The Reagan administration's farm "summit" meeting of farmers and a Who's-Who cast from agribusiness opened yesterday with a call from Secretary of Agriculture John R. Block to think of farm policy in terms of the long-run future.

Public and press were excluded from workshop sessions involving about 70 representatives of farm groups, trade associations, equipment and food firms, who were cautioned by Block that "this is not the time and place" to debate current farm problems.

Rather, Block said, he wanted his hand-picked delegates to "think in terms of decades--not months or even years. We should look at our industry in an almost philosophical sense . . . . We must develop a more responsible approach to the future--one which will give agriculture the flexibility it needs to cope with the volatile world in which it must exist."

"Let me reemphasize that we won't be arriving at any conclusions during this summit. That was never the objective. Instead, we are starting a dialogue," the secretary said. Block said yesterday's sessions produced "spirited debate" with a focus on the importance of exports to U.S. agriculture.

But he declined to say much more about the discussion.

The delegates also heard from Vice President Bush, Secretary of State George P. Shultz and Reagan economic adviser Herbert Stein.

Block convened the two-day session in an atmosphere of growing criticism of the cost of the administration's payment-in-kind (PIK) program to reduce crop surpluses and mounting concern over the record $21 billion-plus that federal farm support programs will cost taxpayers this year.

The Democratic National Committee was quick to exploit the administration's closed-door summit by announcing its plans for a nationwide series of agricultural policy meetings as part of next year's presidential campaign and development of a 1985 farm bill.

Reporters who were turned away from the Block meeting at a luxury hotel flocked to a news conference to hear Texas Agriculture Commissioner Jim Hightower, who will head the DNC agriculture council, DNC Chairman Charles T. Manatt and several farm state Democratic congressmen take turns blistering the current policies that Block's delegates were admonished to avoid. Hightower, saying the president "wouldn't know a hog from a howitzer," charged that Reagan farm programs are bankrupting U.S. farmers while depressing prices and "swindling the general public."

He said Reagan advisers' "idea of a good farm program is 'Hee Haw', but it is no laughing matter."

Hightower denounced the summit as "a bunch of deadheads here trying to design a farm policy behind closed doors." He said the DNC forums, open to all, will aim at developing "a new approach to agriculture that helps our family-sized operators receive a market price high enough to keep them in business."

By department count, less than half of the summit delegates were full-time farmers, a decision Block defended because "the total industry has something at stake."

But the invitation list was top-heavy with representative of agribusiness, banking, manufacturing, grain trading and trade-lobbying firms. While the American Agriculture Movement, for example, got one delegate, General Mills got two; while the American Farm Bureau Federation had one, the tractor-making Deere and Co. had two.

Only one governor (Ted Schwinden of Montana), one consumer-nutrition organization (Public Voice) and one conservation group (National Association of Conservation Districts) attended.