 million and takes up tofive years to build.

The plants consume prodigious amounts of electricity. It takes twice as much energy to produce a ton of paper as a ton of steel.

The mills are also chemical plants, which use millions of tons of chemicals to strengthen and bleach wood fibre, and which extract vats of liquid hydrocarbons from the incoming trees. The tree chemicals are then burned like oil to cook the wood and dry the paper.

Despite the high costs of building mills, profits of the companies have increased sharply since paper prices took a sharp upturn starting in 1973. In the last three years, industry profits have ranged from $1.8 billion to $2.3 billion.

Profits of any industry are always difficult to evaluate because each one has special requirement and accounting methods. The paper giants say they need their earnings and tax advantages in part because of the stringent and costly environmental requirements imposed on them in the last 10 years - especially laws governing sulfur emmissions from coal burning, and purification of the chemical wastes that mills once poured into the nation's waterways.

Yet paper prices rose "more than sufficiently to cover cost changes" in 1974 and 1975, according to a study published last December by the President's Council on Wage and Price Stability. This study noted that the industry's rate of return on equity and its profit margin are "considerably above those achieved in the 1960s."

A substantial amount of the surpluses have been spent in acquisition or expansion. International Paper Co., the world's largest, paid about half a billion dollars in 1975 to acquire an oil company, General Crude. The company hopes to find oil or other minerals on some of International Paper's 8 million acres of timberland.

The vast forests owned or leased by the timber and paper companies constitute a major source of the companies' wealth. The industry wons 67.3 million acreas of commercial timberland, primarily in the South, upper New England and Pacific Northwest. Eight of the companies own one third of the state of Maine, and several million more acres are owned by private landholding companies that work closely with the same companies.

This control of timber resources is pitting the companies against environmental groups in a complex debate over "clearcutting" - the techniqueby which companies mow down whole sections of forest to ankle height.

Criticism of clearcutting, particularly in the West where trees take up to 60 years to grow to full height, may be one factor encouraging an aggressive expansion abroad by several companies.

The Potlatch Corp., which turns out more than 600,000 tons of paper and cardboard a year in the United States, has an ambitiuos project in Western Samoa for cutting large tracts of tropical rain forest to supply a lumber and veneer mill. Other companies are considering major investments in Brazil, which is considered the new timber frontier of the multiples.

The companies have risen to their present economic position on the crest of the country's packaging wave. In the 1950s and 1960s, this packaging revolution had a wide impact on the country's retail system and on national Lifestyles. Packaged convenienced foods filled supermarkets, throwaway milk cartons replaced bottles, linen napkins and cotton dish towels were replaced by paper towels, manufacturers substituted cardboard containiers for wooden shipping crates and high speed copying machines increased the paper blizzard in offices.

For years, America's consumption of paper grew steadily, almost in perfect step with the economy itself. The companies could hardly cut down trees fast enough to feed the mills' need for wood fibre.

Now, however, the signs all point to a slowing down of the paper boom.

The country's output of paper and cardboard slumped from 60 million tons in 1974 to 52 million tons in 1975. It took the industry two years to recover the lost ground. The main reasons was that buyers balked at paying sharply increased prices, and began economizing on their use of paper in various ways.

The Washington Post provided a good example of the economies practiced by big paper users. In 1973, when the cost of a ton of newsprint was $175, the paper used $167,299 tons. This year the paper plans to use only 145,000 tons. The Post accomplished this saving several ways. It slightly reduced the width of the paper that rolls onto its cylindrical presses. It used slightly thinner (and lighter) paper. And it printed six columns of news and nine columns of advertisements, instead of eight of each on a page.

It may say something about national culture and tastes these days that the only sector of the paper economy exhibiting dramatic growth is coated paper - the glossy, clay-treated paper used for slick magazines and annual reports.

Paper company executives say that the magazine industry is flourishing as never before, partly because of the boom in publications catering to lurid sexual interests.

Otherwise, says International Paper's executive vice president Donald Brennan, "the industry is growing at a declining rate."

"Most of the substitution for other materials has already taken place," he adds. One example is the disposable diaper market whose growth has talled off along with the baby boom.

Oddly enough, plastics made from petroleum stocks are penetrating paper markets strongly.

Petroleum cost are high - but energy costs are still higher, and paper plants use more energy than almost any other. "Economics marginally favors plastic systems at the moment," says Brennan. Companies which have diversified into the chemical industry, such as George Pacific, now have an advantage over more conventional paper companies.

The slack growth in most sectors of the paper economy could, ultimately, cause severe disruptions in the industey.Conceivably, however, it could be good news for conservationist.

The U.S. timber and paper industry developed at the turn of the century on a cut-and-move principle. Only a few farsighted companies such as Weyerhaeuser attempted to accumulate land and preserve the forests. Millions of acres of forests were plundered as the country moved west. In the era of the paper boom, there were continued pressures to pillage huge tracts as quickly as possible to keep mills operating.

With the slowing down of the growth in paper consumption, some companies are stressing the importance of efficiency and conservation instead of unbridled production involving haphazard cutting.

"The old wasteful days are gone, and 1975 was a clear signal of that," wrote President George H. Weyerhaeuser, of that company, in the 1975 annual report. "All of us must relearn the arts of conservative and efficient use of the materials which nature and human effort have provided."

On the other hand, critics of timber companies say, the multipulps are stressing the "efficiency" of the controversial clearcutting methods. The critics say the technique despoils the widerness. But the companies insist that they get more wood per acre in this way, and can manage the land better by planting seedlings, much as farmers now plant wheat and corn.

The efficiencies are needed, say company executives, to cope with a number of new pressures beyond the showdown in paper usage. Officials often cited environmental laws as one of these pressures.

"It's Catch 22," says International Paper's Brennan. "In the 1960s, we were ordered to convert coalfired boilers for use with gas and low sulphur oils. Now we are faced with orders to convert back to coal for energy conservation reasons."

The companies also say they are in one of the most competitive businesses in the United States.

"We're out scratching and fighting every day," says Al Cass, a New York City paper merchant who distributes stationery, office paper and annual reports paper for International Paper.

Given the huge amounts of capital and cash needed to operate in the industry, there are a surprisingly large number of companies producing paper. The American Paper Institute in New York City has 150 member companies.

In some respects, though, these numbers are misleading.

For instance, only four major companies - Weyerhaeuser, International Paper, Westvaco and Champion International - make almost all milk carton containiers produced in the United States.

It is not hard to see why that is so. It is a highly complex technology and marketing process.

There are stringent federal sanitary requirements for making and selling this item. The pulp for milk carton paper comes from virgin fibre, usually pine. The pulp is bleached, treated with a polythylene coating to prevent leaking, rolled into spindles and shipped in special wrapping to plants with tile walls and operating room sanitary conditions for conversion into containers. Then the cartons are sold to hundreds of dairies and milk chains.

For the dairies, the number of suppliers is mainly limited to the big four.

That may or may not be a factor in the cost of milk. What is known is that the paper companies currently are embroiled in a number of lawsuits charging price fixing and anticompetitive practices. The Justice Department's antitrust divisions is examining a number of complaints that arose in 1973 and 1974 when paper supplies were extremely tight and some companies allocated supplies to customers.

Still, paper company executives insist that the wealth, profits and power of the companies are overestimated.

For one thing, they note, the vast timberlands they own have little value until the companies bulldoze roads, run in power lines and build installations. Also, the profits and cash pouring into the companies are now large compared to the expense of new mills.

"We have the same problems as the steel industry," Brennan says. "The difference is that their problems are better known to the public."