Despite all the uproar over agricultural marketing orders, which determine how and when you get oranges, nuts, celery and a bushel of other farm products, the Reagan administration has decided to leave them pretty much alone.

Agriculture Secretary John R. Block this week announced new guidelines for administering the 48 federal marketing orders and agreements, calling for some changes but leaving the basic system intact.

Marketing orders, which cover 34 commodities worth about $5.2 billion, became controversial last year when thousands of tons of navel oranges were kept off the market and fed to cattle or left to rot.

The flow of oranges, like most of the other regulated farm products, is governed by a marketing order. Defenders say the system stabilizes supplies and prices; critics call it sophisticated price-fixing.

The Vice President's Task Force on Regulatory Relief included marketing orders on its study list last spring, and USDA did an economic study of the orders, which led to the new guidelines. Block outlined them to administrators this week, saying he would not propose legislation to alter the system, and industries covered by the orders would be expected to apply the guidelines to their programs.

"Marketing orders make a significant contribution to market stability, and I favor these programs," Block said. "But provisions of some orders may pose inefficiencies on the production and marketing system."

Some key points of the guidelines:

Although authorized by law, volume controls that hold prices up and limit the amount of fruits and vegetables that can go to market should be made less restrictive so that consumers may receive more produce.

Orders that bar new producers from entering the market (hops is one crop mentioned by USDA officials) should be eliminated.

Farmers should be allowed to sell directly to consumers when they have overly ripe, less attractive or blemished produce that would not ordinarily be marketed under the order. Limited quantities of some products (California peaches and Florida limes, for example) already can be sold directly.

The guidelines said, however, that if a given industry feels direct marketing of its product is unacceptable, it must present "convincing evidence" to the department.

Block's proposals also would require industry or marketing committees to make clear how they would ultimately dispose of products put into reserve pools, set-asides of overabundant crops that are returned to market in less abundant times.

As the new guidelines were being issued, more controversy was brewing in the navel-orange groves. Two California orange firms, citing increased demand because of the Florida citrus freeze, called on the navel-orange administrative committee to abandon its weekly shipping allocations. Exeter Orange Co. and Sequoia Orange Co. charged that present schedules mean that 32 percent of this year's crop will be withheld from consumers.