The Reagan administration's hopes for a quick turnaround in the sagging farm economy seem increasingly based on the one element that policy makers can't control--the weather.

Addressing farm editors last month, President Reagan picked up on the weather theme. "There is no quick fix for the economy or for our farmers," he said. "Weather will still be a major factor in determining when prices make a recovery."

Agriculture Secretary John R. Block, facing increasing restlessness on the farm and on Capitol Hill, is saying much the same thing. He keeps bravely telling audiences that he sees "glimmers of hope" for better times.

But Block acknowledged to nervous senators last week that bad weather in the 1982 growing season may be the quickest antidote for farmers who face a fourth consecutive year of falling income.

He is encouraging farmers to join his department's acreage reduction program, a supply-control step aimed at cutting planting this year to push up commodity prices. But market experts see the program, if successful, as only a partial solution.

Block is also banking on increased exports to help the income picture, although slumping overseas economies have dimmed demand for U.S. grain. Additionally, the strengthened dollar means that U.S. farmers must sell more just to stay even.

Against this backdrop, congressional jitters are increasing and the secretary undergoes tougher questioning with each subsequent appearance before the agriculture and budget committees.

Last week, for example, at an appropria- tions subcommittee meeting, Sen. Quentin N. Burdick (D-N.D.) asked, "Can farm income rise without legislation?"

"If it is improved," Block said, "it will be from something other than legislation . . . the weather, an export pickup someplace. I fear income may be down again this year."

The income issue has become so sensitive that Block has stopped his department's tradition of publishing monthly net income forecasts. The newest figures circulating in USDA envision a $6 billion drop in 1982 from last year, when farm income, adjusted for inflation, was lower than any time since the Depression.

Block said the forecasts were notoriously inaccurate and he didn't want the administration stuck with them. Added a congressional agriculture aide: "The truth is, the department doesn't want to give validity to the horror stories. The forecasts give credence to what farmers around the country are saying and feeling."

High interest rates and rising production costs, in combination with record-level harvests in 1981 and sagging export income, have put farmers under intense pressure and created fears of widespread farm foreclosures.

Block has insisted that the Farmers Home Administration, contrary to popular belief, is attempting to be as lenient as possible in its credit dealings with the hardest-pressed delinquent farm borrowers. And, he has said repeatedly, FmHA has plenty of money to meet all credit needs.

While the money may be available, figures developed by the Center for Rural Affairs, a Nebraska-based public-interest outfit, suggest that the agency is not lending as much as might be expected as farmers head into spring planting.

"Farmers Home appears to be obligating only about half of what each of the states had been allocated," said Gene Severens of the center. "The worst scenario is that they are not loaning money to new farmers or to delinquents."

The latest data from FmHA, released some weeks ago, indicated that more than half of its farm loans were overdue. The agency reported that the delinquency rate was as high as 80 percent in some states.

From all of this, congressional farm-state Democrats sense a political harvest in an important election year. A group of House Democrats is drafting legislation to increase price support loans and liberalize farm-credit policies.

One of the ringleaders, Rep. Ed Jones (D-Tenn.), chairman of the House agriculture subcommittee on credit and conservation, wondered: "How much more can the American family farmer stand? I can tell you that my farmers in Tennessee have stood about as much as they can stand . . ."

Jones said that he and others would try to come up with "some kind of reasonable alternative to the misguided farm policies which are proving to be disastrous."

Sen. John Melcher (D-Mont.), a member of the Senate Agriculture Committee, plans to introduce a bill this week that would increase the price support loan rates and target price subsidy payments above levels set in the 1981 farm bill, a move he thinks will force market prices upward.